RIVER AND CANAL 
 ENGINEERING 
 
 E,S. BELLASIS 
 
RIVER AND CANAL ENGINEERING 
 
Sy the Same Author 
 
 HYDRAULICS WITH WORKING 
 TABLES. Second Edition. 160 
 illustrations, xii + 3ii pp., 8vo (1911). 
 12/- net. 
 
 PUNJAB RIVERS AND WORKS. 
 Second Edition. 47 illustrations, viii 
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 IRRIGATION WORKS (in the press). 
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RIVER AND CANAL 
 ENGI NEE RI NG 
 
 THE CHARACTERISTICS OF OPEN FLOW- 
 ING STREAMS, AND THE PRINCIPLES 
 AND METHODS TO BE FOLLOWED IN 
 DEALING WITH THEM 
 
 BY 
 
 E. S. pELLASIS, M.lNST.C.E. 
 
 RECENTLY SUPERINTENDING ENGINEER IN. THE IRRIGATION BRANCH OF 
 THE PUBLIC WORKS DEPARTMENT OF INDIA 
 
 ILLUSTRATIONS 
 
 Xottbon 
 
 E. & F. N. SPON, LTD., 57 HAYMARKET, S.W, 
 mew j^orfc 
 
 SPON & CHAMBERLAIN, 123 LIBERTY STREET 
 1913 
 
TABLE OF CONTENTS 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 ARTICLE PAGE 
 
 1. Preliminary Remarks . . 
 
 2. Resume of the Subject . . . . . 1 
 
 3. Design and Execution of Works . . , .3 
 
 4. The Hydraulics of Open Streams . '. . 1 ^ 
 
 CHAPTER II 
 
 RAINFALL 
 
 1. Rainfall Statistics -. '. -.' . .; 6 
 
 2. Available Rainfall . . . . 9 
 
 3. Measurement of Rainfall . . '".. .13 
 
 4. Influence of Forests and Vegetation . y . 14 
 
 5. Heavy Falls in Short Periods . .15 
 
 CHAPTER III 
 
 COLLECTION OF INFORMATION CONCERNING STREAMS 
 
 1. Preliminary Remarks . . . . . 18 
 
 2. Stream Gauges ' . ,. . . . . 19 
 
 3. Plan and Sections . ' . . . . 21 
 
 4. Discharge Observations . . . . .21 
 
 5. Discharge Curves and Tables . . . .23 
 
 6. Small Streams , . . . . .24 
 
 7. Intermittent Streams . . . . . 25 
 
 8. Remarks . 26 
 
vi RIVER AND CANAL ENGINEERING 
 
 CHAPTER IV 
 
 THE SILTING AND SCOURING ACTION OF STREAMS 
 
 ARTICLE PAGE 
 
 1. Preliminary Remarks . . - . . . 27 
 
 2. Rolled Material . . .29 
 
 3. Materials carried in Suspension . . .' .31 
 
 4. Methods of Investigation . . . . .33 
 
 5. Quantity and Distribution of Silt . . .35 
 
 6. Practical Formulae and Figures . . 37 
 
 7. Action on the Sides of a Channel . . .40 
 
 8. Action at Bends . . . . .42 
 
 9. General Tendencies of Streams . . . . 45 
 
 CHAPTER V 
 
 METHODS OF INCREASING OR REDUCING SILTING OR SCOUR 
 
 1. Preliminary Remarks . . . . .48 
 
 2. Increase of Scour or Reduction of Silting . . 48 
 
 3. Production of Silt Deposit . . . .51 
 
 4. Arrangements at Bifurcations . . .53 
 
 5. A Canal with Headworks in a River . . , . 54 
 
 6. Protection of the Bed . . . .58 
 
 CHAPTER VI 
 
 WORKS FOR THE PROTECTION OF BANKS 
 
 1. Preliminary Remarks . . . ..V GO 
 
 2. Spurs . . . . . . ,.- 61 
 
 3. Continuous Lining of the Bank . . . . . 64 
 
 4. Heavy Stone Pitching with Apron . .. .71 
 
 CHAPTER VII 
 
 DIVERSIONS AND CLOSURES OF STREAMS 
 
 1. Diversions . . ' . . . . . 73 
 
 2. Closure of a Flowing Stream . . ..75 
 
 3. Instances of Closures of Streams . ].. .-.,,' 80 
 
TABLE OF CONTENTS vii 
 CHAPTER VIII 
 
 THE TRAINING AND CANALISATION OF RIVERS 
 
 ARTICLE PAGE 
 
 1. Preliminary Remarks . , ; . .84 
 
 2. Dredging and Excavating . . ^ 84 
 
 3. Reduction of Width . . . . . 85 
 
 4. Alteration of Depth or Water-Level . . .88 
 
 5. Training and Canalising . . . . . . 89 
 
 CHAPTER IX 
 
 CANALS AND CONDUITS 
 
 1. Banks . ... . . .92 
 
 2. Navigation Canals . . . . ..93 
 
 3. Locks . . '. . . .96 
 
 4. Other Artificial Channels .. . . . 100 
 
 CHAPTER X 
 
 WEIRS AND SLUICES 
 
 1. Preliminary Remarks . . . .. . . . 102 
 
 2. General Design of a Weir , . ' .. . 105 
 
 3. Weirs on Sandy or Porous Soil . . . 106 
 
 4. Various Types of Weirs . . . . .111 
 
 5. Weirs with Sluices . . . . .115 
 
 6. Falling Shutters .... . . 121 
 
 7. Adjustable Weirs . . -, . . 126 
 
 8. Remarks on Sluices . . . . , .128 
 
 CHAPTER XI 
 
 BRIDGES AND SYPHONS 
 
 1. Bridges . . . . . . 132 
 
 2. Syphons and Culverts . . . . .135 
 
 3. Training Works ... 136 
 
viii RIVER AND CANAL ENGINEERING 
 
 CHAPTER XII 
 
 DRAINAGE AND FLOODS 
 
 ARTICLE PAGE 
 
 1. Preliminary Remarks . . . . . 141 
 
 2. Small Streams . . . ' . .. ,141 
 
 3. Rivers . . . . ./ . 146 
 
 4. Prediction of Floods . . . . 150 
 
 5. Prevention of Floods . ... 153 
 
 6. Lowering the Water-Level . . . .154 
 
 7. Flood Embankments . . . . .156 
 
 CHAPTER XIII 
 
 RESERVOIRS AND DAMS 
 
 1. Reservoirs . 162 
 
 2. Capacity of Reservoirs . . 167 
 
 3. Earthen Dams . . . . . .174 
 
 4. Masonry Dams . . ... 181 
 
 CHAPTER XIV 
 
 TIDAL WATERS AND WORKS 
 
 1. Tides ..... . . 190 
 
 2. Tidal Rivers . ... . . .192 
 
 3. Works in Tidal Rivers . . .196 
 
 4. Tidal Estuaries . .... 197 
 
 5. Works in Tidal Estuaries . . . V 198 
 
 CHAPTER XV 
 
 RIVER BARS 
 
 1. Deltaic Rivers . ... . . . . . . 203 
 
 2. Other Rivers . . . . . .205 
 
 APPENDIX A. Fallacies in the Hydraulics of Streams . 209 
 
 B. Pitching and Bed Protection - . |%- 212 
 
 INDEX 213 
 
PREFACE 
 
 THE object of this book is to describe the principles and 
 practice adopted in the Engineering of Open Streams. 
 If the book seems to be somewhat small for its object, 
 it will, it is hoped, be found that this is due to care 
 in the arrangement and wording. 
 
 Sources of information have been acknowledged in 
 the text, but special mention may be made of lectures 
 given by Professor Unwin at Coopers Hill College, of 
 Harcourt's large work on Rivers and Canals, of the 
 papers 1 by Binnie on rainfall, by Shaw on the closing 
 of the river Tista, by Harcourt on movable weirs and 
 on estuaries, by Strange on reservoirs, and by Ottley and 
 Brightmore, Gore and Wilson, and Hill on the stresses 
 in masonry dams ; of the articles by Bligh 2 on weirs 
 with porous foundations and by Deacon 3 on reservoir 
 capacity, of the Indian Government paper by Spring on 
 " River Control on the Guide Bank System," and of the 
 Punjab Government paper containing Kennedy's remarks 
 on silting and scour in the Sirhind Canal. The two 
 papers last mentioned are not easily accessible, and they 
 contain matter of great interest. The important points, 
 often obscured by masses of detail or figures, have been 
 extracted. 4 
 
 1 Min. Proc. Inst. O.E. 2 Engineering News. 
 
 3 Encyclopedia Britannica. 
 
 4 The paper by Spring in size it is a book will repay perusal by 
 engineers engaged on railway bridges over large shifting rivers. London 
 Agents, Constable & Co. 
 
X RIVER AND CANAL ENGINEERING 
 
 Silting and scour (CHAP. IV.) had already been dealt 
 with in Hydraulics^ but some further information has 
 since come to light and the subject has been treated 
 afresh and the matter re- written. 
 
 1 Hydraulics with Working Tables. Spon, 1912. 
 
 E. S. B. 
 
 CHELTENHAM, 1st May 1913. 
 
RIVER AND CANAL ENGINEERING 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 1. Preliminary Remarks. River and Canal Engineer- 
 ing is that branch of engineering science which deals 
 with the characteristics of streams flowing in open 
 channels, and with the principles and methods which 
 should be followed in dealing with, altering, and con- 
 trolling them. It is not necessary to make a general 
 distinction between natural and artificial streams ; some 
 irrigation canals or other artificial channels are as large 
 as rivers and have many of the same characteristics. 
 Any special remarks applicable to either class will be 
 given as occasion requires. 
 
 2. R6sum6 of the Subject. CHAP. II. of this book 
 deals with the collection of information concerning 
 streams, a procedure which is necessary before any 
 considerable work in connection with a stream can be 
 undertaken, and often before it can even be decided 
 whether or not it is to be undertaken. CHAP. III. deals 
 with rainfall, and describes how rainfall figures and 
 statistics can be utilised by the engineer in dealing 
 with streams. 
 
 CHAP. IV. explains the laws of silting and scouring 
 
 action, a subject of great importance and one to which 
 
 i B 
 
2 RIVER AND CANAL ENGINEERING 
 
 the attention ordinarily given is insufficient. The 
 general characteristics of streams, being due entirely 
 to silting or scouring tendencies, are included in this 
 chapter. CHAP. V. describes how silting or scouring 
 may be, under some circumstances, artificially induced 
 or retarded. 
 
 CHAP. VI. deals with various methods of protecting 
 banks against erosion or damage. CHAP. VII. treats of 
 diversions or the opening out of new channels, and 
 with the opposite of this, viz. the closing of channels, a 
 feat which, when the stream is flowing, is sometimes 
 very difficult to achieve. This chapter also deals with 
 dredging and excavation. 
 
 CHAP. VIII. discusses the subject of the training of 
 streams, a class of work which is generally undertaken 
 in order to make them navigable or to improve their 
 existing capacities for navigation, but may be under- 
 taken for other reasons. The main features of this 
 kind of work are the narrowing and deepening of the 
 stream, often the reduction of the velocity and slope, 
 and generally the raising of the water-level. In this 
 kind of work a channel may be completely remodelled 
 and even new reaches constructed. CHAP. IX. deals 
 with artificial channels of earth or masonry, and includes 
 navigation canals. 1 
 
 In CHAPS. X. and XL the chief masonry works or 
 isolated structures as distinguished from general works 
 which extend over considerable lengths of channel are 
 dealt with, and those principles of design discussed 
 which affect the works in their hydraulic capacities. 
 General principles of design applicable to all kinds of 
 works, such as the thicknesses of arches or retaining 
 
 1 Irrigation canals are dealt with in Irrigation Works (Spon, 1913). 
 
 \ 
 
INTRODUCTION 3 
 
 walls, are not considered ; they can be found in books 
 on general engineering design. 
 
 CHAP. XII. treats of storm waters and river floods, 
 and shows how works can be designed for getting rid 
 of flood water and how floods can be mitigated or 
 prevented, one of the chief measures, the widening of 
 the channel and the lowering of the water-level, being 
 the opposite of that adopted for training works. 
 Embankments for stopping flooding are also dealt with. 
 CHAP. XIII. deals with reservoirs, including the design - 
 of earthen and masonry dams. 
 
 CHAPS. XIV. and XV. deal with tidal waters, river 
 mouths and estuaries, and works in connection with 
 them, viz. the training of estuaries and the methods 
 of dealing with bars, the object being in all cases the 
 improvement of the navigable capacities of the channels. 
 
 3. Design and Execution of Works. After obtain- 
 ing full information concerning the stream to be dealt 
 with, careful calculations are, in the case of any large 
 and important work, made as to the effects which will 
 be produced by it. These effects cannot always be 
 exactly foreseen. Sometimes matters can be arranged so 
 that the work can be stopped short at some stage with- 
 out destroying the utility of the portion done, or so that 
 the completed work can be altered to some extent. 
 
 In works for controlling streams there is, as will 
 appear in due course, a considerable choice of types of 
 work and methods of construction. In practice it will 
 generally be found that there are, in any particular 
 locality, reasons for giving preference to one particular 
 type or kind of work or, at all events, that the choice is 
 limited to a few of them, either because certain kinds of 
 materials and appliances can be obtained more cheaply 
 
 B2 
 
4 RIVER AND CANAL ENGINEERING 
 
 and readily than others, or because works of a particular 
 type have already been successfully adopted there, or 
 because the people of the district are accustomed to 
 certain classes of work or methods of construction. In 
 out-of-the-way places it is often undesirable to avoid 
 any type of work which cannot be quickly repaired or 
 readily kept in order by such means as exist near 
 the spot. 
 
 It is sometimes said that perishable materials, such as 
 trees, stakes, and brushwood, cannot produce permanent 
 results. They can produce results which will last for a 
 long time and which may even be permanent. By the 
 time the materials have decayed, the changes wrought 
 may have been very great, deposits of shingle or silt 
 may have occurred and become covered with vegetation, 
 and there may be little tendency for matters to revert 
 to their former condition. If the expense of using more 
 lasting materials had had to be incurred, the works 
 might never have been carried out at all. On the 
 Mississippi enormous quantities of work have been 
 done with fascines. 
 
 4. The Hydraulics of Open Streams. When any 
 reach of a stream is altered, say by widening, narrowing, 
 or deepening, so that the water-level is changed, there 
 will also be a change in the water-level, a gradually 
 diminishing change, for some distance upstream of the 
 reach. Also in the lowest portion of the reach the 
 change will gradually diminish and it will vanish at the 
 extreme downstream end of the reach. In the next 
 lowest reach there is no change. Thus if it is desired 
 that the change in the water-level shall take full effect 
 throughout the whole of a reach, the change in the 
 channel must be carried further down. If a weir is 
 
INTRODUCTION 5 
 
 built there is no change in the water-level downstream 
 of it except such as may be due to loss of water in the 
 reach upstream of it. The above points are mentioned 
 here because, although they are really questions of 
 hydraulics, they are of much importance and of very 
 general application. 
 
 Matters connected with the hydraulics of open streams 
 seem to lend themselves in a peculiar way to loosely 
 expressed remarks and fallacious opinions. The set of a 
 stream towards a bank is sometimes supposed to pro- 
 foundly affect the discharge of a diversion or branch. 
 Its effect is simply that of " velocity of approach," 
 which, as is well known, is quite small with ordinary 
 velocities, and is merely equivalent to a very small 
 increase of head. Narrow bridges or other works are 
 sometimes said to seriously " obstruct" a stream without 
 any observations being made of the fall in the water 
 surface through the bridge. This fall is the only 
 measure of the real obstruction. 1 
 
 1 See also Appendix A. 
 
CHAPTER II 
 
 RAINFALL 
 
 1. Rainfall Statistics. The mean annual rainfall 
 varies very greatly according to the locality. In 
 England it varies from about 20 inches at Hunstanton 
 in Cambridgeshire, to about 200 inches at Seathwaite 
 in Cumberland ; in India, from 2 or 3 inches in parts 
 of Scinde, to 450 inches or more at Cherrapunji in the 
 Eastern Himalayas. 
 
 Rain is brought by winds which blow across the sea. 
 Hence the rainfall in any country is generally greatest 
 in those localities where the prevailing winds blow 
 from seaward, provided they have travelled a great 
 distance over the sea. Rainfall is greater among hills 
 than elsewhere, because the temperature at great eleva- 
 tions is lower. Currents of moist air striking the hills 
 are deflected upwards, become cooled, and the water 
 vapour becomes rain. This process, if the hills are not 
 lofty, may not produce its full effect till the air currents 
 have passed over the hills, and thus the rainfall on the 
 leeward slopes may be greater than elsewhere, but on 
 the inner and more lofty ranges the rainfall is generally 
 greatest on the windward side. 
 
 Thus the rainfall may vary greatly at places not far 
 apart. An extreme instance of this occurs in the 
 
RAINFALL 7 
 
 Bombay hills, where the mean annual falls at two 
 stations only ten miles apart are respectively 300 inches 
 and 50 inches. 
 
 In temperate climates the rainfall is generally dis- 
 tributed over all the months of the year ; in the tropics 
 the great bulk of the rain often falls in a few months. 
 
 The fall at any one place varies greatly from year to 
 year. To obtain really reliable figures concerning any 
 place, observations at that place should extend over a 
 period of thirty to thirty-five years. The figures of the 
 mean annual fall will then probably be correct to within 
 2 per cent. The degree of accuracy to be expected in 
 results deduced from observations extending over shorter 
 periods is as follows : 
 
 No. of years . 25 20 15 10 5 
 Error per cent. . 3 3 5 815 
 
 These figures were deduced by Binnie (Min. Proc. Inst. 
 C.E., vol. cix.) from an examination of rainfall figures 
 obtained over long periods of time at many places scat- 
 tered over the world. The errors may, of course, be plus 
 or minus. They are the averages of the errors actually 
 found, and are themselves subject to fluctuations. Thus 
 the 15 per cent, error for a five-year period may be 
 16 or 13, the 8 per cent, error for a ten-year period 
 may be 8j or 7|-, with similar but minute fluctations 
 for the other periods. 
 
 Binnie's figures also show that the ratio of the fall at 
 any place in the driest year to the mean annual fall, 
 averages "51 to *68, with a general average of *60, and 
 that the ratio in the wettest year to the mean annual 
 fall averages 1*41 to 175, with a general average of 
 1*51. For India the general averages are *50 and 1*75. 
 
8 RIVER AND CANAL ENGINEERING 
 
 These figures are useful as a means of estimating the 
 probable greatest and least annual fall, but they are 
 averages for groups of places. The greatest fall at any 
 particular place may occasionally be twice the mean 
 annual fall. At some places in India, in Mauritius, and 
 at Marseilles it has been two and a half times the mean 
 annual fall. The least annual fall may. in India, be as 
 low as *27 of the mean. In England the fall in a dry 
 year has, once at least, been found to be only "30 of the 
 mean annual fall. The mean fall (average for all places) 
 in the three driest years is, from Binnie's figures, about 
 76 of the mean annual fall. The figures given above, 
 except when a particular country is mentioned, apply 
 to all countries and to places where the rainfall is heavy, 
 as well as to those where it is light. But in extremely 
 dry places the fluctuations are likely to be much greater. 
 At Kurrachee, with a mean annual fall of only 7*5 inches, 
 the fall in a very wet year has been found to be 373 
 times, and in a very dry year only "07 times the mean 
 annual fall. 
 
 In the United Kingdom the probable rainfall at any 
 place in the driest year may be taken as "63 of the 
 mean annual fall. For periods of two, three, four, five 
 and six consecutive dry years, the figures are "72, 77, 
 *80, '82, and "835. These figures are of importance in 
 calculations for the capacity of reservoirs (CHAP. XIIL, 
 Art. 2). 
 
 When accurate statistics of rainfall are required for 
 any work, the rainfall of the tract concerned must be 
 specially studied and local figures obtained for as many 
 years as possible. Very frequently it is necessary to 
 set up a rain-gauge, or several if the tract is extensive 
 or consists of several areas at different elevations. 
 
RAINFALL 9 
 
 Sometimes there is only a year or so in which to collect 
 figures. In this case the ratio of the observed fall to 
 that, for the same period, at the nearest station where 
 regular records are kept, is calculated. This ratio is 
 assumed to hold good throughout, and thus the probable 
 rainfall figures for the new station can be obtained for 
 the whole period over which the records have been 
 kept at the regular station. The volumes of the British 
 Rainfall Organisation contain a vast amount of informa- 
 tion regarding rainfall. For a large area there should 
 be one rain-gauge for every 500 acres, for a small area 
 more. In the case of a valley there should be at least 
 three gauges along the line of the deepest part one at 
 the highest point, one at the lowest, and one midway 
 as regards height and two gauges half way up the 
 sides and opposite the middle gauge (Ency. Brit. , Tenth 
 Edition, vol. xxxiii.). Some extra gauges may be set up 
 for short periods in order to see whether the regular 
 gauges give fair indications of the rainfall of the tract. 
 If they do not do so some allowances can be made 
 for this. 
 
 2. Available Rainfall. The area drained by a stream 
 is called its " catchment area" or "basin." The avail- 
 able rainfall in a catchment area is the total fall less the 
 quantity which is evaporated or absorbed by vegetation. 
 The evaporation does not chiefly take place directly from 
 the surface. Rain sinks a short distance into the ground, 
 and is subsequently evaporated. The available rainfall 
 does not all flow directly into the streams. Some sinks 
 deep into the ground and forms springs, and these many 
 months later augment the flow of the stream and main- 
 tain it in dry seasons. The available rainfall of a given 
 catchment area is known as the " yield " of that area. 
 
10 RIVER AND CANAL ENGINEERING 
 
 Estimation of the available rainfall is necessary 
 chiefly in cases where water is to be stored in reservoirs 
 for town supply or irrigation. The ratio of the avail- 
 able to the total rainfall depends chiefly on the nature 
 and steepness of the surface of the catchment area, on 
 the temperature and dryness of the air, and on the 
 amount and distribution of the rainfall. The ratio is 
 far greater when the falls are heavy than when they 
 are light. Again, when the ground is fairly dry and the 
 temperature high as in summer in England nearly 
 the whole of the rainfall may evaporate ; but when the 
 ground is soaked and the temperature low as in late 
 autumn and winter in England the bulk of the rain- 
 fall runs off. In the eighteen years from 1893 to 1900 
 the average discharge of the Thames at Teddington, 
 after allowing for abstractions by water companies, was 
 in July, August, and September 12 per cent, of the rain- 
 fall 6*9 inches in its basin, and in January, February, 
 and March 60 per cent, of the fall which was 5*9 inches. 
 The total fall in the year was 26 '4 inches. Some 
 rivers in Spain discharge, in years of heavy rainfall, 
 39 per cent., and in years of light rainfall 9 per cent, 
 of the rainfall (Min. Proc. Inst. C.E., vol. clxvii.). 
 The discharge of a river is not always greatest in the 
 month, or even the year, of greatest rainfall. 
 
 The table opposite gives some figures obtained by 
 comparison of rainfall figures and stream discharges. 
 The case of the area of 2208 acres near Cape Town is 
 described in a paper by Bartlett (Min. Proc. Inst. C.E., 
 vol. clxxxviii.), and it is shown by figures that part of 
 the rainfall in the rainy season went to increase the 
 underground supply which afterwards maintained the 
 flow in the dry season. 
 
RAINFALL 
 
 11 
 
 OJ 
 
 ^.41 
 
 o a 
 
 O 
 
 II 
 II 
 
 
 i i i i O 
 C<1 IO TfH 
 
 O 
 
 ip 
 
 O O5 1^ r^H CO 
 
 CO < (M CO ^ 
 
 !|j 
 
 111 
 
 o S5 
 
 X 
 
 w 
 
 8 
 
 
 If 
 
 I 53 
 
 fa g' 
 
 w o - 
 
 s 
 
 ; 
 
 |i 
 
 Z 
 
 6 
 
 GO O O OO 
 
 s 
 
 ' ' a " 0, ' Q, ' fl ' . 
 
 - 
 
 
 O 
 
 i 
 
12 
 
 RIVER AND CANAL ENGINEERING 
 
 The following statement shows how the available 
 rainfall may vary from year to year. The figures are 
 those of a catchment area of 50 square miles on the 
 Cataract River, New South Wales (Mm. Proc. Inst. 
 C.E., vol. clxxxi.): 
 
 Year. 
 
 Rainfall. 
 
 Available 
 Rainfall. 
 
 Remarks. 
 
 
 Inches. 
 
 Ratio to Total. 
 
 
 1895 
 
 34-1 
 
 84 
 
 Heavy rain falling on saturated area. 
 
 1896 
 
 33-7 
 
 28 
 
 Evenly distributed fall. 
 
 1897 
 
 44-7 
 
 49 
 
 Heavy rains in May. 
 
 1898 
 
 56-4 
 
 45 
 
 ,, ,, February (15 ins.). 
 
 1899 
 
 549 
 
 43 
 
 August (11-5 ins.). 
 
 1900 
 
 26-1 
 
 50 
 
 May and July. 
 
 1901 
 
 37-4 
 
 11 
 
 Evenly distributed fall. 
 
 1902 
 
 29-9 
 
 06 
 
 
 1903 
 
 41-7 
 
 23 
 
 No heavy fall. 
 
 The manner in which the available rainfall may vary 
 from month to month is shown in the following state- 
 ment, which gives the figures for 1905 for the Sudbury 
 River in Massachusetts : 
 
 Month. 
 
 Rainfall. 
 
 Available Rainfall. 
 
 January .... 
 February . 
 March . . , N 
 
 Inches. 
 5-3 
 2-2 
 3-2 
 
 Percentage of fall. 
 48 
 24 
 142 
 
 April . , . 
 May . . . 
 June . . . 
 
 2-7 
 1-3 
 5-0 
 
 104 
 40 
 16 
 
 July ... 
 August . " . 
 September 
 October .... 
 
 5-5 
 
 2-7 
 6-9 
 1-5 
 
 6 
 8 
 31 
 18 
 
 November V . . 
 
 2-1 
 
 23 
 
 December . ~ ; 
 
 4-0 
 
 40 
 
 Total 
 
 42-3 
 
 Average 39*5 
 
RAINFALL 13 
 
 Rankine gives the ratio of the available rainfall to 
 the whole fall as I'O on steep rocks, *8 to '6 on moor- 
 land and hilly pasture, '5 to *4 on flat, cultivated 
 country, and nil on chalk. These figures are only rough. 
 The figures for rocks and pastures are too high. The 
 loss from evaporation and absorption is not propor- 
 tional to the rainfall. It is far more correct to consider 
 the loss as a fairly constant quantity in any given 
 locality but increasing somewhat when the rainfall is 
 great. The available rainfall in Great Britain has 
 generally been overestimated. Sometimes it has been 
 taken as being "60 of the whole fall. More commonly 
 the loss is taken to be 13 to 15 inches. This is correct 
 for the western mountain districts, where the rainfall is 
 about 80 inches and the soil consists chiefly of rocks 
 partly covered with moorland or pasture. In other 
 parts of the country, especially where flat, the loss is 
 often 17 to 20 inches. All the above figures are, 
 however, general averages. The proper estimation of 
 the available rainfall at any place in any country 
 depends a great deal on experience and judgment, and 
 on the extent to which figures for actual cases of 
 similar character are available. Regarding the " run- 
 off" from saturated land during short periods, see 
 CHAP. XII. , Arts. 1 and 2. 
 
 3. Measurement of Rainfall. A rain-gauge should 
 be in open ground and not sheltered by objects of any 
 kind. The ordinary rain-gauge is a short cylinder. 
 This is often connected by a tapering piece to a longer 
 cylinder of smaller diameter. In this the rain is stored 
 safely and is measured by a graduated rod. The 
 measurement can be made more accurately than if the 
 diameter was throughout the same as at the top. In 
 
14 RIVER AND CANAL ENGINEERING 
 
 other cases the water is poured out of the cylinder into 
 a measuring vessel. If the rain-gauge was sunk so that 
 the top was level with the ground, rain falling out- 
 side the gauge would splash into it and vitiate the 
 readings unless the gauge was surrounded by a trench. 
 Ordinarily the top of the gauge is from 1 to 3 feet 
 above the ground. When it is 1 metre above the 
 ground the rain registered is said to be on the average 
 about 6 per cent, less than it should be, owing to the 
 fact that wind causes eddies and currents and carries 
 away drops which should have fallen into the gauge. 
 The velocity of the wind increases with the height 
 above the ground, and so does the error of the rain- 
 gauge. Devices for getting rid of the eddies have been 
 invented by Boernstein and Nipher (Ency. Brit., Tenth 
 Edition, vol. xviii.), but they have not yet come into 
 general use. The Boernstein device is being used 
 experimentally at Eskdalemuir. It would appear that 
 much splashing cannot take place when the ground is 
 covered with grass, and that in such a case the top of 
 the gauge could be 1 foot above the ground, thus 
 making the error very small. 
 
 If the ground is at first level, then rises and then 
 again becomes level, a rain-gauge at the foot of the 
 slope will, with the prevailing wind blowing up the 
 slope, register too much, and a rain-gauge just beyond 
 the top of the slope will register too little (Ency. Brit., 
 Tenth Edition, vol. xxxiii.). 
 
 4. Influence of Forests and Vegetation. When 
 the ground is covered with vegetation, and especially 
 forests, the humus or mould formed from leaves, etc., 
 absorbs and retains moisture. It acts like a reservoir, 
 so that the run-off takes place slowly and the denudation 
 
RAINFALL 15 
 
 and erosion of the soil is checked. The roots of the 
 trees or other vegetation also bind the soil together. 
 Vegetation and forests thus mitigate the severity of 
 floods and reduce the quantity of silt brought into the 
 streams. They also shield the ground from the direct 
 rays of the sun and so reduce evaporation, and thus, 
 on the whole, augment the available rainfall. Forests 
 render the climate more equable and tend to reduce the 
 temperature, and they thus, at least on hills, increase 
 the actual rainfall to some extent. 
 
 If a forest is felled and replaced by cultivation, the 
 ploughing of the soil acts in the same way as the humus 
 of the forest, and the crops replace the trees ; and it has 
 been stated that in the U.S.A. the cultivation is as 
 beneficial as the forests in mitigating floods and check- 
 ing denudation of the soil (Proc. Am. Soc. C.E., vol. 
 xxxiv.). But when forests are felled they are not, at 
 least in hilly country, always replaced by cultivation. 
 Measures to put a stop to the destruction of forests or 
 to afforest or reforest bare land may enter into questions 
 of the regime of streams or the supply of water. On 
 the Rhine, increase in the severity of floods was dis- 
 tinctly traced to deforestation of the drainage area. 
 
 It is usually said that forests act as reservoirs by 
 preventing snows from melting. This is disputed in 
 the paper above quoted, and it is stated that in the 
 absence of forests the snow forms drifts of enormous 
 depth, and these melt very gradually and act as reser- 
 voirs after the snow in the forests has disappeared. 
 
 5. Heavy Falls in Short Periods. When rain 
 water, instead of being stored or utilised, has to be got 
 rid of, it is of primary importance to estimate roughly 
 exact estimates are impossible the greatest probable fall 
 
16 
 
 RIVER AND CANAL ENGINEERING 
 
 in a short time. This bears a rough ratio to the mean 
 annual fall. The maximum observed falls in twenty- 
 four hours range, in the United Kingdom, generally from 
 05 to "10 of the mean annual fall but on one occasion 
 the figure has been *20, and in the tropics from '10 to 
 *25. Actual figures for particular places can be ex- 
 tracted from the rain registers, but the probability of 
 their being exceeded must be taken into account. The 
 greatest fall observed in twenty-four hours in the United 
 Kingdom is 7 inches, and in India 30 inches in the 
 Eastern Himalayas. 
 
 But much shorter periods than twenty-four hours 
 have to be dealt with. The following figures are given 
 by Chamier (Min. Proc. Inst. C.E., vol. cxxxiv.) as 
 applicable to New South Wales, and he considers that 
 they are fair guides, erring on the side of safety, for 
 other countries : 
 
 Duration of fall in hours 
 Ratio of fall to maximum 
 daily fall . 
 
 12 24 
 
 1 
 
 The above figures are probably safe for England. For 
 India the case is far otherwise. The following falls have 
 been observed there : 
 
 Period. 
 
 Fall. 
 
 Rate per Hour. 
 
 Remarks. 
 
 
 Inches. 
 
 Inches. 
 
 
 7 hours 
 
 10 
 
 1-43 
 
 
 4 '5 hours . 
 
 7-7 
 
 1-7 
 
 
 2 hours . 
 
 8 
 
 4 
 
 
 1 hour 
 
 5 
 
 5 
 
 
 20 minutes 
 
 1-6 
 
 4-8 
 
 
 10 minutes 
 
 1 
 
 6 
 
 
RAINFALL 17 
 
 The falls of 1 inch in ten minutes were frequently 
 observed near the head of the Upper Jhelum Canal, a 
 place where the annual rainfall is not more than 30 
 inches (see also CHAP. XII. , Art. 1). In some parts 
 of the Eastern Himalayas, where 30 inches of rain has 
 fallen in a day, it is possible that 8 inches may have 
 fallen in an hour. In England 4 inches has fallen in 
 an hour. The heaviest falls in short periods do not 
 usually occur in the wettest years, and they may occur 
 in very dry years. Nor do they always occur on a very 
 wet day. 
 
CHAPTER III 
 
 COLLECTION OF INFORMATION CONCERNING STREAMS 
 
 1. Preliminary Remarks. The information which is 
 required concerning streams depends on the character 
 of the stream and on the nature of the work which is to 
 be done. For the present let it be supposed that the 
 stream is large and perennial. Other kinds of streams 
 will be dealt with in Arts. 6 and 7. In dealing with a 
 large perennial stream it is nearly always necessary to 
 know the approximate highest and lowest water-levels, 
 and these can generally be ascertained by local inquiry, 
 combined with observations of water marks ; but a higher 
 level than the highest known and a lower level than 
 the lowest known are always liable to occur, and 
 must to some extent be allowed for. If navigation 
 exists or is to be arranged for, the highest and lowest 
 levels consistent with navigation must be ascertained. 
 The highest such level depends chiefly on the heights of 
 bridges. A plan to a fairly large scale is also necessary 
 in most cases. 
 
 If an embankment to keep out floods is to be made 
 along a river which is so large that its flood-level cannot 
 be appreciably affected by the construction of the work, 
 it may be necessary to obtain information only as to the 
 actual flood-levels, and as to the extent to which the 
 
 18 
 
INFORMATION CONCERNING STREAMS 19 
 
 stream is liable to erode its bank. If a length of the 
 bank of a stream has to be protected against scour, it is 
 necessary to know of what materials the bed and bank 
 are composed, and whether the channel is liable to 
 changes and to what extent. It is also desirable to 
 know to what extent the water transports solids, if 
 any. In some kinds of protective work these solids 
 are utilised. 
 
 But in cases where the stream is to be much interfered 
 with, it is necessary to have full information concerning 
 it, not only as regards water-levels, changes in the 
 channel, and transport of solids, but as regards the 
 longitudinal profile and cross-sections, and the dis- 
 charges corresponding to different water-levels. The 
 collection of some of this information, particularly as 
 to the water-levels and discharges at different times of 
 the year and in floods, may occupy a considerable time. 
 
 Methods of ascertaining the quantity of silt carried 
 in the water of a stream are described in CHAP. IV., 
 Art. 4- Remarks regarding the other kinds of infor- 
 mation required the stream being still supposed to be 
 large and perennial are given in Arts. 2 to 5 of this 
 chapter. The degree of accuracy required in the in- 
 formation depends, however, on the importance of the 
 work, and sometimes the procedure can be simplified. 
 Detailed remarks on gauges and on the instruments 
 used and methods adopted for observing discharges and 
 surface slopes, are given in Hydraulics, CHAP. VIII. 
 and Appendix H. 
 
 2. Stream Gauges. Unless the stream being dealt 
 with is an artificial one, it is unlikely that the flow in 
 the reach with which the work is concerned will be 
 
 uniform. The rise and fall of the water at one place 
 
 c2 
 
20 RIVER AND CANAL ENGINEERING 
 
 cannot therefore be correctly inferred from those at 
 another. It will be desirable to have two gauges, 
 either read daily or else automatically, recording the 
 water-level, one near each end of the reach concerned, 
 with intermediate gauges if the reach is very long. If, 
 in or near the reach, there is already a gauge which has 
 been regularly read, it may be sufficient to set up only 
 one new gauge, and to read it only for such a period of 
 time as will give a good range of water-level, and to 
 compare the readings with those of the old gauge. The 
 readings of the new gauge for water-levels outside the 
 range of those observed can then be inferred, but if the 
 stream is very irregular this may involve some trouble 
 (Art. 4). 
 
 In the case of a large stream which shifts its course, 
 the reading of a gauge does not give a proper indica- 
 tion of the water-level. In other words, the distance 
 of the gauge from the two ends of the reach is subject 
 to alteration. The case is the same as if the stream 
 was stable and the gauge was shifted about. In such 
 a stream there ought, if accuracy is required, to be a 
 group of two or more gauges for each point where there 
 would be only one if the stream was not a shifting one. 
 Also, owing to erosion of the bank or the formation of 
 a sandbank, it may often be necessary to shift the 
 gauge. When possible it should be kept in a fixed line 
 laid down at right angles to the general direction of 
 the stream. When shifted, its zero level should be 
 altered in such a way that the reading at the new site 
 at the time of shifting is the same as it was before 
 shifting. When the gauge is moved back to the 
 original site its zero should be placed at its original 
 level, though this may give rise to a sudden jump in 
 
INFORMATION CONCERNING STREAMS 21 
 
 the reading for the reason given in the first sentence 
 of this paragraph. 
 
 3. Plan and Sections. Making a survey and plan, 
 and laying down on it the lines for longitudinal and 
 cross-sections, and taking levels for the sections, are 
 ordinary operations of surveying. If any land is liable 
 to be flooded, its boundaries should be shown on the 
 plan and on some of the cross-sections. Unless the water 
 is shallow, it is necessary to obtain the bed levels from 
 the water-level by soundings, the level of a peg at the 
 water-level having been obtained by levelling. All the 
 sections should show the water-level as it was at some 
 particular time, but the water-level will probably have 
 altered while the survey was in progress, and allowance 
 must be made for this. The pegs at all the cross- 
 sections and on both banks of the stream for the 
 water-levels at opposite banks may not be exactly the 
 same may, for instance, be driven down to the water- 
 level when it is steady, and thereafter any changes in it 
 noted and the soundings corrected accordingly. 
 
 In order to ascertain what changes are occurring in the 
 channel it may be necessary to repeat the soundings at 
 intervals and, if there is much erosion of the bank, to 
 make fresh plans. 
 
 4. Discharge Observations. For a large stream 
 
 it is necessary to observe the discharges by taking 
 cross-sections and measuring the velocity. If there 
 is a sufficient range of water-levels, it will be possible 
 to make actual observations of a sufficient number 
 of discharges. If soundings cannot, owing to the 
 depth or velocity, be taken at high water, they must 
 be inferred from those previously taken, but this 
 does not allow for changes in the channel, which are 
 
22 RIVER AND CANAL ENGINEERING 
 
 sometimes considerable and rapid. If there is not a 
 sufficient range of water-level, the discharges for some 
 water-levels must be calculated from those at other 
 water-levels. In this case observations of the surface 
 slope will be required, and the discharge site should be 
 so selected that no abrupt changes in the channel will 
 come within the length over which the observations are 
 to extend. This length should be such that the fall in 
 the water surface will be great enough to admit of 
 accurate observation. If the cross-section of the stream 
 is nearly uniform throughout the whole of this length, 
 or if it varies in a regular manner, being greatest at one 
 end of the length and least at the other end. the differ- 
 ences in the areas of the two end sections being not 
 more than 10 or 12 per cent., then the velocity and 
 cross-section of the stream can be observed in the usual 
 manner at the centre of the length ; but otherwise 
 they should be observed at intervals over the whole 
 length, or at least in two places, one where the section 
 is small and one where it is great, and the mean taken. 
 Or the velocity can be observed at only one cross-section 
 and calculated for the others by simple proportion and 
 the mean taken. The coefficient C can then be found 
 
 V 
 
 from the formula C = /yF=. To find the discharge for 
 
 V H b 
 
 a higher or lower water-level, the change in the value of 
 C corresponding to the change in E can be estimated by 
 looking out the values of C in tables, and the discharge 
 calculated by using the new values of C and E, and the 
 new sectional area, S remaining unaltered. But if the 
 channel is such that, with the new water-level, a change 
 in S is likely to have occurred, this change must be 
 allowed for. Any such change will be due to the 
 
INFORMATION CONCERNING STREAMS 23 
 
 changed relative effects of irregularities, either in the 
 length over which the observations extended or down- 
 stream of that length. The effect of irregularities in 
 the bed is greatest at low water. The effect of lateral 
 narro wings is greatest at high water. Since a change of 
 10 per cent, in S causes a change of only 5 per cent, in 
 V, it will usually suffice to draw on the longitudinal 
 section the actual water surface observed and to sketch 
 the probable surface for the new water-level. If the 
 whole channel is fairly regular for a long distance down- 
 stream of the discharge site, no slope observations need 
 be made nor need several sections be taken in order to 
 find V. The changed value of C should, however, be 
 estimated in the manner above indicated. For this 
 purpose any probable value of S will suffice. 
 
 5. Discharge Curves and Tables. Ordinarily it 
 will be possible, by plotting the observed discharges as 
 ordinates, the gauge readings being the abscissae, to 
 draw a discharge curve and from it construct a discharge 
 table. Unless the channel is of firm material and not 
 liable to change, there are likely to be discrepancies 
 among the observed discharges, so that a regular dis- 
 charge curve will not pass through all the plotted 
 points. If the discrepancies are not serious, they can be 
 disregarded and the curve drawn so as to pass as near 
 as possible to all the points, but otherwise trouble and 
 uncertainty may arise. The soundings should be 
 compared in order to see whether changes have occurred 
 in the channel. If such changes do not account for the 
 discrepancies, the cause must be sought for in some of 
 the recorded velocities. If no sources of error in these 
 can be found, such as wind, it is possible that the 
 velocity has been affected by a change in the surface 
 
24 RIVER AND CANAL ENGINEERING 
 
 slope owing to some change in the channel downstream 
 of the length. Failing this explanation, the discrepancies 
 must be set down to unknown causes. With an un- 
 stable channel and where accuracy is required, the 
 sectional areas and velocities should be regularly 
 tabulated or plotted so that changes may be watched 
 and investigated. To do this it may be necessary to 
 take surface slope observations, or to set up extra gauges 
 which will show any changes in the slope. 
 
 If, downstream of the discharge site, there is any 
 place where affluents come in and bring varying 
 volumes of water, or where gates or sluices are 
 manipulated, and if the influence of this extends up to 
 the discharge site, the water-level there no longer 
 depends only on the discharge, and a discharge table 
 must be one with several columns whose headings 
 indicate various conditions at the place where the 
 disturbances occur. 
 
 In order to show how the gauge readings and dis- 
 charges vary from day to day throughout the year, a 
 diagram should be prepared showing the gauge readings 
 and discharges as ordinates, the abscissse being the 
 times in days starting from any convenient date as 
 zero. Such a diagram, showing only gauge readings, 
 is given in fig. 56, CHAP. XII. 
 
 6. Small Streams. Small streams will now be con- 
 sidered, those, for instance, which are too small to be 
 navigable and which occasionally run dry or nearly dry. 
 If the water of the stream is to be stored for water 
 supply, power or irrigation purposes, full information as 
 to discharges and silt carried will be required. If the 
 stream is small enough the discharges can be ascertained 
 by means of a weir of planks. The discharge is then 
 
INFORMATION CONCERNING STREAMS 25 
 
 known from the gauge readings. Cross-sections and 
 large scale plans will not be required unless the stream 
 is to be altered or embanked. If the water, instead of 
 being stored, is to be got rid of, as in drainage work, 
 the only information required as to discharges is the 
 maximum discharge. Large scale plans, sections, or 
 information as to silt or water-levels (except as a means 
 of estimating the discharge) will not be required unless 
 the stream is to be altered or embanked. 
 
 In all these cases of small streams the information 
 required is generally, as has been seen, less than in the 
 case of large perennial streams, but it is generally more 
 difficult to obtain. If the stream is ill-defined or its 
 flow intermittent, especially if it is also very small and 
 the place sparsely inhabited, it may be difficult to obtain 
 any discharge figures except those based on figures of 
 rainfall. The method of obtaining such figures has been 
 stated in CHAP. II. The figures required are those of 
 the annual and monthly fall when the water is to be 
 stored, and those of the greatest fall in a short period 
 when the water is to be got rid of. Of course a plan 
 of the catchment area is required. 
 
 7. Intermittent Streams. In the case of large 
 
 streams whose flow is intermittent, the information 
 required will, as before, depend upon the circumstances. 
 Such streams occur in many countries. The difficulty 
 in obtaining information is often very great. To obtain 
 figures of daily discharge a gauge must be set up in the 
 stream and a register kept. The chief difficulty in an 
 out-of-the-way place is likely to be the obtaining 
 correct information as to the maximum discharge. In- 
 formation, derived from reports or from supposed flood 
 marks, as to the highest water-level, may be inaccurate, 
 
26 RIVER AND CANAL ENGINEERING 
 
 and information based on rainfall figures may be 
 extremely doubtful owing to the large size of the 
 catchment area, the absence of rain gauges, and the 
 difficulty, especially if the rain is not heavy, in 
 estimating the available fall. All sources of informa- 
 tion must be utilised and, whenever possible, observa- 
 tions should be made over a long period of time. 
 
 8. Remarks. Very much remains to be done in 
 collecting and publishing information concerning the 
 ratio of the discharges to the rainfall. By observing a 
 fall of rain and the discharge of a stream before and 
 after the fall, it is possible to ascertain the figures for 
 that occasion, but they will not hold good for all 
 occasions. Continuous observations are required. The 
 chief obstacle is the expense. Not only have measuring 
 weirs and apparatus for automatically recording the 
 water-level to be provided, but the weirs would often 
 cause flooding of land involving payment of com- 
 pensation. The most suitable places for making 
 observations are those where reservoirs for water- works 
 exist or are about to be made. 
 
CHAPTER IV 
 
 THE SILTING AND SCOURING ACTION OP STREAMS 
 
 1 . Preliminary Remarks. When flowing water carries 
 solid substances in suspension, they are known as " silt." 
 Material is also moved by being rolled along the bed 
 of the stream. The difference between silt and rolled 
 material is one of degree and not of kind. Material of 
 one kind may be rolled and carried alternately. The 
 quantity of silt present in each cubic foot of water is 
 called the " charge " of silt. Silt consists chiefly of mud 
 and fine sand ; rolled material of sand, gravel, shingle, 
 and boulders. When a stream erodes its channel, it 
 is said to " scour." When it deposits material in its 
 channel, it is said to "silt." Both terms are used 
 irrespective of whether the material is silt or rolled 
 material. A stream of given velocity and depth can 
 carry only a certain charge of silt. When it is carrying 
 this it is said to be " fully charged." 
 
 If a stream has power to scour any particular material 
 from its channel, it has power to transport it ; but the 
 converse is not true. If the material is hard or coherent, 
 the stream may have far more difficulty in eroding it 
 than in merely keeping it moving. And there is gener- 
 ally a little more difficulty even when the material 
 
 is soft. 
 
 27 
 
28 RIVER AND CANAL ENGINEERING 
 
 Silting or scour may affect the bed of a channel or 
 the sides or both. The channel may thus decrease or 
 increase in width or if one bank is affected more 
 than, or in a different manner to, the other alter its 
 position laterally whether or not it is altering its bed 
 level, and vice versa. 
 
 The cross-section of a stream is generally "shallow." 
 i.e. the width of the bed is greater than the combined 
 submerged lengths of the sides, and the action on the 
 bed is generally greater than on the sides. 
 
 Silting and scouring are generally regular or irregular 
 in their action according as the flow is regular or 
 irregular, that is, according as the channel is free or 
 not from abrupt changes and eddies. In a uniform 
 canal fed from a river, the deposit in the head reach 
 of the canal forms a wedge-shaped mass, the depth of 
 the deposit decreasing with a fair approach to uniformity. 
 Salient angles or places alongside of obstructions are 
 most liable to scour, and deep hollows or recesses to 
 silt. Eddies have exceptionally strong scouring power. 
 Immediately downstream of an abrupt change scour 
 is often severe. An abrupt change is one, whether 
 of sectional area or direction of flow, and whether or 
 not accompanied by a junction or bifurcation, which is 
 so sudden as to cause .eddies. The hole scoured along- 
 side of an obstruction may extend to its upstream side, 
 though there is generally little initial tendency to scour 
 there. An obstruction is anything causing an abrupt 
 decrease in any part of the cross-section of a stream, 
 whether or not there is a decrease in the whole cross- 
 section, e.g. a bridge pier or spur. 
 
 Most streams vary greatly at different times both in 
 volume and velocity and in the quantity of material 
 
SILTING AND SCOURING ACTION OF STREAMS 29 
 
 brought into them. Hence the action is not constant. 
 A stream may silt at one season and scour at another, 
 maintaining a steady average. When this happens 
 to a moderate extent, or when the stream never silts 
 or scours appreciably, it is said to be in " permanent 
 regime," or " stable." Most streams in earthen channels 
 
 O ' 
 
 are either just stable and no more, or are unstable. 
 
 Waves, whether due to wind or other agency, may 
 cause scour, especially of the banks. Their effect on 
 the bed becomes less as the depth of water increases, 
 but does not cease altogether at a depth of 21 feet, 
 as has been supposed. Salt water possesses a power of 
 causing mud, but not sand, to deposit. 
 
 Arts. 2, 3, and 6 of this chapter refer to action on 
 the bed of a stream. Action on the sides will be 
 considered in Art. 7. 
 
 Weeds usually grow only in water which has so low 
 a velocity that it carries no silt to speak of, but if any 
 silt is introduced the weeds cause a deposit. The weeds 
 also thrive on such a deposit. 
 
 2. Rolled Material. If a number of bodies have 
 similar shapes, and if D is the diameter of one of them 
 and V the velocity of the water relatively to it, the 
 rolling force is theoretically as V 2 D 2 , arid the resisting 
 force or weight as D 8 . If these are just balanced, D 
 varies as V 2 , or the diameters of similarly shaped bodies 
 which can just be rolled are as V 2 and their weights as 
 V 6 . From practical observations, it seems that the 
 diameters do not vary quite so rapidly as they would 
 by the above law, the weights being more nearly as V 5 . 
 
 Let a stream of pure water having a depth D, and 
 with boulders on its bed, have a velocity V just sufficient 
 to move them very slowly. Any larger boulders would 
 
30 RIVER AND CANAL ENGINEERING 
 
 not be moved. Any smaller boulders would move more 
 quickly. Similarly, fine sand would be rolled more 
 quickly than coarse sand. If the velocity of the stream 
 increases, larger boulders would be moved. Streams are 
 thus constantly sorting out the materials which they 
 roll. If the bed is examined it will be found that large 
 boulders exist only down to a certain point, smaller 
 boulders, shingle, gravel, coarse sand and fine sand 
 following in succession. 
 
 o 
 
 If the water, instead of being pure, is supposed to 
 contain silt, this may affect its velocity it is not, how- 
 ever, known to do so but, given a certain velocity, it 
 is not likely that the rolling power of the stream is 
 much affected by its containing silt. 
 
 It is sometimes supposed that increased depth gives 
 increased rolling power, because of the increased pressure, 
 but this is not so. The increased pressure due to depth 
 acts on both the upstream and downstream sides of a 
 body. It is moved only by the pressure due to the 
 velocity. 
 
 When sand is rolled along the bed of a stream there 
 is usually a succession of abrupt falls in the bed. After 
 each fall there is a long gentle upward slope till the 
 next fall is reached. The sand is rolled up the long 
 slope and falls over the steep one. It soon becomes 
 buried. The positions of the falls of course keep moving 
 downstream. The height of a fall in a large channel is 
 perhaps 6 inches or 1 foot, and the distance between 
 the falls 20 or 30 feet. A fall does not usually extend 
 straight across the bed but zigzags. 
 
 It has sometimes been said that the inclination of the 
 bed of a stream, when high, facilitates scour, the material 
 rolling more easily down a steep inclined plane. The 
 
SILTING AND SCOURING ACTION OF STREAMS 31 
 
 inclination is nearly always too small to have any 
 appreciable direct effect. The inclination of the surface 
 of the stream of course affects its velocity, and this is 
 the chief factor in the case. 
 
 A sudden rise in the bed of a stream does not neces- 
 sarily cause rolled materials to accumulate there, except 
 perhaps to the extent necessary to form a gentle slope. 
 Frequently even this slope is not formed, especially if 
 the rolled material is only sand. The eddies stir it 
 up and it is carried on. The above remarks apply also 
 to weirs or other local rises in the bed. 
 
 3. Materials carried in Suspension. It has long 
 
 been known that the scouring and transporting power 
 of a stream increases with its velocity. Observations 
 made by Kennedy have shown that its power to carry 
 silt decreases as the depth of water increases (Min. 
 Proc. Inst. C.JE., vol. cxix.). The power is probably 
 derived from the eddies which are produced at the bed. 
 Every suspended particle tends to sink, if its specific 
 gravity is greater than unity. It is prevented from 
 sinking by the upward components of the eddies. If V 
 is the velocity of the stream and D its depth, the force 
 exerted by the eddies generated on 1 square foot of the 
 bed is greater as the velocity is greater, and is probably 
 as V 2 or thereabouts. But, given the charge of silt, the 
 weight of silt in a vertical column of water whose base 
 is 1 square foot is as D. Therefore the power of a 
 stream to support silt is as V 2 and inversely as D. The 
 silt charge which a stream of depth D can carry is as 
 V*. V is called the " critical velocity " for that depth, 
 and is designated as V . 
 
 The full charge must be affected by the nature of the 
 silt. The specific gravity of fine mud is not much 
 
32 RIVER AND CANAL ENGINEERING 
 
 greater than that of water, while that of sand is about 
 1 '5 times as great. Moreover, the particles of sand are 
 far larger than the particles of mud. If two streams of 
 equal depths and velocities are fully charged, one with 
 particles of mud and the other with particles of sand, 
 the latter will sink more rapidly and will have to be 
 more frequently thrown up. They will be fewer in 
 number. From some observations referred to by Ken- 
 nedy (Punjab Irrigation Paper, No. 9, " Silt and Scour 
 in the Sirhind Canal," 1904), it appears that in a fully 
 charged stream which carried 33 1 00 of its volume of a 
 mixture of mud and sand of various grades, sand of a 
 particular degree of coarseness formed only -g^Voir f the 
 volume of the water, but that when the same stream 
 was clear and was turned on to a bed of the coarse 
 sand it took up T5 ,^nnj f its volume. It would thus 
 appear that the full charge of silt is less as its coarse- 
 ness and heaviness are greater. This is in accordance 
 with the laws mentioned above (Art. 2, par. l). See 
 also CHAP. V., Art. 2, last paragraph. 
 
 It is probable that fine mud is carried almost equally 
 into all parts of the stream, whereas sand is nearly 
 always found in greater proportion near the bed and, 
 as before remarked, some materials may be rolled and 
 suspended alternately. The charge of mixed silt which 
 a stream can carry is, no doubt, something between 
 the charge which it can carry of each kind separately, 
 but the laws of this part of the subject are not yet 
 fully known. From the observations above referred to, 
 Kennedy concludes that a canal with velocity V will 
 carry in suspension ^Vo to ^^Q f its volume of silt, 
 according as it is charged with sand of all classes or 
 only with the heavier classes. 
 
SILTING AND SCOURING ACTION OF STREAMS 33 
 
 Let a stream be carrying a full charge of any kind 
 of silt. Then if there is any reduction in velocity, 
 a deposit will occur unless there is also a reduction 
 of depth until the charge of silt is reduced again to 
 the full charge for the stream. The deposit generally 
 occurs slowly, and extends over a considerable length 
 of channel. The heavier materials are, of course, 
 deposited first. If a stream is not fully charged, it 
 tends to become so by scouring its channel. It is 
 generally believed that a stream fully charged with silt 
 cannot scour silt from its channel, or bear any intro- 
 duction of further silt. This seems to be correct in the 
 main, but the remarks made in the latter part of the 
 preceding paragraph must be taken into consideration. 
 
 It has been stated (Art. 2} that a weir or a sudden 
 rise in the bed does not necessarily cause an accumula- 
 tion of rolled material. It never causes a deposit of 
 suspended material unless it causes a heading up and 
 reduction of velocity to below the critical velocity. 
 
 4. Methods of Investigation. The quantity of 
 silt in water is found by taking specimens of the water 
 and evaporating it or, if the silt is present in great 
 quantity, leaving it to settle for twelve hours an 
 ounce of alum can be added for every 10 cubic feet of 
 water to accelerate settlement drawing off the water 
 by a syphon, and heating the deposit to dry it. The 
 deposit is then measured or weighed. It is best to 
 weigh it. If clay is filled into a measure, the volume 
 depends greatly on the manner in which it is filled in. 
 When silt deposits in large quantities in a channel, or 
 when heavy scour occurs, the volume deposited or 
 scoured is ascertained by taking careful sections of 
 the channel. 
 
 D 
 
34 
 
 RIVER AND CANAL ENGINEERING 
 
 Silt is best classified by observing its rate of fall 
 through still water. A sand which falls at '10 feet 
 per second is, in India, called class "1, and mixed sand 
 
 FIG. 1. 
 
 which falls at rates varying from *1 to *2 feet per 
 second is called class ^-. Fig. 1 shows a sand separator 
 designed by Kennedy. The scale is -|. It has a syphon 
 action, and the rate of flow can be altered by altering 
 the length of the exit pipe. Suppose it is desired to 
 
SILTING AND SCOURING ACTION OF STREAMS 35 
 
 measure the sand of class "10 and all heavier kinds. 
 The pipe is adjusted so as to give a velocity of *1 foot 
 per second to the upward flowing water, which then 
 carries off all silt of class *10 or finer. All heavier silt 
 falls into the glass tube. It can be separated again by 
 being mixed with water and passed through the 
 instrument again, the velocity of flow through the 
 instrument being increased. 
 
 The quantity of silt present at various depths can be 
 found by pumping specimens of water through pipes. 
 At each change of depth the pipe, delivery hose, etc., 
 should be cleaned. Allowance must be made for the 
 velocity of ascent of the water up the pipe. Suppose 
 this to be 1*4 feet per second. Then the velocity of 
 sand of class *2 would be 1*2 feet per second, and the 
 quantity of sand actually found in the water would 
 have to be increased by one-sixth. 
 
 5. Quantity and Distribution of Silt. The 
 
 quantity of silt present in water varies enormously. 
 Fine mud, even though sufficient to discolour the water, 
 may be so small in volume that it only deposits when 
 the water is still, and even then deposits slowly. In the 
 river Tay, near Perth, the silt was found to be ordinarily 
 f the volume of water, and at low water only 
 I n the river Sutlej at Rupar, near where it issues 
 from the Himalayas, the silt in the flood season is 
 extremely heavy. Out of 360 observations, made at 
 various depths, during the flood seasons of four successive 
 years, in water whose depth ranged up to 12 feet, the 
 silt w^as once found to be 2'1 per cent, by weight of 
 that of the water. It was more than 1*2 per cent, on 
 four occasions, and more than 0'3 per cent, (or 3 in 
 1000) on sixty-four occasions. Generally about one-half 
 
 D2 
 
36 RIVER AND CANAL ENGINEERING 
 
 of the silt was clay and sand of classes finer than *10, 
 about one-third was sand of class rf , and the residue was 
 sand of class rf. The sand of the river Chenab is 
 generally coarser than that of the Sutlej. There are 
 very great differences in the degree of coarseness of 
 river sand. The sand in any river becomes finer and 
 finer as the gradient flattens in approaching the sea. 
 Sea sand has been found to be of class '20. In the 
 Sirhind Canal, which takes out from the Sutlej at Kupar, 
 the maximum quantity of suspended silt observed in 
 the four flood seasons was 07 per cent., on one occasion 
 out of 270, and it exceeded 0*3 per cent, on twenty-five 
 occasions. About 80 per cent, of the silt was clay. 
 
 In another part of the paper quoted, it is stated that 
 the silt suspended in the canal water averaged, during 
 the whole of one flood season, about 17 1 00 of the volume 
 of the water. This would be about X^OTT by weight. 
 The silt deposited in the bed of the canal, in a period of 
 a few days, was sometimes as much as 10 1 00 of the 
 water which had passed along, and occasionally as much as 
 s^j. It was nearly all sand, only about 3 per cent, 
 being clay. Silt of classes finer than 1 gave no trouble, 
 and were to be eliminated in future investigations. In 
 a canal, as in a river, the sand on the bed becomes finer 
 the further from the head. 
 
 Kegarding the distribution of the silt at various 
 depths, in water 5 to 17 feet deep, the quantity of silt 
 near the bed may, when the charge is heavy and 
 consists of mixed silt, be 1^ to 3 times that at the 
 surface. If the charge is fine mud, there is likely to 
 be as much silt at the surface as near the bed, if sand, 
 there may be none at the surface and little in the upper 
 part of the stream. 
 
SILTING AND SCOURING ACTION OF STREAMS 37 
 
 In all cases single observations are likely to show 
 extraordinarily discordant results ; a number of obser- 
 vations must be made at each point and averaged. 
 
 6. Practical Formulae and Figures. A stream 
 
 which carries silt generally rolls materials along its bed. 
 The proportion between the quantities of material 
 rolled and carried is never known, and this makes it 
 impossible to frame an exact formula applicable to such 
 cases, but Kennedy, from his observations on canals 
 fully charged with the heavy silt and fine sand usually 
 found in Indian rivers near the hills, arrived at the 
 empirical formula for critical velocities 
 
 The observations were made on the Bari Doab Canal 
 and its branches, the widths of the channels varying 
 from 8 feet to 91 feet, and the depths of water from 
 2*3 feet to 7*3 feet. The beds of these channels have, 
 in the course of years, adjusted themselves by silting or 
 scouring, so that there is a state of permanent regime, 
 each stream carrying its full charge of silt, and the 
 charges in all being about equal. From further obser- 
 vations referred to above (Art. 3, par. 2) it appears 
 that this kind of silt forms about 3S 1 00 of the volume of 
 the water, and that on the Sirhind Canal, sand coarser 
 than the '10 class, formed 3-5,^017 f the volume of 
 water. 
 
 The formula gives the following critical velocities for 
 various depths : 
 
 D= 1234 5 67 
 
 V =-84 1*30 170 2'04 2'35 2'64 2'92 
 
 D= 8 9 10 
 
 V = 3'18 3'43 3*67. 
 
38 RIVER AND CANAL ENGINEERING 
 
 In Indian rivers not near the hills the silt carried is 
 not so heavy, and the critical velocities are supposed to 
 be about three-fourths of the above. Thrupp (Min. 
 Proc. Inst. C.E., vol. clxxi.) gives the following ranges 
 of velocities as those which will enable streams to 
 carry different kinds of silt. It does not appear that 
 the streams would be fully charged except at the higher 
 figure given for each case. 
 
 D = 1'0 lO'O 
 
 V= 1'5 to 2'3 3'5 to 4'5 (Coarse sand). 
 
 V= '95 to 1-5 2*3 to 3'5 (Heavy silt and fine sand). 
 
 V= '45 to '95 1-2 to 2*3 (Fine silt). 
 
 It cannot be said that the exact relations between D 
 and V are yet known, but it is of great practical 
 importance to know that V must vary with D. The 
 precise manner in which it must vary does not, for 
 moderate changes, make very much difference. In 
 designing a channel a suitable relation of depth to 
 velocity can be arranged for, and one quantity or the 
 other kept in the ascendant, according as scouring or 
 silting is the evil to be guarded against. 
 
 The old idea was that an increase in V, even if 
 accompanied by an increase in D, e.g. simply running 
 a higher supply in a given channel, gave increased silt- 
 transporting power. In a stream of very shallow 
 section this is probably correct, for V increases faster 
 than D' 64 (Hydraulics, CHAP. VI., Art. 2). In a stream 
 of deep section a decrease in D gives increased silt- 
 transporting power. If the discharge is fixed, a change 
 in the depth or width must be met by a change of the 
 opposite kind in the other quantity. In this case 
 widening or narrowing the channel may be proper 
 
SILTING AND SCOURING ACTION OF STREAMS 39 
 
 according to circumstances. In a deep section widening 
 will decrease the depth of water, and may also increase 
 the velocity, and it will thus give increased scouring 
 power. In a shallow section, narrowing will increase 
 the velocity more than it increases D' 64 . In a medium 
 section it is a matter of exact calculation to find out 
 whether widening or narrowing will improve matters. 
 
 If the water entering a channel has a higher silt- 
 charge than can be carried in the channel, some of it 
 must deposit. Suppose an increased discharge to be 
 run, and that this gives a higher silt-carrying power 
 and a smaller rate of deposit per cubic foot of discharge, 
 it does not follow that the deposit will be less. The 
 quantity of silt entering the channel is now greater 
 than before. Owing to want of knowledge regarding 
 the proportions of silt and rolled material, and to 
 want of exactness in the formulae, reliable calculations 
 regarding proportions deposited cannot be made. 
 
 The channels in which the observations above referred 
 to were made have all assumed nearly rectangular cross- 
 sections, the sides having become vertical by the deposit 
 on them of finer silt ; but the formula probably applies 
 approximately to any channel if D is the mean depth 
 from side to side, and V the mean velocity in the whole 
 section. 
 
 If the ratio of V to D differs in different parts of a 
 cross-section, there is a tendency towards deposit in the 
 parts where the ratio is least, or to scour where it is 
 greatest. There is a tendency for the silt- charge to 
 adjust itself, that is, to become less where the above 
 ratio is less, but the irregular movements of the stream 
 cause a transference of water among all parts, and this 
 tends to equalise the silt-charge. 
 
40 RIVER AND CANAL ENGINEERING 
 
 Dubuat gives the following as the velocities close to 
 the bed which will enable a stream to scour or roll 
 various materials. The bed velocity is probably less 
 than the mean velocity in the ratio of about "6 to 1 in 
 rough channels, and about "7 to 1 in smooth channels : 
 
 Gravel as large as peas . . '70 feet per second 
 
 French beans .1*0 ,, ,, 
 
 1 inch in diameter . . 2'25 ,, ,, ,, 
 
 Pebbles i|- inch in diameter . 3*33 ,, ,, 
 
 Heavy shingle .... 4'0 ,, ,, ,, 
 
 Soft rock, brick, earthenware .4*5 ,, ,, ,, 
 
 Rock of various kinds . . 6'0 ,, ,, 
 
 and upward. 
 
 The figures for brick, earthenware, and rock can apply 
 only to materials of exceedingly poor quality. Masonry 
 of good hard stone will stand 20 feet per second, and 
 instances have occurred in which brickwork has with- 
 stood a velocity of 90 feet per second without injury so 
 long as the water did not carry sand and merely flowed 
 along the brickwork. If there are abrupt changes in 
 the stream, causing eddies, or if there is impact and 
 shock, or if sand, gravel, shingle, or boulders are liable 
 to be carried along, velocities must be limited. 
 
 7. Action on the Sides of a Channel. It has been 
 
 seen that the laws of silting and scour on the bed of a 
 channel depend on the ratio of the depth to the velocity. 
 The same laws probably hold good in the case of a 
 gently shelving bank, so that here again V ought to 
 vary as D' 64 . The velocity near the angle where the 
 slope meets the water surface seems to decrease faster 
 than D' 64 . At all events, silt tends to deposit in the 
 angle and the slope to become steep. 
 
SILTING AND SCOURING ACTION OF STREAMS 41 
 
 When the slope is steep the law seems to be different, 
 the tendency for deposit or scour to occur on the bank 
 depending on the actual velocity without much relation 
 to the depth. The velocity very near to a steep bank 
 is always low relatively to that in the rest of the stream. 
 Thus there is often a tendency for silt to deposit on the 
 bank, especially in the upper part, and for the side to 
 become vertical except for a slight rounding at the lower 
 corner. A bank may receive deposits when the bed 
 may be receiving none, and it may have a persistent 
 tendency to grow out towards the stream. The growth 
 of the bank is generally regular, the line of the bank 
 being preserved, but it may be irregular, especially if 
 vegetation, other than small grass, becomes established 
 on the new deposits. 
 
 When scour of the sides of a channel occurs it may 
 occur by direct action of the stream on the sides near 
 the water-level, or by action at or near the toe of the 
 slope, which causes the upper part of the bank to fall in. 
 Such falling in is generally more or less irregular, and 
 the bank presents an uneven appearance. The fallen 
 pieces of bank rnay remain, more or less intact, especially 
 if they are held together by the roots of grasses, etc., 
 where they fell, and prevent further scour occurring along 
 the toe of the slope. Falling in of banks is most liable 
 to occur in large streams and with light soils. It may 
 be caused by the waves which are produced by steamers 
 and boats or, especially in broad streams, by wind. The 
 action on the banks at bends is discussed in Art. 8. 
 
 Thus in designing a channel according to the 
 principles laid down in Art. 6, the question of action on 
 the sides of the channel has to be dealt with as follows. 
 Whether or not the velocity is to be low, relatively to 
 
42 RIVER AND CANAL ENGINEERING 
 
 the depth, i.e. whether or not deposit on the bed is 
 more likely to occur than scour, care can be taken not 
 to make it actually too low, and not to make it actually 
 too high, particularly if the soil is light and friable. 
 With ordinary soils a mean velocity of 3*3 feet per 
 second in the channel is generally safe as regards scour 
 of the sides. Any velocity of more than 3 '5 feet per 
 second may give trouble. A velocity of less than 1 foot 
 per second is likely to give rise to deposit on the sides. 
 
 In channels in alluvial soils the falling in of banks is 
 sometimes said to occur more when the stream is falling 
 than at other times. This has been noticed on both 
 the Mississippi and the Indus. The cause has been said 
 to be the draining out of water which had percolated 
 into the bank, the water in flowing out carrying some 
 sand with it. The effect of this cannot however be 
 great. 
 
 8. Action at Bends. At a bend, owing to the 
 action of centrifugal force and to cross-currents caused 
 thereby, there is a deposit near the convex bank and a 
 corresponding deepening unless the bed is too hard to 
 be scoured near the concave bank. The water-level 
 at the concave bank is slightly higher than at the 
 convex bank. The greatest velocity instead of being 
 in mid-stream is nearer the concave bank. 
 
 As the transverse current and transverse surface slope 
 cannot commence or end abruptly, there is a certain 
 length in which they vary. In this length the radius 
 of curvature of the bend and the form of the cross- 
 section also tend to vary. This can often be seen in 
 plans of river bends, the curvature being less sharp 
 towards the ends. 
 
 When once a stream has assumed a curved form, be 
 
SILTING AND SCOURING ACTION OF STREAMS 43 
 
 it ever so slight, the tendency is for the bend to 
 increase. The greater velocity and greater depth near 
 the concave bank react on each other, each inducing 
 the other. The concave bank is worn away, or 
 becoming vertical by erosion near the bed, cracks, falls 
 in, and is washed away, a deposit of silt occurring at 
 the convex bank, so that the width of the stream 
 remains tolerably constant. The bend may go on in- 
 creasing, and it often tends to move downstream. 
 
 In fig. 2 the deep places are shown by dotted lines. 
 Along the straight dotted line there is no deep place. 
 
 FIG. 2. 
 
 Such a line would be used for a ford. At low water it 
 becomes a shoal. This is the chief reason why a 
 tortuous stream at low water consists of alternate pools 
 and rapids. It is sometimes said that deep water 
 occurs near to a steep hard bank. Such deepening is 
 due to bends or obstructions which give the current a 
 set towards the bank, or it is due to irregularities in the 
 bank which cause eddies. In a straight channel with 
 even and regular banks there is no such deepening. 
 
 When a bend has formed in a channel previously 
 straight, the stream at the lower end of the bend, by 
 setting against the opposite bank, tends to cause 
 another bend of the opposite kind to the first. Thus 
 
44 RIVER AND CANAL ENGINEERING 
 
 the tendency is for the stream to become tortuous and, 
 while the tortuosity is slight, the length, and therefore 
 the slope and velocity, are little affected ; but the action 
 may continue until the increase in the length of the 
 stream materially flattens the slope, and the consequent 
 reduction in velocity causes erosion to cease. Or the 
 stream during a flood may find, along the chord of a 
 bend, a direct route with, of course, a steeper slope. 
 Scouring a channel along this route it straightens itself, 
 and its action then commences afresh. Short cuts of 
 this kind do not, however, occur so frequently as is 
 sometimes believed. In some streams the bends acquire 
 a horse-shoe shape and the neck becomes very narrow 
 and short cuts may then occur. Otherwise they are 
 not common. V increases only as ^/S, and if the 
 country is covered with vegetation it is not easy for a 
 stream to scour out a new channel. 
 
 The effect of bends on the velocity of a stream is not 
 well understood. In case of a bend of 90 the increased 
 resistance to flow when the bend is absolutely sudden (a 
 sudden bend is known as an " elbow ") amounts perhaps 
 
 V 2 
 
 to . Whether it is greater or less in the case of a 
 
 gentle bend of 90 is not known. In the case of a pipe 
 there is a certain radius which gives a minimum resist- 
 ance (Hydraulics, CHAP. V.). The increased resistance 
 at a bend is due partly to the fact that the maximum 
 velocity is no longer in the centre of the stream, and 
 partly to the fact that the velocities at the different 
 parts of the cross-section have to be rearranged at the 
 commencement of the bend and again at its termina- 
 tion. Thus the effect of a bend of 45 is a good deal 
 more than half of that of a bend of 90. Two bends of 
 
SILTING AND SCOURING ACTION OF STREAMS 45 
 
 45, both in the same direction, with a straight reach 
 between them, will cause more resistance than a single 
 bend of 90 with the straight reach above or below 
 the bend. If the two bends of 45 are in different 
 directions the resistance will be still greater. A suc- 
 cession of sharp bends may produce a serious effect, 
 amounting to an increase in roughness of the channel. 
 A succession of gentle bends, of any considerable angle, 
 cannot of course occur within a moderate length of 
 channel. 
 
 When there is head to spare there is clearly no objec- 
 tion to bends, except that the bank may need protection. 
 At a place where the bank has in any case to be pro- 
 tected, e.g. at a weir, there is no objection to an elbow. 
 
 9. General Tendencies of Streams. Since the 
 
 velocity is greater as the area of the cross-section is 
 less, a stream always tends to scour where narrow or 
 shallow, and to silt where wide or deep. The cross- 
 section thus tends to become uniform in size. Suppose 
 two cross-sections to be equal in size but different in 
 shape. The velocities of the two sections will be equal. 
 The tendency of the bed to silt will (Art. 6) be greater 
 at the deeper section and, when silting has occurred on 
 the bed, the section will be reduced and there will be a 
 tendency to scour at the sides. Thus the cross-sections 
 tend to become also uniform in shape. If a bank of 
 silt has formed in a stream, the tendency is for scour 
 to occur. There is also a tendency for silt to deposit 
 just below the point where the bank ends. Hence a 
 silt bank often moves downstream. 
 
 Owing to the tendency to scour alongside of, or 
 downstream of, obstructions (Art. 1), it is clear that a 
 stream constantly tends to destroy obstructions. 
 
46 RIVER AND CANAL ENGINEERING 
 
 There is an obvious tendency for silt to deposit where 
 the bed slope of a stream flattens, and for scour to take 
 place where it steepens (Hydraulics, figs. 16 and 17, 
 pp. 24 and 25), and thus the tendency is for the slope 
 to become uniform. 
 
 In a natural stream flowing from hilly country to 
 a lake or sea, the slope is steepest at the commencement 
 and gradually flattens. There is thus a tendency for 
 the bed to rise except at the mouth of the stream. This 
 rising tends to increase the slope and velocity in the 
 lower reaches, and this again enhances the tendency, 
 described in the preceding article, of the stream to 
 increase in tortuosity. 
 
 When a silt-bearing stream overflows its banks the 
 depth of water on the flooded bank is probably small 
 and its velocity very low, and a deposit of silt takes 
 place on the bank. When the deposit has reached a 
 certain height it acts like a weir on the water of the 
 next flood, which flows quickly over it and, instead of 
 raising it higher, deposits its silt further away from the 
 stream. In this way a strip of country along the stream 
 gradually becomes raised, the raising being greatest 
 close to the stream. The country slopes downwards 
 in going away from the stream. In other words, the 
 stream runs on a ridge. If the bank becomes raised 
 so high that flooding no longer occurs, the raising 
 action ceases, but if, as is likely in alluvial country, 
 the bed of the stream also rises, the action may continue 
 and the ridge become very pronounced. 
 
 Some rivers have very wide and soft channels which 
 are only filled from bank to bank in floods, if then. The 
 deep stream winds about in the channel, and the rest 
 of it is occupied by sandbanks and minor arms. The 
 
SILTING AND SCOURING ACTION OF STREAMS 47 
 
 winding is the result of the velocity being too great for 
 the channel. The streams, especially the main stream, 
 constantly shift their courses by scouring one bank or 
 the other. Now and then the main stream takes a 
 short cut, either down a minor arm or across an easily 
 eroded sandbank. This is a very different matter from 
 a short cut across high ground. The sandbanks receive 
 deposits of silt in floods, but are constantly being cut 
 away at the sides. Such rivers frequently erode their 
 banks to an extraordinary extent. The Indus sometimes 
 cuts into its bank 100 feet or more in a day, and it may 
 cut for half a mile or more without cessation. The 
 tortuosity of such a stream increases as it gets nearer 
 the sea. The actual length of the Indus in the 400 
 miles nearest the sea is 39 per cent, greater than its 
 course measured along the bank. In the reach from 
 the 600th to the 700th mile from the sea, the difference 
 is only 3 per cent. For a detailed description of some 
 such rivers, see Punjab Rivers and Works. 
 
 Sometimes general statements are made regarding 
 silting or scour in connection, for instance, with a stream 
 which is confined between embankments or training 
 walls, or has overflowed its banks or is held up by a 
 weir. It is impossible to say that any such condition, 
 or any condition, will cause silting or scour, unless the 
 velocity depth and silt charge are known. 
 
CHAPTER V 
 
 METHODS OF INCREASING OR REDUCING SILTING 
 OR SCOUR 
 
 l. Preliminary Remarks. Most important works 
 which affect the regime of a stream have some effect on 
 its silting or scouring action, but this is not generally 
 their chief object. Such works will be dealt with in 
 due course, and the effects which they are likely to 
 produce on silting or scouring will be mentioned. In 
 the present chapter only those works and measures will 
 be considered whose chief object is to cause a stream to 
 alter its silting or scouring action. It does not matter, 
 so far as this discussion is concerned, whether the object 
 is direct, i.e. concerned only with the particular place 
 where the effect is to be produced, or indirect, as, for 
 instance, where a stream is made to scour in order that 
 it may deposit material further down the stream. The 
 protection of banks from scour is considered in CHAP. 
 VI. Dredging is dealt with in CHAP. VIII. 
 
 2. Production of Scour or Reduction of Silting. 
 Sometimes the silt on the bed of a stream is artificially 
 stirred up by simple measures, as, for instance, by scrapers 
 or harrows attached to boats which are allowed to drift 
 with the stream, or by means of a cylinder which has 
 claw-like teeth projecting from its circumference and is 
 
 48 
 
INCREASING OR REDUCING SILTING OR SCOUR 49 
 
 rolled along the bed, or by fitting up boats with shutters 
 which are let down close to the bed and so cause a rush 
 of water under them, or by anchoring a steamer and 
 working its screw propeller. It is thus possible to 
 cause a great deal of local scour, but the silt tends to 
 deposit again quickly, and it is not easy to keep any 
 considerable length of channel permanently scoured. 
 The system is suitable in a case in which a local shallow 
 or sandbank is to be got rid of and deposit of silt a 
 little further down is not objectionable. It may be 
 suitable in a case in which the bed is to be scoured 
 while a deposit of silt at the sides of the channel is 
 required, especially if some arrangement to encourage 
 silt deposit at the sides is used (Art. 3, par. 4 ; also 
 CHAP. VI., Art. 3}. 
 
 Holding back the water by means, for instance, of 
 a regulator or movable weir, and letting it in again 
 with somewhat of a rush, will, if frequently repeated, 
 have some effect in moving silt on in the down- 
 stream reach. Regarding the upstream reach, it has 
 been remarked (CHAP. IV., Art. 3) that a weir does not 
 necessarily cause silt deposit. If, in a stream which 
 does not ordinarily silt, a regulator or movable weir 
 causes, when the water is headed up, some silt deposit, 
 the cessation of the heading up not only removes the 
 tendency to silt, but the section of the stream, at the 
 place where the deposit occurred, is less than elsewhere, 
 and there is thus a tendency to scour there. If a 
 regulator is alternately closed and opened, no permanent 
 deposit of much consequence is likely to occur. 
 
 A stream may be made to scour its channel by opening 
 an escape or branch. This causes a draw in the stream, 
 and an increase in velocity for a long distance upstream 
 
50 RIVER AND CANAL ENGINEERING 
 
 of the bifurcation (Hydraulics, CHAP. VII. , Art. 6). 
 This procedure is sometimes adopted on irrigation 
 canals. The escape is generally opened in order to 
 reduce the quantity of water passing down, but it may 
 be opened solely to induce scour or prevent silting. 
 The floor of the escape head is usually higher than the 
 bed of the canal, but this does not interfere with opera- 
 tions except at low supplies. It may (CHAP. IV., Art. 
 2) have some effect on the quantity of rolled material 
 passed out of the escape. 
 
 If there is a weir in the river below the off- take of 
 the canal, and if the escape runs back to the river and 
 thus has a good fall, the scouring action in the canal 
 may be very powerful. 
 
 If the main channel has a uniform slope throughout, 
 the slope of its water surface is greater upstream of the 
 escape than downstream of the escape, and there is thus 
 an abrupt reduction of velocity and possibly a deposit 
 of silt in the main channel below the escape. This may 
 or may not be objectionable. In the case of an irriga- 
 tion canal, it is far less objectionable than deposit in the 
 head of the canal. The best point for the off-take of 
 any escape or scouring channel depends on the position 
 of the deposits in the main channel. The off-take 
 should be downstream of the chief deposits, but as near 
 to all of them as possible. A breach in a bank acts of 
 course in the same way as an escape. 
 
 A stream of clear water when sent down a channel 
 will scour it if the material is sufficiently soft. In the 
 case of the Sirhind Canal, it has already been mentioned 
 (CHAP. IV., Art. 3), that when the river water became 
 clear after the floods the proportion of coarse sand, i.e. 
 sand above the '10 class, carried by the canal water 
 
INCREASING OR REDUCING SILTING OR SCOUR 51 
 
 was about x^W?) by v l ume - This was in the period 
 from 22nd September to 7th October. From 8th to 23rd 
 October the proportion averaged 3-^,^00^ from 24th 
 October to 8th November ^Vcny* anc ^ from 9th to 24th 
 November ^-5,^0^. The reason of this reduction was 
 that the comparatively clear water kept picking up the 
 sand from the bed and moving it on, the finer kinds 
 being moved most quickly. As the coarse sand left on 
 the bed became less in quantity, the water took up less. 
 It appears, however, that the water also picked up some 
 clay which was left, and that the total suspended silt in 
 November was w Vo f the water. All the observations 
 mentioned in this paragraph appear to have been made 
 at Garhi, 26 miles from the head of the canal. 
 
 3. Production of Silt Deposit. Works or measures 
 for causing silt deposit may be undertaken in order 
 to cause silt deposit in specific places where it will be 
 useful, or in order to free the water from silt. Some- 
 times both objects are combined. 
 
 If a stream can be turned into a large pond or low 
 ground a bank being built round it if necessary it can 
 be made to part with some or all of its silt whether 
 rolled or suspended. Even if the pond is so large that 
 the velocity becomes imperceptible, the whole of the 
 suspended matter will not deposit unless it has sufficient 
 time, but the matter which remains in the water is likely 
 to be extremely small in amount. The silting up of 
 marshes, pools, borrow-pits, etc., is now being effected, 
 or should be effected, in places where mosquitoes and 
 malaria are prevalent. 
 
 In the upper or torrential part of a stream, a high 
 dam, provided with a sluice and a high-level waste 
 
 weir, may be built across it. The space above the dam 
 
 E 2 
 
52 RIVER AND CANAL ENGINEERING 
 
 becomes more or less filled with gravel, etc. This has 
 been done in Switzerland (Min. Proc. Inst. C.E., 
 vol. clxxi.). In the U.S.A. long weirs have been 
 built in order to stop the progress of detritus from 
 gold mines. .Such detritus was liable to choke up 
 rivers and damage the adjoining lands. The detritus 
 
 FIG. 3. 
 
 from hill torrents can also be reduced by afforestation 
 of the hill sides. 
 
 When a stream is in embankment irrigation channels 
 are frequently so the bank can be set back (fig. 3), 
 and suspended silt will then deposit on the berms. The 
 object of this arrangement is generally to create very 
 strong banks in low ground. A similar plan can be 
 adopted when the berm is only slightly below the 
 
 FIG. 4. 
 
 water - level and even when it is only occasionally 
 submerged. In this case the deposit of a small bank 
 of silt along the edge of the berm next the stream 
 will prevent the access of fresh supplies of silt-bearing 
 water to the parts further away. Gaps should be cut 
 in the bank of silt at intervals, and cross banks made 
 to form " silting tanks," as shown in fig. 4. The inlets 
 to the tank should be large, and the outlets small, so 
 that the water in the tank may have little velocity. 
 
INCREASING OR REDUCING SILTING OR SCOUR 53 
 
 It is not, however, correct to have the outlet so small 
 unless the water contain very little silt that there is 
 very little flow through the tank. The tanks will 
 generally be silted up most quickly by allowing a good 
 flow through them, even though only a small proportion 
 of the silt in the water is deposited. Eegular banks 
 arranged to form tanks on the above principle can be 
 made behind the original banks of a canal in cases 
 where the original banks were not, for any reason, set 
 back. 
 
 When a channel is made in low ground and the 
 excavation is not sufficient to make the banks, borrow- 
 pits can be dug in the bed of the channel. Such pits 
 should not be long and continuous, but wide bars should 
 be left so that a number of short pits will result. 
 These pits will trap rolled material as well as suspended 
 silt. The object in this case is to free the water from 
 silt and to reduce the size of the channel and thus 
 reduce the loss of water from percolation. 
 
 On the Indus, where it has a strong tendency to shift 
 westwards, long earthen dams or groynes are run out 
 from the west bank across the sandbanks. One object 
 is to cause silt deposit, and so increase the quantity of 
 material which the river will have to cut away, but 
 whether this result is achieved is doubtful. The sand- 
 banks receive deposits in any case. A groyne may 
 increase the deposit on its upstream side, but it cuts 
 off the flood water from its downstream side and so 
 reduces the deposit there. 
 
 4. Arrangements at Bifurcations. At a bifurca- 
 tion, as where a branch takes off from a canal, it is 
 possible to reduce the quantity of rolled material 
 entering the canal by raising its bed or constructing 
 
54 RIVER AND CANAL ENGINEERING 
 
 a weir or " sill " in its head. This arrangement may 
 have great effect in excluding boulders, shingle, or 
 gravel. As regards rolled sand, it has much less effect 
 than might be expected (CHAP. IV., Art. 2). If the 
 canal is reduced in width (fig. 5) there will be eddies 
 below the bed level of the branch. They will stir up 
 the sand and some of it will enter the branch. If the 
 canal is not reduced in width, eddies will be produced 
 in the surface water, and they will affect the bed. 
 
 The above remarks apply also to the case of a canal 
 
 FIG. 5. 
 
 taken off from a river when there are no works in the 
 river. 
 
 5. A Canal with Headworks in a River. In the 
 
 case of a canal taking off from a river and provided 
 with complete headworks, it is possible to do a great 
 deal more. The case of the Sirhind Canal, already 
 referred to (CHAP. IV., Arts. 5 and 6), is a notable 
 example. The canal (fig. 6) is more than 200 feet 
 wide, the full depth of water 10 feet, and the full dis- 
 charge about 7000 cubic feet per second. In 1893 
 when the irrigation had developed, and it became 
 necessary to run high supplies in the summer July, 
 August, and part of September the increase in the silt 
 deposit threatened to stop the working of the canal. 
 
INCREASING OR REDUCING SILTING OR SCOUR 55 
 In the autumn and winter, say from 25th September 
 
 to 15th March, the water entering the canal is clear 
 
56 RIVER AND CANAL ENGINEERING 
 
 and much of the deposit was picked up by it, but not 
 all. In the five years 1893 to 1897 inclusive, the 
 following remedial measures were adopted. Increased 
 use was made of the escape at the twelfth mile. This 
 did some good, but there was seldom water to spare. 
 In 1893 to 1894 the sill of the regulator was raised to 7 
 feet above the canal bed, and it was possible to raise 
 it 3 feet more by means of shutters. This had little 
 effect. The coarsest class of sand was "4, and the velocity 
 of the water, even of that part of it which came up 
 from the river bed and passed over the sill, was over 
 2 feet per second, so that all sand was carried over. 
 In 1894 to 1895 the divide wall, which had been only 
 59 feet long, was lengthened to 710 feet, so as to make 
 a pond between the divide wall and the regulator, 1 
 but probably the leakage through the under-sluices 
 was often as much as the canal supply, and the water 
 in the pond was thus kept in rapid movement and full 
 of silt. The canal was closed in heavy floods. This 
 did some good, but probably the canal was often closed 
 needlessly when the water looked muddy but contained 
 no excessive quantity of sand. The above comments 
 on the measures taken were made by Mr Kennedy when 
 chief engineer. The above measures did not reduce 
 the silt deposits, but the scour in the clear water season 
 improved, probably because higher supplies were run 
 owing to increased irrigation. The deposit in the upper 
 reaches of the canal, when at its maximum about the 
 end of August of each year, was generally more than 
 twenty million cubic feet. From the year 1900 a 
 better system of regulation was enforced, the under- 
 
 1 The regulator runs across the canal head ; the under-sluices are a con- 
 tinuation of the weir, between the divide wall and the regulator. 
 
INCREASING OR REDUCING SILTING OR SCOUR 57 
 
 sluices being kept closed as much as possible, so that 
 there was much less movement in the pond and much 
 less silt in its water. By 1904 the deposit in the canal 
 had. been reduced to three million cubic feet, and no 
 further trouble occurred. 
 
 During the period from 20th September 1908 to 10th 
 October 1908 the quantity of silt in the canal above 
 Chamkour (twelfth mile) decreased from 19,325,800 
 cubic feet to 12,477,600 cubic feet. The quantity 
 scoured away was 6,848,200 cubic feet. During this 
 period no silt entered the canal. The quantity which 
 passed out of the reach in question in suspension was 
 4,183,660 cubic feet, so that 2,664,540 cubic feet of 
 material must have been rolled along the bed. 
 The rolled material was 64 per cent, of the suspended 
 material. During this period the Daher escape, in 
 the twelfth mile, was open, and the mean velocity 
 in the canal just above the escape was about 4 feet 
 per second, the depth of water being about 10 feet. 
 The velocity near the escape was thus greater than 
 the critical velocity for mixed silt (CHAP. IV., Art. 6), 
 and even a long way up the canal it would be in 
 excess of the critical velocity. The water seems to have 
 carried about ^QO of its volume of silt. Whether the 
 above proportions of rolled to suspended matter would 
 hold good in a fully charged stream flowing with the 
 critical velocity it is not easy to say. 
 
 As silt deposits in the pond, the velocity of the water 
 in it, along the course of the main current towards the 
 canal, increases and eventually the water begins to 
 carry coarse sand dangerous for the canal. In order to 
 ascertain when this state of affairs has been reached, two 
 methods of procedure are possible. One is to frequently 
 
58 RIVER AND CANAL ENGINEERING 
 
 test specimens of the water in the pond along the course 
 of the main current and see when it contains more 
 than xg.VoiT f ^s volume of coarse sand. This plan 
 would be troublesome and liable to error, and is rejected 
 by Kennedy, who suggests that the depth and velocity 
 of the water in the pond be frequently observed along 
 the course of the main current. As soon as the velocity 
 exceeds the critical velocity for mixed silt, it is time to 
 close the canal and open the under-sluices and scour 
 out the deposit from the pond. The period in which 
 most silt is believed to have been deposited in the canal 
 is the spring and early summer, say from 15th March 
 to 1st July. This is the time when the snows are 
 melting and the river water is clear. It can then carry 
 more sand than in the rains 1st July to 15th 
 September, when it is muddy. 
 
 Kennedy also suggests that some under-sluices should 
 be provided at the far side of the river, i.e. at the right- 
 hand side of the weir. It would then be possible, by 
 opening them, to let floods pass without interfering 
 with the pond. 
 
 The two spurs or groynes, shown in the plan, were 
 constructed in 1897 so as to cause the stream to flow 
 along the face of the canal regulator and not allow 
 deposits to accumulate there. The depth of silt 
 deposited in a great part of the pond amounted at times 
 to 8 or 10 feet. 
 
 6. Protection of the Bed. It is possible to afford 
 direct protection from scour to the bed of a stream by 
 constructing walls across it, but unless the walls are 
 near together the protection will not be effective. An 
 arrangement used in some streams in Switzerland con- 
 sists of tree trunks secured by short piles and resting on 
 
INCREASING OR REDUCING SILTING OR SCOUR 59 
 
 brushwood. But as long as the walls are not raised 
 above the bed they cannot entirely stop scour, unless 
 extremely close together. If raised above the bed they 
 form a series of weirs. 
 
 The weirs must be so designed that the depth of 
 water in a reach between two weirs is great enough to 
 reduce the velocity down to the critical velocity, or less. 
 The fall in the water surface at each weir being very 
 small, the discharge over the weir can be found by con- 
 sidering it as an orifice extending up to the downstream 
 water surface, and the head being the fall in the surface 
 at the weir. 
 
 To stop scour of the bed by direct protection without 
 raising the water-level, the bed can be paved, a plan 
 adopted in artificial channels with very high velocities. 
 The paving can be of stones, bricks, or concrete blocks. 
 The Villa system of protection, which has been used in 
 Italy, France, and Spain, consists of a flexible covering 
 laid on the bed. Prisms of burnt clay or cement are 
 strung on several parallel galvanized iron wires, which 
 are attached to cross-bars so as to form a grid a few 
 feet square. The grids are loosely connected to one 
 another at the corners, and the whole covering adjusts 
 itself to the irregularities of the bed (Min. Proc. lust. 
 C.E., vol. cxlvii.). 
 
 The special protection or paving required in connec- 
 tion with weirs and such-like works is considered in 
 CHAP. X., Arts. 2 and 3. 
 
CHAPTER VI 
 
 WORKS FOR THE PROTECTION OF BANKS 
 
 l. Preliminary Remarks. The protection of a length 
 of bank from scour may be effected by spurs, which are 
 works projecting into the stream at intervals, or by 
 a continuous lining of the bank. A spur forms an 
 obstruction to the stream (CHAP. IV., Art. 1), and when 
 constructed, or even partly constructed, the scour near 
 its end may be very severe, even though there may be 
 little contraction of the stream as a whole. If the bed 
 is soft a hole is scoured out. Into this hole the spur 
 keeps subsiding, and its construction, or even its main- 
 tenance, may be a matter of the greatest difficulty. A 
 high flood may destroy it. If it does not do so, it may 
 be because the stream has, for some reason, ceased to 
 attack the bank at that place. A continuous lining of 
 the bank is not open to any objection, and is generally 
 the best method of protection. Spurs made of large 
 numbers of rather small trees, weighted with nets filled 
 with stones, have been used on the great shifting rivers 
 of the Punjab which swallowed up enormous quantities 
 of materials. The use of spurs on such rivers has now, 
 in most cases, been given up. If L is the length of a 
 spur measured at right angles to the bank, the length of 
 bank which it protects is about 7 L 3 L upstream and 
 
 60 
 
WORKS FOR THE PROTECTION OF BANKS 61 
 
 4 L downstream, but the spur has to be strongly built, 
 and its cost is, in many cases, not much less than the 
 expense of protecting the whole bank with a continuous 
 lining. 
 
 Whatever method is adopted, a plan, large enough to 
 show all irregularities, should always be prepared, and 
 the line to which it is intended that the bank shall be 
 brought marked on it. 
 
 Sometimes natural spurs exist as, for instance, where a 
 tree projects into a stream or has fallen into it, and the 
 holes between the spurs may be deep, so that a continuous 
 
 FIG. 7. 
 
 protection would be expensive. Or there may be trees 
 standing in such positions that, if felled, they will be 
 in good places for spurs. In cases such as the above, 
 spurs may be suitable even in a stream with a soft 
 channel. 
 
 Regarding the use of spurs or groynes for diversion 
 works or for reducing the width of a stream, see CHAP. 
 VII. , Art. 1, and CHAP. VIII, Art. 3. 
 
 2. Spurs. A spur may be made of 
 
 (a) Loose stone, which may be faced with rubble 
 
 above low- water level (fig. 7). 
 
 (b) Layers of fascines weighted with gravel or 
 
 stones. 
 
 (c) Earth or sand closely covered with fascines. 
 
62 
 
 RIVER AND CANAL ENGINEERING 
 
 (d) A double line of stakes with fascines or 
 
 brushwood laid between them (fig. 8). 
 
 (e) A single line of stakes with planking or 
 
 basket work on its upstream side, or with 
 twigs or wattle laid horizontally and passed 
 in and out of the stakes, as in fig. 20. 
 (/) A single tree with the thick end of the trunk 
 on the bank arid with stakes, if necessary, 
 to prevent the current from moving it. 
 
 777777?? 
 
 FIG. 8. 
 
 (g) A number of small trees heaped together and 
 weighted with nets full of stones. 
 
 (h) A layer of poles and over them a layer of 
 fascines on which are built walls of fascine 
 work so arranged as to form cells or hollow 
 rectangular spaces which become filled 
 with silt. 
 
 (i) Large fascines running out into the stream 
 and having their inner ends staked to the 
 bank while the outer ends float, other 
 fascines being added over them and pro- 
 jecting further into the stream, and the 
 whole eventually sinking. 
 Combinations of the above are also used, for instance, 
 
WORKS FOR THE PROTECTION OF BANKS 63 
 
 (d) or (e) may be used for the upper portion, the 
 foundation being (a) or (c). 
 
 Instead of running out at right angles to the bank 
 a spur may be inclined somewhat downstream. This 
 
 FIG. 9. 
 
 somewhat reduces the eddying and scour round the end. 
 The ends of a system of spurs should be in the line 
 which it is intended that the edge of the stream shall 
 have (fig. 9). The tops of short spurs are usually above 
 
 FIG. 10. 
 
 high flood level. Sometimes spurs are made to slope 
 downwards (fig. 10), and they then cause less disturbance 
 of the water and less scour than if built to the form shown 
 by the dotted line. Such spurs are sometimes combined 
 with a low wall running across the bed of the stream, 
 
 FIG. 11. 
 
 the whole forming a "profile 7 * of the cross-section to 
 which it is intended to bring the channel. Regarding 
 such walls, see CHAP. V., Art. 6. When a spur is long it- 
 may have small subsidiary spurs (fig. 11) to reduce the 
 rush of water along it ; or its end may have to be pro- 
 
64 RIVER AND CANAL ENGINEERING 
 
 tected in the same manner as the advancing end of a 
 closure dam (CHAP. VIL, Art. #). 
 
 The following is a curious case of misconception of 
 the action of spurs. In 1909 the river Indus was erod- 
 ing its right bank and threatening to destroy the town 
 of Dera Ghazi Khan. A clump of date palms formed 
 a promontory and resisted erosion to some extent. A 
 suggestion was made by an engineer of eminence who 
 had formerly been consulted in the case to the effect 
 that the date palms be removed, the reason given being 
 that they caused disturbance and scour. On this 
 principle spurs would have to be made not to protect 
 a bank but to cause it to be eroded. 
 
 3. Continuous Lining of the Bank. The lining or 
 protection of a bank may be of stone or brick pitching 
 (figs. 12 and 13), loose stone (fig. 14), fascines (fig. 15), 
 turfing, plantations, brushwood, or of other materials 
 laid on the slopes. Before protecting a bank it is best 
 to remove irregularities and bring it to a regular line. 
 This can generally be done most easily by filling in 
 hollows, but sometimes it is done by cutting off pro- 
 jections. It is also necessary to make the side slope 
 uniform. Where the slope is as shown by the dotted 
 lines in figs. 12 to 14, filling in can be effected, but 
 cutting away the upper part of the slope is also feasible. 
 Such cutting away has been proposed as a remedy in 
 itself in cases where the steep upper part of the slope 
 was falling in, but it is not much of a remedy. 
 
 Stone pitching may rest, if boats are required to come 
 close to the bank, on a toe wall of concrete, as in fig. 13, 1 
 or otherwise on a foundation of loose stone, as in fig. 12. 
 When concrete is used the bed is dredged to such a 
 
 1 See also Appendix B. 
 
WORKS FOR THE PROTECTION OF BANKS 65 
 
 depth as will provide against undermining by scour. 
 Sloping boards attached to piles are placed along the 
 front face and the concrete is thrown in under water. 
 The slope of stone or brick pitching is usually from 
 
 FIG. 12. 
 
 2 to 1 to 1 to 1, but it may be as steep as \ to 1. The 
 earth behind the pitching must be well rammed in 
 layers. In order to prevent the earth from being eaten 
 away by the water which penetrates through the inter- 
 
 77//Y 
 
 FIG. 13. 
 
 stices of the stone or brick, a layer, 3 to 6 inches thick, 
 of gravel or ballast is placed over the earth and rammed. 
 When loose stone is used, dredging is not necessary, but 
 the stone is allowed to gradually sink down and more is 
 added at the top. A certain proportion of the stones 
 should be of large size. 
 
66 
 
 RIVER AND CANAL ENGINEERING 
 
 When fascining is used, long twigs are made into 
 bundles and tied up at every 2 feet so as to form 
 fascines about 4 to 6 inches thick, and these are laid 
 
 FIG. 14. 
 
 on the slopes and secured by pegs driven in at short 
 intervals, between the fascines. 
 
 Sometimes the pitching or loose stone is not carried 
 up to the top of the bank, or even up to high flood-level, 
 
 FIG. 15. 
 
 and the bank above the pitching is protected by turfing 
 the pieces of turf being placed on edge normally to 
 the slope if very steep (fig. 14) or laid parallel to the 
 slope if it is not very steep or, above ordinary water- 
 level, by plantations of osiers or willows which obstruct 
 
WORKS FOR THE PROTECTION OF BANKS 67 
 
 the water and tend to cause silting, and whose roots 
 bind the banks together. 
 
 Another method of using fascines is to lay them on 
 the slopes with their lengths normal to the direction of 
 the stream. The upper end of a fascine is above low 
 
 FIG. 16. 
 
 water, and the lower end extends down to the bed of 
 the stream. Sometimes large ropes made of straw, or 
 rough mats made of grass, are laid on the slopes and 
 pegged down, or mattresses of fascines are laid on the 
 slopes and weighted with stones. 
 
 A deep recess in the bank (fig. 16) can be filled in, 
 before the protection is added, with earth well rammed. 
 
 FIG. 17. 
 
 On the Adige the filling material consisted (fig. 17) of 
 faggots filled with stones, small cross dams being made 
 at intervals, as shown by the dotted lines, to arrest flood 
 water and cause it to deposit silt. At the back of the 
 berm, poplar or willow slips were planted, and these 
 grew up and their roots held the bank together. This 
 system succeeded well. 
 
 A method of protection which is suitable when the 
 
 F2 
 
68 RIVER AND CANAL ENGINEERING 
 
 water contains much silt is what is known in India as 
 bushing. Large leafy branches of trees are cut and 
 hung, as shown in fig. 18, by ropes to pegs. They must 
 be closely packed so as not to shake. At first they 
 require looking after, but silt rapidly deposits and the 
 branches become fixed and no longer dependent on the 
 
 FIG. 18. 
 
 ropes. If the work is carefully done, the result is a 
 smooth, regular, and tenacious berrn, as per dotted line 
 in the figure. 
 
 Another method, used on canals, is to make up the 
 bank with earth and to revet it with twigs or reeds, as 
 shown in fig. 19. The foundation must be taken down 
 
 well below bed-level, otherwise the work may slip. This 
 kind of work cannot be done except when the canal is 
 dry. 
 
 If the bank consists of sand or of very sandy soil, it 
 must in any case have a flat side slope such as 3 to 1. 
 If the sand is in layers alternating with firm soil, it is 
 a good plan to dig out some of the sand and to replace 
 it with clods of hard earth. 
 
WORKS FOR THE PROTECTION OF BANKS 69 
 
 Staking (fig. 20) may be used, the stakes being one 
 or two feet apart from centre to centre, and long twigs 
 laid horizontally being passed in and out of the stakes, 
 or bushing filled in behind the stakes. But bushing 
 alone is cheaper and nearly as good. 
 
 For protecting the banks of the Indus it has been 
 proposed (Punjab Rivers and Works, CHAP. IV.) to use 
 trees in exactly the same manner as bushing, the trees 
 being grown in several rows parallel to the river so that 
 whenever the river, by eroding its bank, comes up to 
 the lines of trees the first row will fall in. The first row 
 
 FIG. 20. 
 
 would be chained to the second, which would take the 
 place of the pegs used in bushing. The other rows 
 would remain as a reserve. 
 
 The Villa system of bed protection (CHAP. V., Art. 6) 
 has also been successfully used for bank protection on 
 the Scheldt, and on the Brussels-Ghent Canal, the 
 prisms being about 10x10x4 inches, and having 
 overlapping joints. The bands of prisms are placed 
 in position by a boat, the bands unrolling over a drum. 
 The boat is provided with an oscillating platform carry- 
 ing rollers at its end. A thin layer of gravel is laid 
 over the bank and is pressed down by the rollers before 
 the prisms are laid on it (Min. Proc. Inst. C.E., vol. 
 cxxxiv., and vol. clxxv.). 
 
70 RIVER AND CANAL ENGINEERING 
 
 In the case of the river mentioned in CHAP. XL, 
 Art. 3, where extremely high velocities were met with, 
 cylindrical rolls of wire-netting were made, each 50 feet 
 long and 5 feet in diameter, and filled with boulders. 
 These rolls can be used for bank protection. The 
 netting was made by wires 6 inches apart, crossing 
 each other at right angles and tied together at the 
 crossings by short pieces of wire. 
 
 On ship canals a berm (fig. 21) is frequently made a 
 few feet below the water-level. It serves as a foundation 
 for the pitching, which need not usually extend down to 
 
 FIG. 21. 
 
 more than 5 feet below the water-level. Below that 
 the wash has little or no effect on the banks. On 
 ordinary navigation canals a similar berm is sometimes 
 made one or two feet in width and a foot or less below 
 the water-level and rushes are planted on it. 
 
 Sometimes a bank has been protected by a kind of 
 artificial weed, consisting of bushes or branches of trees 
 attached to ropes. The end of the rope is fastened to 
 the bank and the weeds float in the stream alongside 
 the bank. 
 
 To protect a bank from ice, which exercises an uplift- 
 ing force on pitching, use has been made of a covering 
 of a kind of reinforced concrete consisting of slabs of 
 
WORKS FOR THE PROTECTION OF BANKS 71 
 
 concrete with wires embedded in it, and fastened to the 
 bank by wires, 20 inches long, running into the bank, 
 these wires being embedded in mortar so as to act like 
 stakes. 
 
 4. Heavy Stone Pitching with Apron. On the 
 great shifting rivers of India a system of bank protec- 
 tion is adopted, consisting of a pitched slope with an 
 apron (fig. 22). The system is used chiefly in connec- 
 tion with railway bridges or weirs, but it has been used 
 in one instance, that of Dera Ghazi Khan, for the 
 protection of the bank near a town. When, as is usual, 
 the flood-level is higher than the river bank, an artificial 
 
 
 FIG. 22. 
 
 bank is made. In any case the bank is properly 
 aligned. The pitching has a slope of 2 to 1, and 
 consists of quarried blocks of stone loosely laid, the 
 largest blocks weighing perhaps 120 Ibs. The apron 
 is laid at the time of low water on the sandbank or 
 bed of the stream. If necessary, the ground is specially 
 levelled for it. It is intended to slip when scour occurs. 
 The following dimensions of the apron are given by 
 Spring (Government of India Technical Paper, No. 153, 
 " River Training and Control on the Guide Bank System," 
 1904). The probable maximum depth of scour can be 
 calculated as explained in CHAP. XL, Art. 3. If this 
 depth, measured from the toe of the slope pitching is D, 
 and if T is the thickness considered necessary for the slope 
 
72 RIVER AND CANAL ENGINEERING 
 
 pitching, then the width of the apron should be 1*5 D. 
 and its thickness 1*25 T next the slope and 2*8 T next 
 the river. It will then be able to cover the scoured 
 slope to a thickness of 1'25 T. This thickness is made 
 greater than T because the stone is not likely to slip 
 quite regularly. The thickness T should, according to 
 Spring, be 16 inches to 52 inches, being least with a 
 slow current and a channel of coarse sand, and greatest 
 with a more rapid current and fine sand ; but since the 
 sand is generally finer as the current is slower, it would 
 appear that a thickness of about 3 feet would generally 
 be suitable. Under the rough stone there should be 
 smaller pieces or bricks. Along the top of the bank 
 there is generally a line of rails so that stone from 
 reserve stacks, which are placed at intervals along the 
 bank, can be quickly brought to the spot in case the 
 river anywhere damages the pitched slope. 
 
 For the special protection to banks required near 
 weirs and similar works, see CHAP. X., Arts. 2 and 3. 
 
CHAPTER VII 
 
 DIVERSIONS AND CLOSURES OF STREAMS 
 
 1. Diversions. When a stream is permanently 
 diverted the new course is generally shorter than the 
 old one, and the diversion is then often called a cut-off. 
 The first result of a cut-off is a lowering of the water- 
 level upstream and a tendency to scour there, and to silt 
 downstream of the cut-off. Fig. 23 shows the longi- 
 
 FIG. 23. 
 
 tudinal section of a stream after a cut-off A B has been 
 made. The bed tends to assume the position shown by 
 the dotted line. If both the diversion and the old 
 channel are to remain open, the water-level at the 
 bifurcation will be lowered still more, and the tendency 
 to scour in the diversion will be reduced. 
 
 If the material is soft enough to be scoured by the 
 stream, it is often practicable to excavate a diversion to 
 a small section and to let it enlarge itself by scour. 
 This operation is immensely facilitated if the old channel 
 can be closed at the bifurcation. The question whether 
 the scoured material will deposit in the channel down- 
 
 73 
 
74 RIVER AND CANAL ENGINEERING 
 
 stream of the diversion must be taken into consideration ; 
 also the question whether the diversion will continue to 
 enlarge itself more than is desirable. The velocity in 
 the diversion will be a maximum if its section is of the 
 " best form," i.e. if its bed and sides are tangents to 
 a semicircle whose diameter coincides with the water 
 surface, but this may not (CHAP. IV., Art. 6) be the 
 section which will give most scour. In order to prevent 
 the enlargement of the diversion taking place irregularly, 
 the excavation can be made as shown in fig. 24, water 
 being admitted only to the central gullet. The side 
 gullets should not be quite continuous, but unexcavated 
 portions should be left at intervals, so that if the water 
 
 FIG. 24. 
 
 in scouring out the channel breaks into the gullet, it 
 will not be able to flow along it until it has broken in 
 all along. 
 
 If a diversion is made, not with the object of lowering 
 the water-level but merely in order to shorten the 
 channel, the increased velocity caused by the steepened 
 slope may be inconvenient. In this case a weir or weirs 
 can be added (CHAP. VIII., Art. 4). 
 
 If the water contains sufficient silt to enable the 
 abandoned loop to be silted up within a reasonable time, 
 it may be desirable to do this. The silting up may, for 
 instance, increase the value of the land. The loop 
 should be closed at its upper end. Water entering the 
 lower end will cause a deposit there. When the lower 
 end is well obstructed by silt, the upper end should be 
 opened. 
 
DIVERSIONS AND CLOSURES OF STREAMS 75 
 
 The set of the stream, due, for instance, to a bend at 
 the point where a diversion takes off has very little to 
 do with the quantity of water which goes down the 
 diversion. The only effect of the set of the stream is 
 a slight rise of the water-level as compared with the 
 opposite bank. Similarly, the angle at which the 
 diversion takes off is only of importance in giving, in 
 some cases, a velocity of approach whose effect is 
 generally small. The distribution of the water between 
 the diversion and the old channel really depends on 
 their relative discharging capacities. If the required 
 quantity of water does not flow down a diversion it 
 can be dredged. 
 
 Sometimes a long spur is run out to send the water 
 towards the off-take of a diversion. The effect of this 
 is very small it merely causes a set of the stream, 
 unless its length is so great that it amounts to something- 
 like a closure dam. It is sometimes said that it is easier 
 to lead a river than to drive it. This remark is probably 
 based on the fact that spurs, such as those under con- 
 sideration, generally produce little effect, whereas the 
 excavation of a diversion or the deepening of a branch 
 by dredging it, is more likely to produce some result. 
 There is, however, no certainty about this. Sometimes 
 too much is expected of such channels. Calculations 
 are not always made as to the scouring power of the 
 stream, nor is account always taken of the fact that as 
 the cut scours its gradient flattens. 
 
 2. Closure of a Flowing Stream. The closure of 
 
 a flowing stream by means of a dam is usually attended 
 with some difficulty and sometimes with enormous 
 difficulty. There may be little trouble in running out 
 dams from both banks for a certain distance, but as soon 
 
76 RIVER AND CANAL ENGINEERING 
 
 as the gap between the dams becomes much less than 
 the original width of the stream, the water on the up- 
 stream side is headed up and there is a rush of water 
 through the gap, which tends to deeply scour the bed 
 and to undermine the dams. The smaller the gap 
 becomes the greater is the rush and scour. 
 
 The closure is most easily effected at or near to the 
 place where the stream bifurcates from another. Then, 
 as the gap decreases in width, some of the water is 
 driven down the other stream and it does not rise so 
 much. Eventually all the water goes down the other 
 stream, and the total rise is only so much as will enable 
 this other stream to carry the increased discharge. If 
 the closure is not effected near a bifurcation, the rise of 
 the water will go on even after the closure is completed, 
 and it will not cease, unless the water escapes or breaks 
 out somewhere, until it has risen to the same level as 
 that to which it would have risen if the closure had 
 been at the bifurcation, or perhaps not quite to the 
 same level, since there may still be a slight slope in the 
 water surface and a small discharge which percolates 
 through the dam. Sometimes in such a case it is 
 possible to arrange for temporary escapes or bifurca- 
 tions, which will be shallow and therefore easily closed, 
 after the main closure has been completed. 
 
 A closure is, of course, far more easily effected where 
 the bed is hard than where it is soft. Very often it is 
 best to close temporarily at such a place or near a 
 bifurcation, even if the permanent dam has to be 
 elsewhere, and then to construct the permanent dam 
 in the dry channel, or in the still water, and remove 
 the temporary one or cut a gap in it. 
 
 Generally the best method to adopt in a closure is to 
 
DIVERSIONS AND CLOSURES OF STREAMS 77 
 
 cover the bed of the channel beforehand unless it is 
 already hard enough with a mattress or floor, such 
 that it cannot be scoured as the gap closes. A floor 
 may consist of a number of stones or sandbags dropped 
 in from boats or by any suitable means, and placed 
 with care so that there shall not be gaps or mounds. 
 Sandbags should be carefully sewn up. A mattress 
 may be made of fascines laid side by side and tied 
 together, floated into position, weighted and sunk. 
 Even a carpet made of matting or cloth and suitably 
 weighted has sufficed in some cases. If the scour is 
 
 o 
 
 likely to be such that stones or sandbags will be carried 
 away, the stones may be placed in nets, baskets, or crates. 
 Sandbags may also be placed in nets. Probably the 
 long rolls of wire-netting filled with stones, described in 
 CHAP. VI., Art. 3, would be better than anything, and 
 the diameter could be reduced somewhat. The floor 
 or mattress need not usually extend right across the 
 stream. It must cover a width much greater than 
 perhaps twice as great as the width of the gap is likely 
 to be when scour begins. Its length, measured parallel 
 to the direction of the stream, must be such that severe 
 eddies in the contracted stream will have ceased before 
 the stream reaches its downstream edge. It need not 
 extend to any considerable distance upstream of the 
 line of the dam. 
 
 The dams when started from the banks can generally 
 be of simple earth or gravel, or loose stones, but before 
 they have advanced far they will probably require 
 protection at the ends by stones, or by staking and 
 brushwood, or by fascines. As soon as the dams have 
 advanced well onto the mattress and their ends have 
 been well protected, it is best to cease contracting the 
 
78 RIVER AND CANAL ENGINEERING 
 
 stream from the sides and to contract it from the 
 bottom by laying a number of sandbags across the gap 
 so as to form a submerged weir. In this way the rush 
 of water is spread over a considerable width of the 
 stream. The weir is then raised until it comes up 
 above water. Leakage can be stopped by throwing in 
 earth, or gravel, or bundles of grass on the upstream 
 side. Sometimes it is best to construct the mattress 
 over the whole width of the stream, and to effect the 
 closure entirely by a weir, carrying each layer right 
 across before adding another. The banks of the stream, 
 if not hard, can be protected by sandbags, stones, 
 staking or fascining. 
 
 The chief cause of failures of attempts to close flowing 
 streams is neglect to provide a proper floor or mattress. 
 The stones or other materials may be of insufficient 
 weight or not closely laid, or the extent of the floor 
 may be insufficient. In a soft channel and deep water 
 loose stones in almost any quantity may fail unless a 
 mattress of fascines is laid under them. Another cause 
 of failure is running short of materials, such as sandbags. 
 Allowance should be made for every contingency, in- 
 cluding making good any failure of parts of the 
 work. Enormous sums of money have been wasted, 
 and vast inconvenience, loss and trouble incurred, 
 in futile attempts to close breaches in banks, or gaps 
 in dams. 
 
 Sometimes the gap is closed by sinking a barge loaded 
 with stones, or by sinking a "cradle" or large mattress 
 made of fascines, taken out to the site by four boats, 
 one supporting each corner, and then loaded with stones 
 and sunk. Another method is to run out a floating 
 mattress of fascines from one side of the gap to the 
 
DIVERSIONS AND CLOSURES OF STREAMS 79 
 
 middle and sink it, then to proceed similarly on the 
 other side, and so on. 
 
 An excellent plan, when it can be adopted, is to have 
 more than one line of operations, so that the heading up 
 of the water is divided between them. 
 
 In India closures of streams having depths of 6 or 8 
 feet are effected by means of rough trestles made from 
 trunks of small trees and placed at intervals in the 
 stream like bridge piers, one leg of the trestle inclined 
 upstream and one downstream. Each pair of adjacent 
 trestles is connected by a number of rough, horizontal 
 poles. Against these are placed bundles of brushwood. 
 Earth is at the same time collected and is rapidly added 
 at the last. The chief danger is the undermining of 
 the bed by scour. This is prevented by driving in 
 stakes and placing brushwood against them. Closures 
 of small channels or of breaches in the banks of canals 
 are effected by means of staking and brushwood. 
 Where dangerous breaches are liable to occur, it is a good 
 plan to have a barge, fitted up with a small pile-driver 
 and carrying a supply of sheet piles, ready at a 
 convenient spot. 
 
 Hurdle dykes, first used on the Mississippi, were 
 employed on the Indus in 1902 to close partially the 
 main channel of the river. There were to be three 
 dykes, each dyke consisting of three lines of very long 
 piles some were 60 feet long, driven into the bed of 
 the stream, which was to be protected with mattresses 
 made of fascines and extending right across it, with 
 their heads above flood-level. The idea was not to 
 wholly stop the flow of the water, but to obstruct it 
 so much that silt would deposit, the channel become 
 choked up, and the water find a course down another 
 
80 RIVER AND CANAL ENGINEERING 
 
 channel. The work was begun in March 1902, and was 
 in progress in May of the same year when an unusually 
 early flood put a stop to it. The dykes had at this 
 time advanced considerable distances from the right 
 bank of the stream, but none had been completed. 
 Two dykes out of the three were for the most part 
 carried away. The river, however, took a new course, 
 starting from a point far upstream, the western channel 
 became a creek, and the remains of the dykes were 
 soon embedded in silt. 
 
 In any case in which the provision of a proper 
 mattress has been omitted, or when the mattress has 
 been destroyed, or when a breach has occurred in 
 an embankment, whenever, in short, it is evident 
 that the gap cannot be closed until some other 
 escape for the water is provided, it may be possible 
 to provide such an escape by cutting partly through 
 the dam or embankment on the downstream side 
 at another place, and thoroughly protecting the 
 place and extending the protection downstream and 
 away from the dam or embankment. The water 
 can then be let in, and the closure of the old gap 
 attempted. If a closure is effected, the protected gap 
 can then be closed. Sometimes it may be desirable 
 to make such a protected gap beforehand and with 
 deliberation. 
 
 Dams for closing streams which are dry can be made 
 similarly to flood embankments (CHAP. XII., Art. 7). 
 Sand does very well, provided it is protected by a 
 covering of clay or by fascining. 
 
 3. Instances of Closures of Streams. In 1904 
 the Colorado River broke into the Salton Sink a valley 
 covering 4000 square miles. Unsuccessful attempts 
 
DIVERSIONS AND CLOSURES OF STREAMS 81 
 
 were made to close the stream by two rows of piles 
 with willows and sandbags between them, by a gate 
 200 feet long, supported on 500 piles, and by twelve 
 gates each 12 feet wide. A " rock-fill" dam was then 
 constructed on a mattress 100 feet wide and 1*5 feet 
 thick. The river, which was 600 feet wide, broke 
 through, but was stopped by the construction of three 
 
 FIG. 25. 
 
 parallel rock-fill dams in the gap (Min. Proc. Inst. 
 C.E., vol. clxxi.). 
 
 At the site of the railway bridge over the river Tista in 
 Bengal, it was necessary to close the main stream 
 (fig. 25), which flowed at the left side of the channel, 
 while the bridge had been built at the right. The bed 
 was of sand, width 500 feet, depth 6 feet, and discharge 
 3700 cubic feet per second. The first attempt to close 
 
 the stream was made at M N, a floor of stone 200 feet 
 
 G 
 
82 RIVER AND CANAL ENGINEERING 
 
 long, 20 feet wide, and 2 feet thick, being laid in the 
 middle of the stream, and dams of earth, sandbags, and 
 stones being run out from each bank. As the gap 
 decreased in width the bed was torn up and the work 
 failed. The heading up was 3 feet 9 inches. It was 
 recognised later that the site should have been at the 
 bifurcation higher up, and that the stone floor should 
 have been laid on a mattress. 
 
 In the next working season the dams C D and E F G 
 were made. The dam C D was of earth. Two walls, 
 each consisting of a double Hne of bamboos with the 
 spaces between the lines filled with bundles of grass 
 weighted with earth, were run out 50 feet in advance 
 of the earthwork near the lines of the toes of the 
 slopes. Along the line of the upper wall a mattress of 
 broken bricks 10 feet in width, and 1 foot thick, was 
 laid, and was kept 50 feet in advance of the wall. A 
 total length of 1000 feet of embankment was made in 
 five months and pitched on its upstream side. The 
 end was strongly protected by a mass of stone. The 
 embankment F G was of earth. The dam E F consisted 
 of three lines of piles driven 10 feet into the bed. A 
 mattress weighted with stones extended for 20 feet 
 upstream of the dam and 40 feet downstream. A gap 
 of 150 feet was left at D E, and was not protected by a 
 floor of any kind. A channel, parallel to F G and 
 extending to K, had been dug to a width of 200 feet. 
 During the floods the heading up at D E was about 2 '5 
 feet, and the water was 30 feet deep. The line E F was 
 greatly damaged and was repaired. The cut F G K 
 gradually enlarged, and by the end of the floods more 
 water was going down it than down the main stream. 
 The gap D E was finally closed by means of a line of 
 
DIVERSIONS AND CLOSURES OF STREAMS 83 
 
 bamboos and grass, the bed being protected by a carpet, 
 100 x 50 feet, made of common cloth weighted with sand- 
 bags. The success of the operations turned on the 
 scouring out of the cut F G K. It is remarkable that 
 the gap D E did not become wholly unmanageable in 
 the floods (Min. Proc. Inst. C.E., vol. cl.). 
 
 G 2 
 
CHAPTEK VIII 
 
 THE TRAINING AND CANALISATION OF RIVERS 
 
 1 . Preliminary Remarks. When a stream is trained 
 or regularised it is generally made narrower, but some- 
 times narrow places have to be widened. Deepening 
 has also very frequently to be effected. The object of 
 training is generally the improvement of navigation, but 
 it may be the prevention of silt deposit. Some natural 
 arms of rivers which form the head reaches of canals in 
 the Punjab are wide and tortuous, and they are some- 
 times trained. Training often includes straightening or 
 the cutting-oif of bends, as to which reference may be 
 made to CHAP. VII. 
 
 2. Dredging and Excavating. When a flowing 
 stream is to be deepened, the work is usually done by 
 dredgers. Dredgers can remove mud, sand, clay, 
 boulders, or broken pieces of rock. The "bucket ladder" 
 dredger is the commonest type. The " dipper" dredger 
 is another. Both these can work in depths of water 
 ranging up to 35 feet. The "grab bucket" dredger 
 can work up to any depth and in a confined space. 
 The " suction dredger " draws up mud or sand mixed 
 with water. A dredger may be fitted with a hopper or 
 movable bottom, by means of which it can discharge the 
 dredged material this, however, involves cessation of 
 
 84 
 
THE TRAINING AND CANALISATION OF RIVERS 85 
 
 work while the dredger makes a journey to the place 
 where the material is to be deposited or it can dis- 
 charge into hopper barges or directly on to the shore 
 by means of long shoots. For small works in compara- 
 tively shallow water the " bag and spoon " dredger, 
 worked by two men, can be used. 
 
 When rock has to be removed under water it is 
 blasted or broken up by the blows of heavy rams pro- 
 vided with steel -pointed cutters. 
 
 In widening a channel the excavation can be carried 
 down in the ordinary way to below the water-level, a 
 narrow piece of earth, like a wall, being left to keep the 
 water out. If the channel cannot be laid dry, the work 
 can be finished by dredging. 
 
 Regarding methods by which the stream is itself 
 made to deepen or widen its channel, reference may be 
 made to CHAP. V. 
 
 3. Reduction of Width. If a channel which is to 
 be narrowed is not a wide one, the reduction in width 
 can be effected by any of the processes described under 
 bank protection (CHAP. VI.). But in a wide channel, 
 reduction of the width by any direct process is generally 
 impracticable. The expense would generally be pro- 
 hibitive. Earth, if filled in, is liable to be washed away 
 unless protected all a]ong. Reduction in the width of 
 a large channel is nearly always effected either by 
 groynes (fig. 26) or by training walls (fig. 27). Spurs 
 or short groynes for bank protection have been already 
 described (CHAP. VI. , Art. 2). Groynes for narrowing 
 streams are made in the same way and of the same 
 materials, but are longer. They are at right angles to 
 the stream or nearly so. Groynes in the river Sutlej 
 have been mentioned in CHAP. V., Art. 5, and are shown 
 
86 
 
 RIVER AND CANAL ENGINEERING 
 
 in fig. 6, p. 55. Whether groynes or training walls are 
 used, the object is to confine the stream to a definite 
 zone and to silt up the spaces at the sides. These 
 spaces when partly silted can be planted with osiers or 
 
 FIG. 26. 
 
 with anything which will grow when partly submerged, 
 and this will assist in completing the silting. 
 
 A training wall can be made of any of the materials 
 used for groynes. In order to silt up the spaces between 
 
 FIG. 27. 
 
 each wall and the adjacent bank of the stream, other 
 walls are run at intervals across them. Usually the 
 training walls and cross walls are carried up only to 
 ordinary water-level, sometimes only to low-water level. 
 Floods can thus spread out and submerge the walls and 
 deposit silt. If the walls are carried up too high it may 
 
THE TRAINING AND CANALISATION OF RIVERS 87 
 
 be necessary, in order to give room for floods, to space 
 them too far apart, and this, as will be seen below, is 
 objectionable. 
 
 The difference between training walls and groynes is 
 one of degree rather than one of kind. The material 
 most commonly used is, in either case, loose stone with 
 pitching, if desired, above low- water level, but it may 
 be wattled stakes. If the water of the stream contains 
 silt at all stages of the supply, gaps can be left in 
 training walls so that silt deposit may occur at all 
 times and not only in floods. If the walls are of 
 wattled stakes, water will pass through them, and it 
 may not be necessary to leave any gaps. Groynes are 
 frequently made with T-heads (fig. 26), and they are 
 thus equivalent to training walls with long gaps in 
 them. The edge of the narrowed channel usually forms 
 somewhat as shown in the figure. If the groynes are 
 placed so near together as to give a regular channel, the 
 cost is not likely to be much less than that of training 
 walls. 
 
 The alignment of training walls or groynes should be 
 such as will give the best channel consistent with 
 economy in cost. The best channel is generally that 
 which is most free from sharp bends. It is assumed for 
 the present that no cuts or diversions of such lengths 
 as to materially alter the gradient are to be made, but 
 that a certain amount of choice of alignment is afforded' 
 by the reduced width of the trained channel and by 
 small diversions or easings of bends. It is sometimes 
 said that straight reaches are objectionable because the 
 stream will tend to wander from side to side and cause 
 shoals, whereas in a bend there will be no such 
 tendency. The difficulty as to shoaling will be greatest 
 
88 RIVER AND CANAL ENGINEERING 
 
 at low water, but it is likely to be serious only when the 
 width between the training walls is too great. If the 
 width cannot be reduced to such an extent as to do 
 away with the trouble, it may be better to adopt a 
 curved course. The width between the training walls 
 should generally be the same throughout, whether the 
 reaches are straight or curved, but in view of the 
 preceding remarks it may be desirable, where a reach 
 cannot be otherwise than straight and where shoaling is 
 feared, to give the straight portion a reduced width 
 with of course a greater depth, and similarly to reduce 
 the width at reverse changes of curvature. In curves 
 which are at all sharp the curvature should be rather 
 sharper in the middle of the curve than at the ends 
 (CHAP. IV., Art. 8). 
 
 4. Alteration of Depth or Water-Level. When 
 the width of a stream is altered, the depth of water 
 the gradient being supposed to be unchanged must 
 alter in the opposite manner. A narrowing of the 
 channel by training necessitates an increase in the 
 depth of water, and the same remark applies if an arm 
 of the stream is closed. The increase in depth may be 
 effected either by raising the water-level or by lowering 
 the bed as may be convenient or both. If the bed 
 is to be lowered and is of hard clay, it may be necessary 
 to dredge it and, when this has been done, training may 
 be unnecessary. If the bed is of soft mud, a dredged 
 channel is likely to fill up again, and training alone will 
 be the method to adopt. If the bed is moderately hard, 
 say compact sand, it may be suitable to train the 
 channel first and then to dredge if necessary. In any 
 case, shoals of hard material may have to be dredged 
 or rocks, whether these form shoals or lateral obstruc- 
 
THE TRAINING AND CANALISATION OF RIVERS 89 
 
 tions, to be blasted or otherwise broken up (Art. 
 2). In cases where it is desired to raise the water- 
 level without any lowering of the bed, training is of 
 course necessary. In any case in which the bed is 
 likely to scour to a lower level than is desired, or if the 
 bed is to be raised, the measures described in CHAP. V. , 
 Art. 6, may be adopted, but they are hardly likely to 
 be suitable and satisfactory in all cases. 
 
 5. Training and Canalising. The steps so far 
 described, together with any of those described in 
 CHAPS. V. and VI. , exhaust the list of what can be 
 done so long as only the cross-section of a stream is 
 
 dealt with. This is often called the " regulation " of a 
 stream, though " training" is a more satisfactory term. 1 
 
 A mere alteration of the cross-section of a stream will 
 not always afford a solution of the problem to be solved. 
 Frequently a change of gradient is required. The 
 gradient can be steepened by means of straightenings, 
 or flattened by introducing weirs, or perhaps by adopt- 
 ing a course somewhat more circuitous than was 
 intended. This extended scope of operations is known 
 as canalising in the case of a river, and remodelling in 
 the case of a canal. 
 
 Suppose that it is desired to alter the cross-section of 
 a stream, at ordinary water-level, so as to reduce the 
 width and increase the depth (fig. 28). If the mean 
 depth is doubled, the new width will be about equal to 
 
 1 On Indian canals the term " regulation " is applied to the control of the 
 discharge at the regulators or off-take works. 
 
90 RIVER AND CANAL ENGINEERING 
 
 ^2- of the old width (Hydraulics, CHAP. VI, Art. 2). 
 If this gives too narrow a channel, it may be desirable 
 to flatten the gradient. If it gives too wide a channel, 
 the gradient can be steepened or a greater depth 
 adopted. While the width and depth of the stream will 
 be fixed so as to be suitable for the navigation, the ratio 
 of depth to velocity should be so arranged, if this is 
 possible, as to minimise trouble connected with silting 
 or scour (CHAP. IV., Art. 6}. A remodelled channel is, 
 in short, designed in exactly the same way as a new 
 channel. The depth of water exercises the greatest 
 effect on the discharge, and the gradient the least. The 
 weak point in a scheme which includes weirs is the 
 difficulty of dealing with floods. A scheme perfect in 
 all other respects may be vitiated because of the 
 obstruction, caused by weirs, to the passage of floods. 
 The difficulty is got over by means of movable weirs. 
 The whole subject of weirs is dealt with in CHAP. X. 
 
 Training or canalising should not be effected in any 
 reach of a stream without regard to other reaches. A 
 mere local lowering of the water-level by dredging may 
 accentuate the effect of a shoal at the upper end of the 
 reach. 
 
 When the water-level is raised by a weir or by 
 narrowing the channel though in the latter case the 
 raising may not be permanent it is generally best to 
 commence the work from the upstream end. The 
 raising of the water-level will then not interfere with 
 the execution of the rest of the work. But in a case of 
 widening, where the water-level upstream of the work 
 is lowered, the work can conveniently be begun at the 
 downstream end, and the remark applies also to a case of 
 straightening, provided that the new channel is not so 
 
THE TRAINING AND CANALISATION OF RIVERS 91 
 
 small that it at first causes no lowering. In any case in 
 which there is a doubt whether the whole of the scheme 
 will be carried out, the reach to be dealt with first can 
 be decided on according to circumstances. There is no 
 general reason for selecting an upstream or downstream 
 reach, except that any raising or lowering of the 
 water-level will extend upstream of the reach and not 
 downstream of it (CHAP. L, Art. 4)- 
 
 Training walls and groynes, if made with stakes or 
 fascines or any materials except stone, require careful 
 watching and maintenance. 
 
CHAPTER IX 
 
 CANALS AND CONDUITS 
 
 1. Banks. All banks which have to hold up water 
 should be carefully made. The earth should be de- 
 posited in layers and all clods broken up. In high 
 banks the layers should be moistened and rammed. 
 The dotted lines in fig. 29 show two possible courses 
 of percolation water. The vertical height from the 
 
 FIG. 29. 
 
 water-level to the ground outside the bank, divided 
 by the length of the line of percolation is the hydraulic 
 gradient, as in the case of a pipe, and this gradient is 
 more or less a measure of the tendency to leakage. 
 A bank which has water constantly against it nearly 
 always becomes almost water-tight in time. The time 
 is less or greater according as the soil is better, and 
 according to the amount of care with which the bank 
 is made. 
 
 The side slopes of banks vary with the soil. Generally 
 they are 1|- to 1, but they are sometimes 2 to 1 or even 
 
 92 
 
CANALS AND CONDUITS 93 
 
 3 to 1 if the soil is bad or sandy, or if great precautions 
 against breaches are to be taken. 
 
 Leakage can sometimes be stopped by throwing chaff 
 or other finely divided substances into the water at the 
 site of the leak. In other cases it is necessary to dig 
 up part of the bank, find the channel by which the 
 water is escaping, and fill it up by adding earth and 
 ramming. On some navigable canals in France it was 
 at one time the custom to lay the reach dry, when a 
 bad leak occurred, and to dig away the bank and lay 
 slabs of concrete or puddle over the place. This plan 
 was abandoned, and instead of it sheet piles are driven 
 in. They are then withdrawn one at a time and, if any 
 leakage occurs, the space is filled with concrete. 
 
 The dimensions of a bank should depend to some 
 extent on the head of water against it and on the 
 volume of the stream whose water it holds up. A 
 breach is obviously more serious the greater the volume 
 of the escaping water. This volume depends on the size of 
 the stream and on its velocity. In navigation canals in 
 England the bank on the side opposite the towing-path 
 is usually 4 to 6 feet wide and 1^ feet above the water. 
 In irrigation canals in India the bank of a very large 
 canal is 2 feet above the water and 20 feet wide, while 
 that of a small canal with 6 feet of water is 8 or 10 feet 
 wide and 1^ feet above the water, and that of a small 
 distributary channel with 3 feet of water is 4 feet wide 
 and 1 foot above the water. The soil is often poor. 
 
 Further remarks, which apply to banks of special 
 height or special importance, are given under Embank- 
 ments (CHAP. XII. , Art. 6). 
 
 2. Navigation Canals. A navigation canal is 
 sometimes all on one level, but generally different 
 
94 RIVER AND CANAL ENGINEERING 
 
 reaches are at different levels, the change being made 
 by means of locks. A " lateral " canal the most 
 common kind runs along a river valley more or less 
 parallel to the river. It is frequently cheaper to 
 construct such a canal than to canalise the river. A 
 " summit " canal crosses over a ridge and connects 
 two valleys. A navigation canal requires a supply of 
 water to make good the losses which occur by lockage, 
 leakage, or absorption and evaporation. A canal may 
 be of any size, according to the size of the boats which 
 are to be used. There is always room, except in short 
 reaches where the expense of construction has to be 
 kept down, for two boats to pass one another. 
 
 A lateral canal obtains water from the river or from 
 the small affluents which it crosses. For a summit 
 canal it may be necessary to provide storage reservoirs. 
 The canal crosses the ridge where it is low, and the 
 reservoirs are made on higher ground. Keservoirs may 
 be required also for other canals to hold water for use 
 in dry seasons or in order to fill the canal quickly when 
 laid dry for repairs. 
 
 In tropical countries weeds grow profusely in canals 
 which have still or nearly still water. Traffic tends 
 to keep them down, but they have to be cleared 
 periodically. 
 
 In designing a barge canal the chief considerations 
 generally are that it shall not be in such low ground or 
 so near a river as to be liable to damage by floods, that 
 it shall not traverse very permeable soil or gravel this 
 is often found near a river, that the material excavated 
 shall be as nearly as possible equal to that required, at 
 the same place, for embankment, and that as far as 
 possible high embankments, which are very expensive 
 
CANALS AND CONDUITS 95 
 
 to construct and are more or less a source of danger, 
 shall be avoided. The side slopes of the banks of a 
 navigation canal depend on the nature of the soil. 
 They are generally l^ to 1, but the inner slope may be 
 2 to 1. The banks are generally 1^ or 2 feet above the 
 water-level, the width of the bank on the towing-path 
 side ranging from 8 to 16 feet, but being generally 12 
 feet and the width of the other bank 4 to 6 feet. The 
 width of a canal is made sufficient for two boats to 
 pass, and the depth is l|- to 2 feet greater than the 
 draught of the boats used. In some cases the banks 
 are protected by pitching for short lengths, but generally 
 they are merely turfed. The sides near the water 
 surface wear away, so that the side slope becomes 
 steeper in the upper part and flatter in the lower part. 
 The resistance of a boat to traction in a canal is given 
 by the formula 
 
 p . 8 ' 46 
 
 
 where r is the resistance in a large body of water and A 
 and a are the areas of the cross-sections of the canal 
 and of the immersed part of the boat. When A is six 
 times a, R, is only 6 per cent, more than r. In practice 
 A is never less than six times a. 
 
 Kegarding methods of protecting banks, see CHAP. VI. 
 
 A ship canal is a barge canal on a large scale. The 
 speed of ships has to be strictly limited to avoid damage 
 to the banks. 
 
 The Manchester Ship Canal takes in the waters of the 
 Irwell and the Mersey, and conveys them for several 
 miles. It is thus a canalised river for part of its course. 
 Below that it is a tidal stream, the tide being admitted 
 
96 RIVER AND CANAL ENGINEERING 
 
 at its lower end where it joins the estuary of the Mersey, 
 and passing out higher up where it leaves the estuary 
 after skirting it. This circulation of water is beneficial 
 to the estuary. 
 
 The Panama Canal might have been constructed at 
 one level, but the cost of this, and the time occupied, 
 would have been double that of making it a summit 
 canal. The water of the river Chagres is to be 
 impounded to form a lake of great extent that will not 
 only supply water for lockage but will itself form part 
 of the high-level reach of the canal, and ships will be 
 able to traverse it at greater speed than in the rest of 
 the canal. 
 
 Some Indian irrigation canals have been constructed 
 so as to be navigable. The increase in cost has 
 usually been enormously in excess of any resulting 
 benefits. 
 
 3. Locks. An ordinary lock is shown in fig. 29A. 
 The space above the head gates is called the " head bay," 
 and that below the tail gates the " tail bay." The 
 floor of the lock is often an inverted arch. Sometimes 
 the floor is of cast-iron. The "lift wall" is generally 
 a horizontal arch. The gates when closed press at 
 their lower ends against the "mitre sills"; and the 
 vertical "mitre posts" at the edges of the gates meet 
 and are pressed together. The gate, in opening and 
 closing, revolves above the cylindrical "heel post" 
 which stands in the " hollow quoin" of the lock wall 
 and when fully open is contained in the " gate recess." 
 
 A lock is always strongly built, of masonry or 
 concrete. The walls have to withstand the earth 
 pressure when the lock is laid dry for repairs. The 
 floor has to withstand the scouring action from the 
 
CANALS AND CONDUITS 
 
 97 
 
 sluices. Regarding the upward pressure of the water 
 when the lock is empty, see CHAP. X., Art. 3. The lift 
 or difference in the water-levels of the two reaches of a 
 barge canal is generally from 4 to 9 feet, but occasion- 
 ally it is much more. 
 
 The gates of small locks are generally of wood and 
 
 FIG. 29A. 
 
 are counterbalanced. Those of large locks are of wood 
 or steel, and the weight is generally taken by rollers. 
 Ordinary wood should not be used if the Teredo navalis 
 exists in the waters. An iron gate, if enclosed on all 
 sides by plating, is buoyant, and the rollers and anchor 
 straps which hold the upper ends of the heel posts 
 are thus relieved of much weight. The gates of the 
 Panama Canal locks are 110 feet long and 7 feet thick, 
 and the height ranges from 48 feet to 82 feet. 
 
98 
 
 RIYER AND CANAL ENGINEERING 
 
 The sluices for filling and emptying a lock are placed 
 in the gates or in the walls. The gates and sluices 
 are generally worked by hydraulic power or by 
 electricity. 
 
 Locks are frequently arranged in flights. There are, 
 in a few instances, 20 to 30 locks in a flight, the total 
 lift being 150 to 200 feet. By this means the number 
 of gates is reduced, the tail gates of one lock being the 
 head gates of the rest, and there is a saving in labour 
 in working the locks. 
 
 Let L be the volume of water contained in a lock 
 between the levels of the upper and lower reaches, and 
 let B be the submerged volume of a boat. The 
 " lockage " or volume of water withdrawn from the 
 upper reach of the canal is shown in the following 
 statement : 
 
 
 
 
 
 
 Lockage. 
 
 Reference 
 Number 
 of Case. 
 
 Number 
 of Boats. 
 
 Direction 
 of Travel. 
 
 Lock or 
 Locks 
 Found 
 
 Lock or 
 Locks 
 Left 
 
 
 Single Flight of ra 
 
 
 
 
 
 
 Lock. 
 
 Locks. 
 
 1 
 
 1 
 
 Down. 
 
 Empty. 
 
 Empty. 
 
 L-B 
 
 L-B 
 
 2 
 
 1 
 
 ( 
 
 Full. 
 
 55 
 
 -B 
 
 -B 
 
 3 
 
 4 
 
 1 
 1 
 
 Up. 
 
 Emptv. 
 Full." 
 
 Full. 
 
 55 
 
 L + B 
 L + B 
 
 mL + B 
 L + B 
 
 5 
 
 2n 
 
 Up and 
 down 
 
 Going 
 down, full. 
 
 Going 
 down, 
 
 nL 
 
 mnL 
 
 
 
 alternately. 
 
 Going up, 
 
 empty. 
 
 
 
 
 
 
 empty. 
 
 Going up, 
 
 
 
 
 
 
 
 full. 
 
 
 
 6 
 
 n 
 
 Down. 
 
 Empty. 
 
 Empty. 
 
 nL-nE 
 
 nL - nE 
 
 7 
 
 n 
 
 
 
 Full. 
 
 5) 
 
 (n-l)L 
 
 (n-l)L 
 
 
 
 
 
 
 -nE 
 
 -nB 
 
 8 
 
 n 
 
 Up. 
 
 Empty. 
 
 Full. 
 
 nL + nE 
 
 (m + n - 1 )L 
 
 9 
 
 n 
 
 
 Full. 
 
 
 nL + nE 
 
 nL + nE 
 
 10 
 
 \ n 
 
 Down. 1 
 Up. / 
 
 5) 
 
 55 
 
 (2n-l)L 
 
 (m + 2n-2)L 
 
 
 
 
 
 
 i 
 
CANALS AND CONDUITS 99 
 
 In the case of a single lock, if two boats are to pass 
 through, one descending and one ascending (cases 2 and 
 3), the descending boat would be passed through first if 
 the lock were full, and the ascending boat first if empty ; 
 
 in either case, the total lockage is L, or for each boat. 
 
 This also appears from case 5. Cases 6 to 10 show that 
 if a long train of boats descends, even though the lock 
 is full for the first boat or if a long train ascends even 
 the lock is empty for the first boat, the total lockage 
 is nearly L per boat. Thus in a single lock, boats 
 should pass up and down alternately so far as this may 
 be possible. 
 
 In the case of a flight of m locks, a single boat in 
 descending uses no more water than if there were only 
 one lock, the same water passing from lock to lock, but 
 in ascending it uses more. In the case of a number 
 (2n) of boats going up and down alternately (case 5), 
 the lockage is m n L, the lockage per lock per boat being 
 
 , but in the case of a long train of boats descending 
 
 followed by an equal train ascending (cases 7 and 8), 
 the lockage is less. If n is supposed to be equal to m, 
 the average lockage per boat is as follows : 
 
 m = I 
 Lockage _ L 
 per boat , ~2 
 
 2 
 L 
 
 3 
 7L 
 6 
 
 4 
 5L 
 
 5 
 13L 
 
 6 
 4L 
 
 Infinity 
 3L " 
 
 2 
 
 4 
 
 10 
 
 3 
 
 Thus in a case where n and m are very large, the 
 average lockage per boat, when the boats pass up and 
 down in trains, is to the lockage per boat, when the 
 single boats pass up and down alternately through m 
 single locks all at different places, as 3 is to m. The 
 
 H2 
 
100 RIVER AND CANAL ENGINEERING 
 
 reason for the difference, which may appear puzzling, is 
 that when the locks are at different places they are 
 worked independently of one another. 
 
 Sometimes a lock is provided with intermediate gates 
 which provide . a short lock for short vessels. In the 
 Manchester Ship Canal, alongside each lock there is 
 another of smaller size to be used for small vessels and 
 thus save lockage. At the Eastham lock, where the 
 Manchester Ship Canal descends into the estuary of the 
 Mersey, there is, below the tail gates, an extra pair of 
 gates opening towards the estuary, so that the lock can 
 be worked when the water of the estuary is higher than 
 that in the canal. Water can be economised by means 
 of a " side-pond," into which the upper portion of the 
 water from a lock can be discharged and utilised again 
 when the lock has to be filled. If two locks are built 
 side by side, each acts as a side-pond to the other. Two 
 flights of locks can be built side by side. 
 
 Sometimes instead of a lock there is an inclined plane, 
 up or down which are drawn on rails caissons containing 
 water in which the boats float. The rails extend below 
 the water-levels of the two reaches, and the caissons can 
 thus be run under the boats. " Lifts " have also been 
 constructed by which the boats can be lifted bodily and 
 swung over from one reach to the other. 
 
 4. Other Artificial Channels. The method of 
 calculating the discharges of channels in which water is 
 to flow is a question of hydraulics. The principles and 
 rules to be followed, in the design of earthen channels, 
 have been stated in CHAP. IV., Art. 6, and in CHAP. 
 VIII., Art. 5. The design of banks has been dealt with 
 in Art. 1 of this Chapter. For conveying water for 
 the supply of towns, or for other purposes, masonry con- 
 
CANALS AND CONDUITS 
 
 101 
 
 duits are often used. A usual form is shown in fig. 30. 
 The curving of the profile of the cross-section gives an 
 
 FIG. 30. 
 
 increased sectional area and hydraulic radius, and hence 
 an increased discharge. 
 
CHAPTER X 
 
 WEIRS AND SLUICES 
 
 1. Preliminary Remarks. -- Every structure which 
 interferes at all with a stream causes an abrupt change 
 in the stream (CHAP. IV., Art. 1). At an abrupt change 
 there are always eddies, and these have a peculiar scour- 
 ing effect. This effect is greatest where the velocity of 
 the stream is abruptly reduced as where, for instance, 
 after being contracted by an obstruction, it expands 
 again or where it falls over a weir or issues from a sluice 
 opening. In all cases of this kind the protection of the 
 structure from scour is of primary importance. 
 
 The site of a weir or other permanent structure should, 
 if the stream is unstable, be in a fairly straight reach, 
 or at least not be immediately downstream of a bend. 
 This is because of the tendency of bends to shift down- 
 stream (CHAP. IV., Art. 8). There is no particular 
 advantage in selecting a narrow place. A narrow 
 place is likely to be deep or it may be liable to 
 widen. In a hard and stable stream there is no restric- 
 tion as to site. 
 
 Weirs are frequently constructed for purposes of 
 navigation, as mentioned in CHAP. VIII. They are also 
 used in streams which are not navigable in order that 
 
 the gradient may not be too steep, and in irrigation 
 
 102 
 
WEIRS AND SLUICES 103 
 
 canals for the same reason. They are used both in 
 rivers and canals in order that the water-level may be 
 raised and water drawn off by branch channels for 
 purposes of manufactures, water-power or irrigation. 
 
 Upstream of a weir there is more or less tendency for 
 silt to deposit, but it by no means follows that there will 
 be deposit (CHAP. IV., Art. 2, last par., and Art. 3, 
 last par.). When deposit of sand or mud is feared, small 
 horizontal passages, known as " weep holes," may be left 
 in the weir at the level of the upstream bed. In the 
 old Nile barrages iron gratings were provided, but they 
 were needlessly large. 
 
 An inherent defect of an ordinary weir is that 
 it obstructs the passage of 
 floods. The obstruction may 
 or may not be of consequence. 
 Sometimes it is of great con- 
 sequence. Attempts have 
 been made to partially 
 remedy the evil by placing FlG - 31< 
 
 the weir obliquely to the stream, thus giving it a 
 greater length. At ordinary water-levels the flow 
 over the crest of the weir is normal to its length, 
 or nearly so. Supposing that the water has to be 
 held up to a given level, the crest of the weir must 
 be higher, because of its greater length, than if it 
 were normal to the stream. In a flood the water has 
 a high velocity and flows over the weir in a direction 
 nearly parallel to the axis of the stream, so that the 
 effective length of the weir is not much greater than if 
 it were normal to the stream, and, its crest being higher, 
 it obstructs the flood as much. Oblique weirs are 
 usually made as in fig. 31. If made in one straight 
 
104 RIVER AND CANAL ENGINEERING 
 
 line, there might be excessive action on the bank at the 
 lower end. 
 
 If the weir is lengthened, not by being built obliquely 
 but by a widening of the stream at the site, the crest 
 has to be raised and nothing is gained. 
 
 The only arrangement by which a weir can be made 
 to hold up water when a stream is low and to let floods 
 pass freely, consists in having part of the weir movable, 
 i.e. consisting of gates, shutters or horizontal or vertical 
 timbers, which can be withdrawn to let floods pass, and 
 can be manipulated to any extent so as to regulate the 
 amount of water passing A familiar instance of a 
 movable weir is the one which is usually placed across 
 a mill stream, the wooden gates working in grooves in 
 the masonry. 
 
 Above a weir in Java, 162 feet long, there was a 
 great accumulation of shingle in the bed of the river, 
 and the head of a canal taking off above the weir 
 became choked. The crest of the weir on the side away 
 from the canal was raised 5j feet and the crest sloped 
 gradually down, a length of 43 feet on the side next 
 the canal remaining as it was. This was quite success- 
 ful. It was practically a contraction of the river near 
 the canal off-take, and this must have caused scour, so 
 that the bed became lower than the floor of the canal 
 head and the shingle was not carried in. The shingle, 
 however, is said to have been carried over the weir 
 (Min. Proc. Inst. C.E., vol. clxv.). 
 
 A lock is an adjunct to a weir, used when navigation 
 has to be provided for. The lock may be placed close 
 to the weir or it may be in a side channel, the upstream 
 end of the lock being about in a line with the weir. 
 Locks have already been discussed in CHAP. IX., Art. 3. 
 
WEIRS AND SLUICES 
 
 105 
 
 Frequently a "salmon ladder" has to be provided. 
 It consists of a series of steps or a zigzag arrangement 
 so that the velocity of the water is not too great for 
 the fish to ascend. 
 
 2. General Design of a Weir. Unless the bed 
 and sides of the channel are of rock, a weir has side 
 walls and rests on a strong floor or "apron." These 
 need not extend far upstream, but must extend some 
 way downstream because of the scouring action of the 
 water. 1 A common type of weir is shown in fig. 32. 
 The downstream face is made sloping, so that the water 
 
 FIG. 32. 
 
 may not fall vertically and strike the floor below the 
 weir. The thickness and length of the floor depend on 
 the volume of water to be passed and on the height 
 which it will fall and on the nature of the soil, and are 
 generally matters of judgment, though rules regarding 
 them, applicable to certain special cases, are given in 
 the next article. 
 
 The upper corners of the weir should be rounded. This 
 prevents their being worn away ; but the rounding of 
 the upstream corner has another advantage. If the 
 corner is sharp, the stream springs clear from it and the 
 weir holds up the water higher, especially in floods. 
 With small depths of water the difference is less, and it 
 vanishes when there is only a trickle of water. Thus a 
 
 1 See also Appendix B. 
 
106 RIVER AND CANAL ENGINEERING 
 
 crest rounded on the upstream side holds up low-water 
 nearly as well as a sharp-edged crest, but lets floods 
 pass more freely. Any batter given to the upstream 
 face has a similar advantage. The rounding is of more 
 importance as the batter is less. For similar reasons, 
 the upstream wing walls should be splayed or even 
 curved so as to be tangential to the side wall, and not 
 built normally to the stream. These advantages are 
 sometimes lost sight of. The downstream walls are 
 splayed to reduce the swirl. 
 
 The body of the weir may be of rubble and the face- 
 work of dressed stone. In large weirs the stones are 
 sometimes do welled together. Where, as in many parts 
 of India, stone is expensive, brick is used for small 
 weirs, the crest and faces being brick on edge. 
 
 Downstream of the floor, unless the channel is of 
 very hard material, there is paving or pitching of the 
 bed and pitching of the sides, and these may terminate 
 in a curtain wall. The bank pitching may be of any of 
 the kinds described in CHAP. VI., Art. 3, and the bed 
 paving as described in CHAP. V., Art. 6, but downstream 
 of a weir the eddying is continuous and the lap of the 
 water on the bank is ceaseless, and good methods are 
 necessary. Sometimes planking, laid over a wooden 
 framing or attached to piles, is used instead of paving 
 and pitching. 
 
 In case the height of a weir is great relatively to its 
 thickness, the danger of its being overturned must be 
 considered. To be safe against overturning, the result- 
 ant of the pressure on the weir must pass through the 
 middle third of its base (see fig. 62, CHAP. XIII. ). 
 
 3. Weirs on Sandy or Porous Soil. If the channel 
 is very soft or sandy the weir may be built on one or 
 
WEIRS AND SLUICES 
 
 107 
 
 more lines of wells. The wells are not so much to 
 support the weir as to form a curtain and prevent 
 streams, due to the hydraulic gradient A E (fig. 
 33), from forming under the structure and gradually 
 removing the soil. It is assumed in the case repre- 
 sented by the figure that the maximum head occurs 
 when the downstream channel is dry. Any removal of 
 soil from under the weir may cause its destruction. 
 The wells should be as close together as possible, and 
 the spaces between them carefully filled up with brick- 
 work or concrete to as great a depth as possible, and 
 
 FIG. 33. 
 
 below that by piles. Instead of wells, lines of sheet 
 piling cast-iron or wood can be used. A good fit 
 should be made, but it is not necessary that the joints 
 should be absolutely water-tight. The object is to 
 flatten the hydraulic gradient by increasing the length 
 travelled by the water from B E to B L G H E. Of 
 course, no flattening occurs at a point where the curtain 
 is not water-tight, but if only small interstices exist, 
 none but small trickles of water can pass, and the 
 interstices will probably soon be choked up, just as the 
 sand in a filter bed becomes clogged and has to be 
 washed. In any case, no important stream could 
 develop otherwise than round the toe of the curtain. 
 It has been stated that when a curtain is water-tight 
 
108 RIVER AND CANAL ENGINEERING 
 
 the water follows the line BLMGHKE, but this 
 requires proof. Another plan is to cover the bed and 
 sides of the channel with a continuous sheet of concrete 
 extending upstream of the weir from B to D thus 
 flattening the hydraulic gradient from A E to F E. 
 Instead of concrete, clay puddle can be used with 
 pitching over it. The choice between the different 
 methods depends largely on questions of cost and 
 facility of construction. It has been said that a certain 
 amount of leakage occurs under structures such as the 
 Okla weir (Art. 4), which nevertheless remains un- 
 damaged. There have, however, been cases in which 
 failures of works have occurred, especially when there 
 has been a great difference between the water-levels of 
 the upstream and downstream reaches, from no other 
 apparent cause than the passage of water underneath 
 the works. 
 
 Weirs in porous soils have been discussed by Bligh 
 (Engineering News, 29th December 1910), who gives 
 the following as safe hydraulic gradients (5) or ratio of 
 the greatest head A B to the length BE: 
 
 Fine silt and sand as in the Nile . 1 in 18 
 Fine micaceous sand as in Colorado 
 
 and Himalayan rivers . . 1 in 15 
 
 Ordinary coarse sand . . . L in 12 
 
 Gravel and sand . . . 1 in 9 
 
 Boulders, gravel and sand . . 1 in 4 to 1 in 6 
 
 These figures are probably quite safe enough even for 
 the most important works and for those where the 
 heading up is constant. For small works or for 
 regulators (Art. 5} where the heading up is not constant, 
 steeper gradients are permissible. Much also depends 
 
WEIRS AND SLUICES 
 
 109 
 
 on the condition of the water. If it contains much silt, 
 all interstices will probably become choked up. The 
 hydraulic gradient in the case of the Narora weir 
 across the Ganges was 1 in 1 1 . The weir failed after 
 working for twenty years. It was rebuilt with a 
 gradient of 1 in 16. In the Zifta and Assiut regulators 
 on the Nile the gradients are 1 in 16 '4 and 1 in 21. 
 
 ISO from 
 Drop Wai I 
 
 NARORA WEIR AS ORIGINALLY BUILT. 
 
 -l-Z04',c-l5.7 
 NARORA WEIR AS RECONSTRUCTED. 
 
 Rip-rap 
 
 FOUNDATION OF THE ZIFTA REGULATOR, RIVER NILE. 
 
 Regarding the upward pressure on the floor due to 
 the hydrostatic pressure from the head A B, there is a 
 theory that the weight of a portion of the floor at any 
 point P should be able to balance the pressure due to 
 a head of water P R. This, supposing the masonry to 
 be twice as heavy as water, would give a thickness of 
 floor equal to half P R. According to Bligh, the 
 theoretical thickness ought, for safety, to be increased 
 
110 RIVER AND CANAL ENGINEERING 
 
 by one-third. Practically the thickness need not, in 
 most cases, be made even so great as is given by the 
 theoretical rule. On canals in the Punjab it is certainly 
 less. Water passing through soil or fine sand does 
 not exert anything like the pressure which it exerts 
 when passing through a pipe. It acts in the same 
 manner as in a capillary tube. It is only in coarse 
 sand or gravel or boulders that water flows as in a pipe. 1 
 If the tail water covers the floor, the weight of a portion 
 of floor is reduced by the weight of an equal volume of 
 water. If the foundation of any part of the floor is 
 higher than B E, the upward pressure on it is reduced 
 because the water has to force its way upwards through 
 the soil. 
 
 Bligh also states as an empirical rule that in order to 
 provide efficiently against scour the length of floor B E 
 
 A. /IT 
 
 should be - / , where H is the maximum head A B ; 
 s\J 13 
 
 and he points out that in a case where this length is 
 less as it usually is than that necessary to give a 
 hydraulic gradient of the requisite flatness, according to 
 the rule previously quoted, it is better to add an up- 
 stream floor B D, which may be of puddle and there- 
 fore cheap, than to add to the downstream floor a 
 length E C which must be of masonry or concrete, and 
 that this arrangement, by shifting the line of hydraulic 
 gradient from AE to FE, gives a reduced upward 
 pressure on the downstream floor. 
 
 The length EN to which pitching, if of f ' rip-rap" 
 
 type, should extend is given by Bligh as _ /_ /JL 
 where q is the maximum discharge in cubic feet per 
 
 1 Irrigation Works, CHAP. L, Art. 4- 
 
WEIRS AND SLUICES 
 
 111 
 
 second passing over a 1-foot length of the weir, and H 
 is the head A B. 
 
 4. Various Types of Weirs. The type of weir 
 shown in fig. 32 may be varied by steepening or 
 flattening the slopes of one or both faces. Flattening 
 increases the cost but gives a greater spread for the 
 foundations. It may, however, be combined with a 
 decrease in the width of the crest. Flattening of the 
 downstream slope reduces the shock of the water on 
 the floor, but the slope itself, .especially the lower 
 portion, has to stand a good deal of wear, and the 
 
 
 FIG. 34. 
 
 length exposed to this is increased. Flattening the 
 upstream slope facilitates the passage of floods. The 
 same result is obtained by making the crest slope 
 upwards (fig. 34). In a small stream or in an 
 irrigation distributing channel, a weir may be a 
 simple brick wall with both faces vertical and corners 
 rounded. 
 
 Weirs in America are often built of crib-work filled 
 with stones. Weirs are also made of sheet piling filled 
 in with rubble, and the top may be protected by sheet 
 iron. A weir made on the Mersey in connection with 
 the Manchester Ship Canal works was so made. There 
 were three rows of piles and- the filling in the back part 
 was of clay. 
 
 Sometimes the downstream faces of weirs used to be 
 
112 
 
 RIVER AND CANAL ENGINEERING 
 
 made curved (figs. 35 and 36), the object being to 
 reduce the shock of the falling water, but the advantage 
 
 FIG. 35. 
 
 gained is not very appreciable, and this type of weir is 
 not very common. 
 
 The Okla weir (fig. 37) across the river Jumna near 
 
 FIG. 36. 
 
 Delhi was built about thirty-eight years ago on the 
 river bed, which consisted of fine sand. The depth of 
 water over the crest in floods is 6 to 10 feet. The 
 
 ' " f ?^7^?* J ^ > V - 'T' r T. 7f^. ff "^^"^^ 
 
 Scale I Inch = 40 Feet 
 
 9 *0 
 
 _ 8(0 Feet 
 
 FIG. 37. 
 
 material, except the face-work and the three walls, is 
 dry rubble. 
 
 When the reach of channel downstream of a weir 
 has a bed-level much lower than that of the upstream 
 
WEIRS AND SLUICES 
 
 113 
 
 reach this is often the case in irrigation canals, the 
 work is known as a "fall" or "rapid." At a fall the 
 water generally drops vertically, and a cistern (fig. 38) 
 is provided. The falling water strikes that in the 
 cistern and the shock on the floor is greatly reduced. 
 An empirical rale for the depth of the cistern, measured 
 from the bed of the downstream reach, is 
 
 K=H+ VHVD, 
 
 where H is the depth of the crest of the fall below the 
 upstream water-level, and D is the difference between 
 the upstream and downstream water-levels. At some 
 
 FIG. 38. 
 
 old falls on Indian canals the water, as it begins to 
 fall into the cistern, is made to pass through a grating 
 which projects with an upward inclination from the 
 crest of the weir at the downstream angle. This splits 
 up the water and reduces the shock, but rubbish is 
 liable to collect. 
 
 In the usual modern type of canal fall in India the 
 weir has no raised crest, and the water is held up by 
 lateral contraction of the waterway just above the fall. 
 The opening through which the water passes is trape- 
 zoidal (fig. 39), being wide at the water-leve] and narrow 
 at the bed-level. In a small channel there is only one 
 opening, but in a large canal there are several side by 
 
 side, so that the water falls in several distinct streams. 
 
 i 
 
114 
 
 RIVER AND CANAL ENGINEERING 
 
 The curved lip shown in the plan is added to make the 
 water spread out and cause less shock to the floor. 
 The dimensions of the openings are calculated so that 
 however the supply in the canal may vary, there is 
 never any heading up or drawing down. The detailed 
 method of calculation for finding C F and the ratio of 
 A B to B C is given in Hydraulics, CHAP. IV. In 
 cases where it is only necessary for the notch to be 
 accurate when the depth of water ranges from B C to 
 three-fourths B C, it will suffice to calculate as follows : 
 Let 6 be the bed width of the canal, and let Q be the 
 discharge and B the mean width of the stream when 
 
 FIG. 39. 
 
 the depth of water is B C. Decide on the number of 
 notches, and let W be the width of a notch calculated 
 as if it were to be rectangular, i.e. by the ordinary weir 
 formula. Increase the width to W'=1'05 W. Then 
 make the notch trapezoidal, keeping the mean width W, 
 and making the bottom width w (or CF), such that 
 
 vy7 = ft- The top width of the notch is of course 
 
 increased as much as the bottom width is reduced. 
 A rapid has a long downstream slope, which is 
 expensive to construct and difficult to keep in repair, 
 especially as the canals can only be closed for short 
 periods. Rapids exist in large numbers on the Bari 
 Doab Canal in India, the face-work consisting in many 
 
WEIRS AND SLUICES 
 
 115 
 
 cases of rounded undressed boulders with the inter- 
 stices filled up by spawls and concrete which stand the 
 wear well. Rapids have again been used on the more 
 modern canals in places where boulders are obtainable, 
 and where deep foundations would have given trouble 
 in unwatering. The upstream face of a rapid is 
 vertical, or has a steep slope. 
 
 5. Weirs with Sluices. The long weirs built across 
 Indian rivers below the heads of irrigation canals 
 generally extend across the greater part of the river 
 bed. In the remaining part generally the part nearest 
 the canal head there is, instead of the weir, a set of 
 
 FIG. 40. 
 
 openings or " under-sluices " (fig. 40) with piers having 
 iron grooves in which gates can slide vertically. The 
 piers may be twenty feet apart and five feet thick. The 
 gates are worked by one or more " travellers," which 
 run on rails on the arched roadway. The traveller is 
 provided with screw gearing to start a gate which sticks. 
 When once started it is easily lifted by the ordinary 
 gears. The gates descend by their own weight. The 
 gate in each opening is usually in two halves, upper and 
 lower, each in its own grooves, and both can be lifted 
 clear of the floods. In intermediate stages of the river 
 these gates have to be worked a good deal. (See also 
 CHAP. V., Art. 5.) Usually the weir has, all along its 
 crest, a set of hinged shutters, which lie flat at all 
 
 seasons, except that of low water in the river. 
 
 i2 
 
116 
 
 RIVER AND CANAL ENGINEERING 
 
WEIRS AND SLUICES 117 
 
 The canal head consists of smaller arched openings, 
 provided with gates working in vertical grooves and 
 lifted by a light traveller. If the floor of the canal 
 head is higher than the beds of the river and the canal, 
 it may be said to be a weir, but otherwise the canal head 
 is merely a set of sluices without a weir. 
 
 The barrage of the Nile at Assiut (fig. 41), and the old 
 barrages of the Rosetta and Damietta branches, consist 
 of sets of sluices without weirs. At Assiut there are 
 piers five metres apart and gates working in grooves 
 like those, above described, at Indian headworks. 
 
 FIG. 42. 
 
 The "dam" across the Ravi, at the head of the 
 Sidhnai Canal in the Punjab, also consists of sluice 
 openings without a weir. The piers are connected by 
 horizontal beams (fig. 42), against which, and against a 
 sill at their lower ends, rest a number of nearly vertical 
 timber " needles," fitting close together, which can be 
 removed when necessary by men standing on a foot- 
 bridge. In floods the needles are all removed and laid 
 on the high-level bridge (not shown in the drawing), 
 the foot-bridge being then submerged. With needles 
 the span between two piers can be greater than would be 
 possible with a gate. Needles can be used up to a length 
 
118 RIVER AND CANAL ENGINEERING 
 
 of 12 or 14 feet, excluding the handle which projects above 
 the horizontal beam. They can be of pine, about 5 inches 
 deep in the direction of the stream, and 4 inches thick. 
 
 Where a branch takes off' from a canal in India there 
 are usually no fixed weirs but two sets of piers one in 
 the canal and one in the branch, with openings and 
 gates like those at the canal heads, or else with wider 
 openings and needles. These works are called regu- 
 lators. The gates are worked by travellers or by fixed 
 windlasses or racks and pinions. Very small gates for 
 distributaries are often worked entirely by screw gear- 
 ing. For the smaller branches the gates are replaced 
 by sets of planks or timbers lying one above another 
 and removed by means of hooks. They are replaced by 
 means of the hooks or by being held in position some 
 little height above the water, and dropped. They are 
 finally closed up by ramming. 
 
 In the case of either planks or needles, leakage can be 
 much reduced by throwing shavings or chopped straw 
 into the water upstream of them. 
 
 Needles can be provided on their downstream sides 
 with eye-bolts just above the level of the beam against 
 which their upper ends rest. They can then be attached 
 by chains or cords to the beam or to the next pier, and 
 cannot be lost when released. They can be released by 
 a lever which can be inserted under the eye-bolt. By 
 pushing the head of a needle forward and inserting a 
 piece of wood under it, a little water can be let through. 
 In this way, or by removing needles here and there, the 
 discharge can be adjusted with exactness. 
 
 At a needle weir in an Indian canal all the needles in 
 one opening are reported to have broken simultaneously. 
 A possible explanation is that one needle broke and that 
 
WEIRS AND SLUICES 119 
 
 the velocity thus set up in the approaching stream 
 caused the others to break. On another occasion when 
 a canal was dry all the needles were blown down. 
 
 Sometimes the beam or bar against which the upper 
 ends of the needles rest is itself movable. At Ravenna, 
 in Italy, the bar between any two piers has a vertical 
 pivot at one pier and can swing horizontally. Its other 
 end is held by a prolongation of the next bar, near to 
 its pivot. If the end bar of the weir is released, each 
 bar in turn is released automatically. 
 
 At Teddington on the Thames the oblique weir, 480 
 feet long, has thirty-five gates, which extend over half 
 the length of the weir. They are worked by travellers 
 which run on a foot-bridge. The openings do not 
 extend down to the river bed, but are placed on the 
 top of a low weir. The other half of the weir is fixed. 
 The gates are raised to let floods pass. 
 
 At Richmond on the Thames the arrangements are 
 similar, the gates being counterbalanced to admit of easy 
 and rapid raising. When raised they are tilted into a 
 horizontal position so as not to obstruct the view. 
 
 In Stoney's sluice gates a set of rollers is interposed 
 between the gate and the groove. The rollers are 
 suspended from a chain, one end of which is attached to 
 the top of the gate and the other end to the groove. 
 The rollers thus move up or down at half the rate of 
 the gate, and some of them are always in the proper 
 position for taking the pressure. Escape of water 
 between the gate and the groove is prevented by a rod 
 which is suspended on the upstream side of the gate 
 close to its end, and is pressed by the water against the 
 pier. Stoney's sluice gates, with spans ranging up to 
 30 feet, have been used on the Manchester Ship Canal 
 
120 RIVER AND CANAL ENGINEERING 
 
 for the sluices by which the water of the river Weaver 
 is passed across the canal, and at locks for passing the 
 flood waters of the Irwell and Mersey down the canal. 
 The gates are balanced by counterweights. 
 
 Frame weirs/ used chiefly on rivers in France but 
 also in Belgium and Germany, are a modification of the 
 needle and plank arrangements above described. For 
 the masonry piers there are substituted iron frames or 
 trestles, which are hinged at the floor-level so that, when 
 the timbers have been removed, the frame can be turned 
 over sideways and lie flat on the floor, thus leaving the 
 waterway absolutely clear from side to side of the 
 stream. The foot-bridge which rests on the frames is 
 removed piece by piece. The frames are raised again 
 by means of chains attached to them. In order that 
 the frames may not be too heavy they are spaced 3 to 
 4 feet apart, or very much nearer than when masonry 
 piers are used. Horizontal planks can thus be used of 
 shorter lengths than the needles, and they can be made 
 up into greater widths so that the leakage is less. 
 
 A further modification consists in placing the bridge 
 platform above flood-level, and in hinging the frames to 
 it instead of to the floor. The frame turns about a 
 horizontal axis parallel to the length of the weir. A 
 weir of this kind can be used for greater depths of water 
 than the ordinary frame weir. 
 
 In some cases the horizontal planks are connected 
 together by hinges so that they form a " curtain." The 
 curtain is raised by rolling it up by means of a traveller. 
 It admits of rapid and accurate adjustment of the water- 
 level, but there is considerable scouring action below a 
 curtain when it is somewhat raised. 
 
 1 Min. Proc. Inst. C.E., vols. Ix. and Ixxxv. 
 
WEIRS AND SLUICES 121 
 
 6. Falling Shutters. In The"nard's system, first 
 used in France, a shutter (fig. 43) is hinged at its lower 
 edge and is held up by a strut. When the lower end 
 of the strut is pushed aside it slides downstream and 
 the shutter falls flat. To enable the shutter to be 
 raised again an upstream shutter, which ordinarily lies 
 flat and is held down by a bolt, is released, and it is 
 then raised by the current to the extent permitted by 
 a chain attached to it. The downstream shutter is then 
 raised. Thenard's system was not much used in France 
 because the river had to fall to a level somewhat too 
 low for navigation before the shutters could be raised. 
 
 FIG. 43. 
 
 The sudden jerk on the chain of the upstream shutter 
 is also liable to do damage. The system has been 
 adopted on some of the long weirs which cross Indian 
 rivers downstream of the heads of irrigation canals. 
 To prevent damage by shock, a hydraulic brake was 
 designed by Fouracres. It consists of a piston which 
 travels along a cylinder and drives water out through 
 small holes. The shutters are placed on the top of the 
 fixed weir, where they usually lie flat, except in the low 
 water season, any adjustments of the river discharge 
 being effected by means of the under-sluices. 
 
 In the Chanoine system of falling shutters (fig. 44), 
 used first in France, the shutter is hinged at a point 
 rather higher than the centre of pressure. The hinge 
 
122 RIVER AND CANAL ENGINEERING 
 
 is supported by a vertical trestle, which is hinged at its 
 lower end and is supported by a strut which slides in a 
 groove and rests against a stop. When the water rises 
 to a certain height above the top of the shutter, it is 
 turned by the force of the water into a horizontal 
 position. The struts can then be pushed sideways out 
 of the stops by means of a " tripping bar," which lies 
 along the floor parallel to the line of shutters and is 
 worked from the bank. The struts, trestles, and 
 shutters then fall flat. To close the weir the shutters 
 are first raised into the horizontal position which they 
 
 FIG. 44. 
 
 occupied before falling, by means of a hook worked 
 from a boat or by chains attached to a foot-bridge 
 running across the river upstream of the weir. They 
 can then be easily closed by a boat-hook. The water 
 closes them of itself if it falls low enough. 
 
 When the shutters fall a great rush of water occurs. 
 To obviate this a valve is made in the upper half of 
 the shutter. It consists of a miniature shutter on the 
 same principle as the main shutter. The pivot of the 
 main shutter is made at such a height that the shutter 
 
 o 
 
 will not turn over when only a small depth of water 
 flows over it. Instead of this the valve comes into 
 operation. The valve also facilitates the raising of the 
 
WEIRS AND SLUICES 123 
 
 shutter. Again, instead of the tripping bar, which 
 would sometimes have to be of great length or be liable 
 to damage owing to stones jamming in its teeth, the 
 shutter can be released by pulling the strut upstream 
 so that .it falls into a second groove, down which it 
 slides. When a tripping bar is used, its teeth can be so 
 arranged that the shutters are released a few at a time, 
 first singly, then in twos and threes. Sometimes there are 
 gaps of a few inches between one shutter and the next, 
 and the gaps can be closed by needles if necessary. 
 Chanoine shutters can be very rapidly lowered, and 
 
 FIG. 45. 
 
 they are used in France and in the U.S.A. in places 
 where sudden floods occur. They are also used for 
 navigation " passes " where most of the heavy traffic is 
 downstream and where it is too heavy to be dealt with 
 in a lock. A foot-bridge across the stream or across 
 the navigation pass is always an assistance, but some- 
 times it cannot be used when there is much floating 
 rubbish or ice. With a foot-bridge the cost is greater 
 than that of a needle weir. 1 
 
 In the Bear Trap weir (fig. 45) the upstream shutter 
 rests against the downstream one. Both are raised by 
 admitting water from the upper reach, by means of a 
 culvert, through an opening in the side wall, and they 
 are made to fall by placing this opening in communica- 
 
 1 Rivers and Canals, Harcourt. 
 
124 RIVER AND CANAL ENGINEERING 
 
 tion with the downstream instead of the upstream 
 reach. This kind of shutter is only suitable for passes 
 of moderate width, and it is rather expensive on account 
 of the culverts. 1 
 
 Shutters with fixed supports are used on the Irwell 
 and Mersey. A fixed frame is built across the stream 
 (fig. 46) and the shutters are hinged to it. When the 
 water rises to a certain height above its top, the shutter 
 turns into a horizontal position, but as this causes a 
 severe rush of water the shutter is usually raised by a 
 chain attached to its lower end and worked from the 
 bank. When in a horizontal position, it is held there by 
 a ratchet. When the stream falls the ratchet is released 
 and the shutter is closed by the stream. This kind of 
 shutter cannot be used where there is navigation. 
 
 On the weir 4000 feet long across the river Chenab 
 at Khanki in the Punjab, the falling shutters, 6 feet 
 high and 3 feet wide, are hinged at the base and held 
 up by a tie-rod on the upstream side. The trigger 
 which releases the rod is actuated by means of a wire 
 rope carrying a steel ball, and worked by a winch from 
 the abutment of the weir or from one of the piers, 
 which are 500 feet apart. A winch is fixed on the 
 top of each pier, and communication with the piers is 
 effected by means of a cradle slung from a steel wire 
 rope, which rests on standards and runs across the weir. 
 The wire rope which carries the steel ball passes over a 
 series of forks, one on each shutter. When one trigger 
 has been released, that shutter falls and the ball hangs 
 loose. A further haul on the rope causes it to actuate 
 the trigger of the next shutter, and so on. If it is 
 desired to drop only some of the shutters, the rope is 
 
 1 Rivers and Canals, Harcourt. 
 
WEIRS AND SLUICES 125 
 
 passed over the forks of those shutters only. The 
 
 FIG. 46. 
 
 shutters can be raised by means of a crane which runs 
 
126 
 
 RIVER AND CANAL ENGINEERING 
 
 along the weir on rails downstream of the shutters or, 
 if the water is too high to allow of this, by a crane in 
 the stern of a boat which is moored upstream of the 
 weir and allowed to drop down. 
 
 7. Adjustable Weirs. Drum weirs, invented by 
 Desfontaines, have been used in France and Germany. 
 Two paddles (fig. 47) are fixed on a horizontal axis and 
 can turn through about 90, the lower paddle, which 
 should be slightly the larger, working in a " drum," 
 which is roofed over and can, by means of sluices, be 
 
 FIG. 47. 
 
 placed in communication with either the upper or lower 
 reach of the stream. According as the upper paddle is 
 to be raised or lowered, water is admitted from the 
 upper reach above or below the lower paddle, the water 
 on its other side being at the same time placed in com- 
 munication with the lower reach. On the weirs first 
 made on the Marne, the height of the upper paddle was 
 3 feet 7J inches, and there were, in a weir, a number of 
 pairs of paddles, each being 4 feet 1 1 inches wide. By 
 having sluices at both abutments communicating with 
 both reaches, and by opening or closing each of them 
 more or less, the various paddles can be made to take up 
 different positions, and thus perfect control over the 
 
WEIRS AND SLUICES 127 
 
 discharge is obtained by simply turning a handle to 
 control a sluice gate. A weir has since been made with 
 a single pair of paddles extending right across the 
 opening (33 feet), and the height of the upper paddle is 
 over 9 feet. 1 
 
 The chief objection to drum weirs is the necessity for the 
 hollow or drum, which renders the work very expensive, 
 except when only a small depth of water is held up. 
 
 The old sluice gates of the Nile barrages were made 
 
 FIG. 48. 
 
 segmental (fig. 48), turned on pivots in the piers, 
 and were raised by chains. 
 
 In some factories in Bavaria and Switzerland there 
 are self-acting shutters which revolve on a horizontal 
 axis at the lower edge, and are counterbalanced by 
 cylindrical weights which roll on ways in the side wall. 
 This arrangement is suitable when there is only one span, 
 which can, however, be as great as 30 feet. An adjust- 
 able weir used at Schweinfurt on the Maine, consists of 
 a hollow iron cylinder, 59 feet long and 10 feet in 
 diameter, running across the stream. The cylinder is 
 pear-shaped in cross-sections, and can be made, by 
 means of mechanism, to revolve, the water passing over 
 it. Another kind used at Mulhausen on the Rhine 
 
 1 Rivers and Canals, Harcourt. 
 
128 RIVER AND CANAL ENGINEERING 
 
 consists of a hollow iron cylinder 85 feet long and 9*8 
 feet in diameter. The whole cylinder can be raised by 
 winches (Min. Proc. lust. C.E., vols. cliii. and clvi.). 
 
 8. Remarks on Sluices. In all kinds of sluice 
 openings or regulators, the principles of design as regards 
 protection of the bed and sides, splaying and curving 
 of walls and piers, thickness of floor, and prevention of 
 the formation of streams under the structure are the 
 same as laid down for weirs. 
 
 In order that a pier may be safe from being over- 
 turned by the pressure of the water when the gates or 
 timbers are down, the resultant of its weight, including 
 that of anything resting on it, and of the water pressure 
 on it, must pass through the middle third of its length. 
 This generally occurs when there is an arched roadway. 
 Otherwise it must be arranged for by prolonging the 
 base of the piers downstream, and giving the down- 
 stream side a batter or steps. 
 
 The floor should usually be placed at a level some- 
 what lower than the mean bed-level of the stream. 
 The bed may possibly be lowered in course of time. 
 Lowering the floor also gives a greater thickness of 
 water cushion to take the shock of water falling over 
 the gates or planks. It is convenient to build, on the 
 floor, a low wall or sill, reaching up to the level of the 
 bed or thereabouts, and running across from pier to 
 pier under the line of gates or needles. The height of 
 the gates or needles can thus be reduced, and there is 
 little chance of silt or stones collecting and interfering 
 with them. In the case of needles the wall must be 
 strong enough to resist their horizontal pressure. If 
 ever the bed is lowered, the wall can easily be cut down 
 or removed. 
 
WEIRS AND SLUICES 129 
 
 Sluices with gates are, of course, used in connection 
 with works other than weirs or regulators, as, for 
 instance, in reservoirs or locks, or generally for com- 
 munication between any two bodies of water. The 
 gate may or may not be wholly submerged. If it is 
 not wholly submerged, planks can be used. Needles 
 can be used if the flow is always in one direction and 
 never in the reverse direction. In all cases protection 
 downstream of the opening is required. 
 
 In designing a set of sluice openings or regulators, it 
 is sometimes the custom to make the total area of water- 
 way the same as that of the stream in its unobstructed 
 condition. There is no particular reason why it should 
 be the same. In a description of the Assiut Barrage 
 (Min. Proc. Inst. C.E., vol. clviii., p. 30), it is mentioned 
 that one of the reasons for placing the floor lower than 
 the river bed was that the width of the waterway of 
 the barrage was less than that of the river. The bed 
 has to be heavily protected in any case, and the proper 
 principle is to fix a velocity which is considered to 
 be safe and, the maximum discharge being known, to 
 determine the area of the waterway accordingly. In 
 the case of a very wide river like the Nile, with a well- 
 defined channel, it is inconvenient to make the distance 
 between the abutments of a work much less than the 
 width of the channel, but so far as velocity is concerned, 
 the floor need not usually be lower than the bed. The 
 protection given to the channel on the upstream side 
 of the barrage (fig. 41) seems to be rather greater than 
 necessary. The thickness of the floor (9 feet 10 inches) 
 seems excessive. The thickness originally proposed 
 was much less. 
 
 Of the many kinds of apparatus described in this 
 
 K 
 
130 RIVER AND CANAL ENGINEERING 
 
 chapter each possesses some advantages and disadvan- 
 tages. Gates require a bridge with powerful lifting 
 apparatus, and are suitable for large bodies of water and 
 great depths. Comparing needles with planks, the 
 former can be worked by one man and admit of rapid 
 removal, and require far fewer piers. Planks require 
 two men, and are sometimes liable to jam, but obstruct 
 floating rubbish less than needles, and in shallow water 
 give rise to less leakage. Whether needles or planks 
 are used, masonry piers are most suitable where sand or 
 gravel are liable to accumulate on the floor, or where 
 there is much floating rubbish. The hinged frames 
 are suitable in other cases. Falling shutters of the 
 Chanoine type admit of very rapid lowering, and can 
 be used without a foot-bridge. The drum weir is 
 perfect in action, but its cost is high. 
 
 At any system of sluices the regulation should be so 
 arranged as to minimise the chances of damage to the 
 bed and banks where this is at all likely to occur. If 
 the gates are opened only near one side of the structure, 
 there will be a rush of water on that side, and serious 
 damage may occur. The opening should be done 
 symmetrically and, as far as possible, distributed along 
 the whole length. 
 
 Until experience has shown it to be unnecessary, 
 soundings should be taken at regular periods of time 
 downstream of every important work where scour can 
 occur. When scour is found to have occurred at any 
 particular part of the work, the rush of water at such 
 places should, as far as possible, be prevented, and a 
 chance given for silting to occur. 
 
 Unless experience shows that damage is not likely to 
 occur, a stock of concrete blocks, sandbags, or other 
 
WEIRS AND SLUICES 131 
 
 suitable materials should be kept on the spot ready for 
 use. Life-buoys should be provided on any work where 
 large volumes of water are dealt with, especially if it is 
 unfenced in any part, or if any of the men employed 
 are casual workers. 
 
 Kegarding works for preventing a river from shifting 
 its course so as to damage or destroy a weir or similar 
 work, see CHAP. XL, Art. 3. 
 
 K2 
 
CHAPTER XI 
 
 BRIDGES AND SYPHONS 
 
 1. Bridges. Bridges are of many kinds. In this book 
 only those parts of them are considered which are 
 exposed to the stream. If a bridge has piers, there must 
 be some disturbance of the water. The disturbance will 
 be least when the area of the waterway of the bridge is 
 at least as great as that of the stream, and when its 
 shape is as nearly as possible the same. For small 
 streams, a single span clearing the whole stream may be 
 adopted, especially when the channel is of soft material, 
 but for a large stream the cost of intermediate piers, 
 even with a certain amount of protection for them or 
 with deep foundations, will be more than counterbalanced 
 by the smaller thickness of arch or depth of girder. 
 
 Generally a bridge has vertical abutments which limit 
 the waterway, but it may have land-spans, and in this 
 <case the stream as it rises can spread out. Piers and 
 abutments should be so designed that abrupt changes in 
 the section of the stream are, as far as possible, avoided, 
 the piers being rounded or boat-shaped at both ends 
 and the abutments suitably curved (fig. 49). Boat- 
 shaped piers, besides presenting the neatest appearance, 
 cause the least amount of disturbance. 
 
 A bridge can be made safe against scour either by 
 
 132 
 
BRIDGES AND SYPHONS 133 
 
 giving deep foundations to the piers and abutments 
 or by adding a floor and, if necessary, pitching. The 
 former course is usually adopted and is the best. But 
 in a case in which the discharge of a stream is to be 
 increased or has been underestimated, it is often far 
 easier to add a floor to an existing bridge than to in- 
 crease the span of the bridge. In order to increase the 
 waterway the floor can be " dished," i.e. made at a level 
 lower than the bed of the stream l and gradually sloped 
 
 FIG. 49. 
 
 up the slopes being pitched both upstream and 
 downstream of the bridge, to meet the bed. 
 
 In any case in which the water rises above the crown 
 of the arch, the bridge becomes a syphon, and a floor 
 is probably necessary unless the foundations are very 
 deep, or unless the rise of water above the crown is 
 temporary. 
 
 In the case of Indian rivers which have soft channels, 
 and are ordinarily of moderate width but are subject 
 to occasional floods when the width of the stream is 
 multiplied several times and becomes very great, it is 
 
 1 The foundations of piers and abutments should be deep enough to 
 allow of this. 
 
134 
 
 RIVER AND CANAL ENGINEERING 
 
 the rule to make the span of a railway bridge far less 
 than this greater width. The stream during floods 
 scours out a deep channel through the bridge with great 
 rapidity, and no heading up worth mentioning occurs. 
 The foundations of the piers are very deep, being 
 frequently 50 feet below the lowest point of the river 
 bed which can be found anywhere within several miles 
 of the bridge. The span of the bridge can be arrived 
 at by considering a general cross-section of the river as 
 it is when in high flood, and assuming that scour to the 
 
 xxxx 
 
 FIG. 50. 
 
 depth of the lowest point, found as just explained, will 
 take place in one-third of the span of the bridge. The 
 span can then be so fixed as to give no heading up. It 
 is not assumed that there will be no increase in velocity 
 through the bridge. The velocity in the deep scoured 
 portions will be increased. The piers are protected by 
 loose stone (fig. 50). The spans vary from 100 to 250 
 feet. The bridge over the river Chenab at Wazirabad had 
 originally sixty-four spans of 1 45 feet each. The number 
 of spans has since been reduced to twenty-eight. With 
 a very long bridge, the current of the shifting stream is 
 more likely to strike the bridge obliquely, though this is 
 not the chief reason for reducing the length. Long 
 
BRIDGES AND SYPHONS 
 
 135 
 
 spans, say 250 feet, have been found to be better than 
 shorter spans ; the cost of the stone protection round the 
 piers is of course less (Government of India Technical 
 Paper, No. 153, "River Training and Control on the 
 Guide Bank System," by Sir F. J. E. Spring, C.I.E., 
 1904). 
 
 2. Syphons and Culverts. Syphons are used to 
 pass drainage channels or other streams under canals 
 or other lines of communication. In the case of a 
 masonry syphon under a stream which may be dry 
 while the syphon is full, the weight of the arch and its 
 solid load must be riot less than the upward pressure of 
 
 \\\\\\V\\\\\\\\\\\\\\\\ 
 FIG. 51. 
 
 the water passing through the syphon. The channel 
 sometimes has a vertical drop at the upstream side 
 (fig. 51) and a slope at the downstream side. The 
 slope enables any solid materials to be carried through, 
 and facilitates cleaning out and unwatering. The drop 
 at the upstream side does not give rise to any shock on 
 the floor when the syphon is full, but a slope is prefer- 
 able if there is room for it, and it causes less loss of 
 head. 
 
 A. culvert which is liable to run full and which 
 has a steep approach channel (fig. 52) may become 
 suddenly drowned on the upstream side. As soon 
 as the water rises to the crown of the arch, the 
 wet border of the culvert increases and this reduces 
 the velocity and discharge. The water coming 
 
136 
 
 RIVER AND CANAL ENGINEERING 
 
 down the approach channel then rises abruptly, 
 and the increased section of the stream causes 
 a reduced velocity of approach, and this further 
 reduces the discharge through the culvert. The 
 heading up continues until the difference in the up- 
 stream and downstream water-levels is great enough 
 to readjust matters (Min. Proc. lust. C.E., vol. 
 clxxxvi.). The possibility of this heading up occurring 
 should be attended to in the design. In the case of a 
 culvert in a railway embankment where heavy floods 
 have to be passed, the culvert may be made bell- 
 
 FIG. 52. 
 
 mouthed by a curved embankment constructed on 
 its upstream side. 
 
 3. Training Works. The object of the upstream 
 and downstream protections already described (CHAP. 
 X.) is to prevent damage to the structure owing to the 
 disturbance caused by the structure itself. When a 
 river is given to shifting its course (CHAP, IV., Art. 9) 
 and cutting away its banks, protection of another kind is 
 required. The stream, if left to itself, may cut away 
 one bank upstream of the structure for a long distance, 
 and eventually damage, or destroy by undermining, the 
 upstream pitching and the abutment itself. This is 
 known as outflanking. If in the neighbourhood of the 
 
BRIDGES AND SYPHONS 
 
 137 
 
 line AB (fig. 53) there is nothing for the river to 
 damage, if, for instance, the structure is a weir with a 
 canal, if any, only on the opposite bank of the river, 
 and if the land is of no particular value, the case could 
 conceivably be met by protecting the abutment on 
 all sides, but even then there might be a chance of 
 the erosion of the bank continuing until the stream 
 had formed a connection at C with the downstream 
 reach. This, of course, in the case of a weir, would 
 render the work useless and might even destroy it. 
 
 In the case of a bridge carrying a 
 road or railway, or of a syphon or 
 aqueduct carrying a canal or other 
 stream, it is wholly inadmissible to 
 allow the stream to cut away even as 
 far as the point A for fear of its 
 severing the line of communication. 
 Thus in every case it is practically 
 necessary to prevent any serious 
 erosion of the bank upstream of the 
 structure. In ordinary cases it is sufficient to protect 
 the bank C D by any of the methods given in CHAP. 
 VI., Art. 3, the protection being turned inwards, as 
 shown at D, to prevent the end of it being damaged. 
 
 In the case of railway bridges across the great 
 shifting rivers of India, protection used at one time to 
 be afforded by various systems of spurs. This has now 
 been abandoned in favour of Bell's guide banks 
 (fig. 54), which are found to be far more satisfactory. 
 These guide banks are discussed in the paper by Spring 
 quoted above (Art. 1). The spaces behind the guide 
 banks become filled with water, at least during floods, 
 and are meant to be silted up. An opening in the 
 
 
 C 
 A 
 
 s 
 
 t 
 
 FIG. 53. 
 
138 
 
 RIVER AND CANAL ENGINEERING 
 
 railway embankment should be provided at A, and 
 another on the opposite side of the river, to ensure a 
 constant flow of water (CHAP. V., Art. 3), but they 
 should not be large enough to cause high velocity. 
 The chief danger to which a guide bank is liable is 
 outflanking when the stream assumes the position 
 shown. To guard against this danger it is necessary 
 
 FIG. 54. 
 
 to have very strong and massive heads to the guide 
 banks. When the bank of the eroding stream, down- 
 stream of the guide bank head, becomes a semicircle or 
 thereabouts, the stream takes a short-cut across the 
 sandbank, and to encourage this an artificial cut can be 
 dug, at the season of low water, on any suitable line. 
 
 If the guide banks were made with an increased 
 width of opening at the upper end, this would reduce 
 the chance of outflanking but would increase the danger 
 
BRIDGES AND SYPHONS 
 
 139 
 
 from a direct attack such as indicated, in the figure, on 
 the left bank. It has been suggested that the width at 
 the upstream end should be less than at the bridge, but 
 this seems undesirable. Probably the form shown in 
 fig. 54 is the proper one. The length of the guide bank 
 upstream of the bridge is made about equal to the span 
 of the bridge between the two guide banks. If made 
 less than this, the river might cut into the line of 
 
 DfctHA PORCST 
 
 FIG. 55. 
 
 railway. The length of guide bank downstream of the 
 bridge is generally 300 to 500 feet, being greater as the 
 velocity of the river is greater and the sand of its bed 
 finer. 
 
 The Bengal Dooars Railway runs near the foot of 
 the Bhutan Himalayas, and crosses some broad river 
 channels which, after the excessively heavy rains which 
 occur, are filled by streams of very high velocities. 
 One such channel or set of channels (fig. 55), more than 
 half a mile wide, is provided with a bridge whose 
 
140 RIVER AND CANAL ENGINEERING 
 
 waterway consists of ten spans of 60 feet each. The 
 railway embankment across the remainder of the 
 channel having been breached in many places in 1903, 
 protection was afforded by T-headed spurs and other 
 groynes, the first arrangement, which withstood the 
 floods of 1904, being as shown in the figure. The 
 triangular apex of the A-shaped groyne, south-east of 
 the bridge, was added in 1905 because, in its absence, 
 the water struck the bridge obliquely. After the 
 addition there was a great deposit of silt in the 
 neighbourhood of the four T-headed spurs. Next year 
 the river, in a great flood, rose over the top of the 
 railway embankment near these spurs, and finally caused 
 a breach 600 feet wide. The embankment was after- 
 wards raised. The velocity through the bridge seems 
 to have approached 18 feet per second. The bridge 
 had at first no floor. A floor was added, but was much 
 damaged by the floods (Min. Proc. lust. C.E., vol. 
 clxxiii.). The level of the floor is not given, but it 
 would seem to have been desirable to make it at a very 
 low level. The rising of the stream over the railway 
 embankment was attributed to the silting up near the 
 T-headed spurs. The addition of the triangular portion 
 above referred to would seem to have somewhat assisted 
 this process. If all the trouble could have been fore- 
 seen, it might have been best to build an additional 
 bridge 2000 feet south-east of the existing bridge. 
 The groynes were composed of the wire-network rolls, 
 described in CHAP. VI., Art. 3, piled pyramid fashion. 
 
CHAPTER XII 
 
 DRAINAGE AND FLOODS 
 
 1. Preliminary Remarks. Arts. 2 and 3 of this 
 Chapter deal with the calculation of flood discharges. 
 Art. 2 dealing with small streams, in which the water 
 has to be got rid of, and Art. 3 with large streams. 
 The remaining articles discuss the methods of predicting 
 floods and of preventing them from doing damage. 
 When the discharge figures have been arrived at in any 
 case, the necessary masonry works can be designed in 
 accordance with the principles described in CHAPS. X. 
 and XL For remarks regarding the design of channels 
 and banks, see CHAP. IX., Art. 4, and also Art. 6 of the 
 present Chapter. 
 
 In England, land near a stream or flooded area is 
 said to be " awash " when the flood water rises to 
 within 3 feet of the surface of the ground. The 
 drainage of such land is apt to be unsatisfactory. If 
 land is flooded or awash, it may be desirable to shift 
 the outfall of a branch drain to a point lower down in 
 the main outfall. 
 
 2. Small Streams. In dealing with small streams, 
 such as branch drains or natural streams not far from 
 their sources, the engineer is concerned only with their 
 maximum discharges. He has to design culverts, 
 
 141 
 
142 RIVER AND CANAL ENGINEERING 
 
 bridges or syphons to pass the streams under roads or 
 other works, or to design channels or waste weirs for 
 them. In a settled country there may be already some 
 works in existence on the same stream, and these may 
 form a guide, or it may be possible to obtain local 
 information as to the height or volume of floods. Even 
 in such a case rainfall figures will be most useful. In 
 districts where there is no settled population, and in 
 any case where the stream is ill- defined, and the flow 
 fitful, the rainfall figures may afford the only, or at 
 least far the best, means of estimating the discharge. 
 
 The rainfall to be considered in all these cases is the 
 maximum likely to fall in a short period of time. The 
 catchment areas dealt with are small, say 5 square 
 miles or less. It must be assumed that the fall of rain 
 extends to all parts of the catchment area, and that its 
 duration is sufficient for the water from all parts of it 
 to reach the site of the work. The different valleys or 
 divisions of the catchment area should be considered 
 separately, and regard must be had, not only to the 
 area of each division, but to its length and declivity 
 measured along the course of the stream which drains 
 it. On these two factors depend the time taken by the 
 rain water to reach the site of the work. The rate at 
 which the rain water flows over the ground into rills or 
 small subsidiary streams may be taken to be J mile per 
 hour in flat land, and 1 mile per hour on steep hill 
 sides. The velocity of the current in the rills and 
 larger streams is generally 2 to 4 miles per hour. It 
 can, when necessary, be calculated roughly from the 
 size and slope of the stream. To be on the safe side, 
 the highest probable figure can be taken. 
 
 The time taken by the water to flow from the 
 
DRAINAGE AND FLOODS 143 
 
 farthest points of the catchment area to the site of the 
 work having been arrived at as above, the next thing is 
 to estimate the probable maximum intensity of the 
 rainfall during that time over the whole catchment area. 
 The only figures immediately available will be the 
 mean annual rainfall, or perhaps the maximum fall in 
 twenty-four hours, but it has been shown (CHAP. II., 
 Art. 5} how the probable maximum fall over a shorter 
 period may be estimated. 
 
 The next thing to be calculated is the " run-off," i.e. 
 the probable proportion of the rainfall which will at 
 once run off. This may be less than the proportion 
 which will eventually become "available," because some 
 of it may go to feed the underground supply from 
 which springs are fed. The proportion running off a 
 small area in a short time would, under most circum- 
 stances, be rather difficult to estimate, but in the case 
 under consideration, only the probable maximum figure 
 is required. This occurs when the ground is saturated. 
 Under these circumstances the ratio of the run-off to 
 the total fall may be somewhat as follows : 
 
 Steep rocky hillsides . . ;< *70 to "90 
 
 Ordinary hills . . . .. '50 ,, 75 
 
 Undulating country . . . *40 ,, *50 
 
 Flat country . . . . '30 5 , *35 
 
 The figures can be increased when the surface is 
 specially hard or frozen, and decreased when it is soft, 
 sandy, covered with woods or vegetation, or cultivated. 
 Whether or not the above procedure is necessary in 
 its entirety depends chiefly on the size of the proposed 
 work and on the degree of inconvenience likely to 
 arise from any wrong estimation of the discharge. 
 
144 RIVER AND CANAL ENGINEERING 
 
 In designing syphons to carry torrents across the 
 Upper Jhelum Canal in the Punjab, the discharge from 
 a catchment area of 79 square miles was found to be 
 about 4000 cubic feet per second. This is at the rate of 
 about 5000 cubic feet per second per square mile, and is 
 equivalent to a run-off of 7*8 inches in an hour. The 
 catchment area was among low hills, not far from the 
 Himalayas, and the declivities of the rills were very 
 steep. The superintending engineer, Mr K. E. Purves, 
 states l that the discharge observations were reliable, 
 and that falls of rain of an inch in ten minutes occur 
 not infrequently, even though the fall in twenty-four 
 hours might not exceed 2 or 3 inches. In order to 
 account for the discharge in the case under considera- 
 tion, it would at first seem to be necessary to assume 
 not only that a fall at the rate of 7 '8 inches per hour 
 had occurred, but that the whole of it had run off. It 
 is not, however, necessary to assume quite so much. 
 The ground being saturated, the rain falling in a period 
 of five minutes might be reaching the discharge site 
 with little loss. A suddenly increased fall at the rate 
 of 6 inches per hour might then occur, and the water 
 travelling more quickly and with hardly any loss, would 
 overtake that already passing the site. This case seems 
 to show that for a very small catchment area the whole 
 of the fall, and more, must be allowed for. 
 
 The Chief Engineer of the Punjab did not accept 
 the above figures. 2 He remarked that observations 
 taken under great difficulties as to time and place 
 are liable to error, and he considered that an allow- 
 
 1 Report on the Revised Estimate, Upper Jhelum Canal. 
 
 2 Revised Estimate of the Upper Jhelum, Upper Chenab, and Lower Bari 
 Doab Canals. 
 
DRAINAGE AND FLOODS 
 
 145 
 
 ance of rainfall at the rate of 4 '8 inches per hour 
 a rate which had been observed elsewhere and a 
 run-off of 75 of the fall would be sufficient. He 
 accepted a discharge of 2000 cubic feet per second for 
 catchment areas of less than 5 square miles, assuming 
 the run-off to be 75 of the fall, but afterwards increased 
 the figure to 2400 cubic feet per second. The chief 
 engineer did not overlook the fact that in the designs 
 for the drainage aqueducts a free-board of 5 feet had 
 been allowed, and perhaps this led to an acceptance 
 of an estimated discharge less than would otherwise 
 have been accepted. It does not seem to be at all 
 certain that the figure put forward by Mr Purves 
 was far wrong. When the original project estimate 
 for the Upper Jhelum Canal was framed, the irrigation 
 engineers had had no experience of small and steep 
 catchments, and no one had suspected that the dis- 
 charge per square mile would be anything like the 
 above. The sums of money provided for works for the 
 passages of torrents had to be increased in ratios varying 
 from 2*5 to 1 to 6 to 1. 
 
 The following statement shows the figures for other 
 small catchment areas in the neighbourhood of the 
 Upper Jhelum Canal : 
 
 Catchment 
 Area. 
 
 Discharge per 
 square mile. 
 
 Run-off. 
 
 Sq. miles. 
 79 
 
 Cub. ft. per sec. 
 5000 
 
 Inches. 
 
 7'8 
 
 1-47 
 
 3825 
 
 5-82 
 
 2-96 
 
 2214 
 
 3-46 
 
 In the south-east of New South Wales flood dis- 
 
 L 
 
146 
 
 RIVER AND CANAL ENGINEERING 
 
 charges of 135 and 84 cubic feet per second have been 
 found for catchment areas of '91 and 2*5 square miles 
 respectively in broken country. 
 
 3. Rivers. It is possible to apply the methods of 
 the preceding article to large catchment areas, but the 
 results would be quite unreliable. If the calculations 
 were made so as to err on the side of safety, the result- 
 ing discharges would often be enormous. The following 
 table shows some figures based on actual flood discharges. 
 None of the localities have excessive rainfalls, though 
 most are liable to occasional very heavy falls. In 
 mountainous districts in the North of England and in 
 Scotland the flood discharges per square mile of catch- 
 ment area have been found to vary from 64 to 320 
 cubic feet per second. 
 
 <D 
 
 
 
 
 Flood Dis- 
 
 
 Referenc 
 Number 
 
 Country. 
 
 Locality. 
 
 Catchment 
 Area. 
 
 charge per 
 sq. mile of 
 Catchment 
 Area 
 
 Remarks. 
 
 
 
 
 Sq. miles 
 
 Cub. ft. per sec. 
 
 
 1 
 
 India. 
 
 Upper 
 
 5 to 10 
 
 1613 
 
 
 
 
 Jhelum. 
 
 
 
 
 2 
 
 j 
 
 Nagpur. 
 
 6-6 
 
 480 
 
 
 3 
 
 South Africa. 
 
 Near Cape 
 
 34-5 
 
 78 
 
 
 
 
 Town. 
 
 
 
 
 4 
 
 i ) 
 
 Near Port 
 
 35 
 
 640 
 
 Estimated. 
 
 
 
 Elizabeth. 
 
 
 
 
 5 
 
 New South 
 
 South-East 
 
 49 
 
 37 
 
 
 
 Wales. 
 
 District. 
 
 
 
 
 6 
 
 India. 
 
 Upper 
 
 56 
 
 1000 
 
 
 
 
 Jhelum. 
 
 
 
 
 7 
 
 5) 
 
 35 
 
 174 
 
 550 
 
 
 8 
 
 New South 
 
 South-East 
 
 418 
 
 11-2 
 
 
 
 Wales. 
 
 District. 
 
 
 
 
 9 
 
 India. 
 
 Kali Nadi 
 
 2593 
 
 51 
 
 Estimated 
 
 
 
 Stream. 
 
 
 or more 
 
 roughly. 
 
DRAINAGE AND FLOODS 147 
 
 The tendency of the figures in column 5 of the table 
 is to decrease as the catchment area increases. This 
 tendency has long been known, and attempts have 
 been made to found on it formulae for calculating- 
 flood discharges. One such formula is Q = c M* 
 where Q is the flood discharge in cubic feet per 
 second and M is the area of the catchment in square 
 miles. The formula is roughly correct, c being 
 a constant for catchment areas of not dissimilar 
 characters and with rainfalls not differing much. But 
 for other cases there is no knowing how c may 
 vary, and this renders the formula practically use- 
 less. The author of another such formula quotes cases 
 Nos. 5 and 8 in the above table, and the two cases 
 mentioned at the end of Art. 2 as agreeing fairly 
 well with the result of his formula. The tendency 
 just mentioned is due to the fact that every river 
 is composed of tributaries which have their own 
 small catchment areas but are, when measured to the 
 general outlet or point where the discharge is under 
 consideration, of very different lengths, to the im- 
 probability of heavy rainfall occurring over all these 
 small areas at such times as to cause the different flood 
 waves to arrive simultaneously at the outlet, and to the 
 facts that in the case of the longer tributaries the flood 
 waves flatten out (Hydraulics, CHAP. IX., Arts. 3 and 
 4) so as to arrive more gradually, and that, unless rain 
 is also falling all along their courses, these longer 
 tributaries undergo losses from evaporation and absorp- 
 tion. But occasionally it happens that the various flood 
 waves do arrive at the outlet more or less simultaneously, 
 and that the rainfall continues so long and is so widely 
 
 distributed though not necessarily of the same intensity 
 
 L 2 
 
148 RIVER AND CANAL ENGINEERING 
 
 as that which caused the flood that the flood waves do 
 not flatten out and that losses in the channels do not 
 occur. Floods can thus vary to an extraordinary degree 
 in severity, and formulae are quite useless. This is why 
 floods occur surpassing all previous records, as, for 
 instance, the recent floods in Paris. However severe a 
 flood may be, it can never be said that the maximum 
 has, even probably, been attained unless it can be shown 
 that the rainfall has been so heavy, so long continued, 
 and so distributed that anything worse is not likely 
 to occur. 
 
 The best method of estimating the flood discharge of 
 a large perennial stream is to ascertain, by local inquiry, 
 the height to which it is known to have risen, and to 
 take cross-sections of the channel and calculate the 
 discharge (CHAP. III., Arts. 4 and 5). In designing 
 works, allowance can be made for a flood exceeding any 
 known before. This method applies also to a case in 
 which a river is formed by the junction of two or more 
 large tributaries. It is possible that the tributaries 
 have not, within the memory of man, been in high flood 
 simultaneously. If so, the chances of this occurring are 
 no greater and no less than if the stream was composed 
 merely of a number of small affluents. Remarks 
 regarding intermittent streams are given in CHAP. III., 
 Art. 7. 
 
 Since an acre contains 43,560 square feet, and a twelfth 
 of this is 3630, it follows that a fall of 4 inches of rain, 
 of which 1 inch runs off, in an hour, gives a discharge 
 of 3630 cubic feet per hour, or about 1 cubic foot per 
 second. This is 640 cubic feet per second for a square 
 mile. The figures in column 5 of the above table show 
 that the run-off was, in the cases quoted, generally far 
 
DRAINAGE AND FLOODS 149 
 
 less than 1 inch. In case No. 4 it was 1 inch, and in 
 case No. 2 it was f inch. 
 
 In the case of the Kali Nadi (No. 9 in the table) an 
 aqueduct to carry the Lower Ganges Canal over the 
 stream was being designed. The flood discharge, 
 estimated from the supposed flood-level and cross-section 
 of the stream was (Min. Proc. Inst. C.E., vol. xcv.) 
 26,352 cubic feet per second. The discharge, estimated 
 by assuming a fall of 6 inches of rain in twenty -four 
 hours over the catchment area then believed to be 
 3025 square miles and a run-off of '25 of the fall, was 
 114,950 cubic feet per second. This figure was rejected 
 on the ground that the rainfall would not be continuous 
 over so large an area as 3025 square miles. An allow- 
 ance of 7 cubic feet per second per square mile was 
 made and, a fresh survey having shown that the catch- 
 ment area was only 2593 square miles, a discharge of 
 18,000 cubic feet per second was allowed for. The 
 aqueduct was built, about the year 1875, with five 
 arched spans of 35 feet each, the total area of the 
 waterway being about 3000 square feet. The length 
 of the piers and abutments was 212 feet, the width of 
 the canal carried over the aqueduct being 192 feet. In 
 1884 the aqueduct was partly destroyed by a flood 
 whose discharge was about 44,000 cubic feet per second. 
 In July 1885 it was wholly destroyed by a flood whose 
 discharge was estimated at 132,475 cubic feet per second, 
 but was probably more. The discharge must have been 
 more than 51 cubic feet per second per square mile. 
 The aqueduct was rebuilt with a waterway of about 
 15,000 square feet. Below the aqueduct there was a 
 bridge which had been standing for a hundred years. 
 Its waterway was only 1146 square feet. It was not 
 
150 RIVER AND CANAL ENGINEERING 
 
 much damaged by the flood of 1884, but much of the 
 water passed round it, breaking through the embanked 
 roadway or pouring over it. It is understood that the 
 bridge was destroyed by the flood of 1885. 
 
 This case shows the necessity for making every 
 possible allowance in calculating flood discharges for 
 important works. The smallness of the discharge, as 
 calculated from the cross-section of the stream, was 
 probably owing to its being dry when the survey was 
 made, so that the velocity could not be observed, 
 but it is probable that such a discharge as wrecked 
 the aqueduct had never before passed down the 
 stream. 
 
 4. Prediction Of Floods. At any place high up 
 on the course of a stream, the occurrence of a flood can 
 often be predicted when rain storms often accompanied 
 in the tropics by lightning can be seen to be occurring 
 towards the sources of the stream. For any station 
 lower down the stream and for precise information in 
 any case, the readings of gauges higher up the stream 
 can be telegraphed. If the station is at a great distance 
 from the gauge and if there is railway communication, 
 the readings can be sent by post. 
 
 In order to be able to predict the time of the arrival 
 of a flood at the lower station the reading of a gauge 
 there, and also of that at the upper station, should be 
 taken at frequent intervals. In the case of large rivers 
 and distances of hundreds of miles, the interval may be 
 six or even twelve hours, but in other cases it should be 
 much less. If the readings are plotted, as in fig. 56, 
 oblique lines can be drawn to connect the saliences and 
 depressions, and the time taken by each change can thus 
 be readily seen. When the upper part of the stream 
 
DRAINAGE AND FLOODS 
 
 151 
 
 is formed by two or more important tributaries there 
 should be a gauge in each. 
 
 As to what constitutes a flood, the gauge diagram of 
 a river (fig. 56) is generally such that a line can be 
 sketched as shown dotted. The rises above this line 
 are floods. The maximum flood discharge of a Northern 
 Indian river is estimated roughly as being 100 times 
 the low-water discharge. Leslie's rule for floods in the 
 British Isles is that if all the daily discharges of a 
 stream during the year are ranged in order of magni- 
 
 FIG. 56. 
 
 tude, the discharges of the upper quarter are considered 
 to be floods. 
 
 In India it is sometimes arranged that a telegram 
 shall, in the low-water stage of the river, be sent from 
 the upper station when a rise of 2 feet occurs in 
 twenty-four hours or any less period, with a further 
 telegram for any such subsequent rise. The telegram 
 states the exact reading on the gauge and whether the 
 water is rising steady or falling. This is given as 
 indicating the procedure that may be followed where 
 the telegraph has to be used, but when long and 
 frequent telegrams are not desirable. 
 
 The advancing end of a flood wave may, while the 
 
152 RIVER AND CANAL ENGINEERING 
 
 wave is rising and being formed, travel rapidly, but 
 when the wave has been formed it travels at the 
 ordinary rate of flow of the risen stream. The advanc- 
 ing end of a trough may, while it is being formed, 
 travel rapidly, but after formation it travels at the 
 ordinary rate of the fallen stream (Hydraulics, CHAP. 
 IX., Arts. 3 and 4)- Thus the rate at which a change in 
 water-level travels down a stream depends at first on 
 the amount of the rise or fall, but afterwards on the 
 water-level of the risen or fallen stream. 
 
 By taking the above facts into consideration and 
 noting the actual times obtained from the diagram, it 
 will be possible to arrive at the probable time that will 
 be taken by any change. It will also be possible to 
 predict the height of the flood. If it is worth while, 
 an empirical formula can be got out. If there are 
 tributaries, each with a gauge, the matter will be more 
 difficult. Probably the floods in the tributaries will 
 arrive at different times, but even in such cases empirical 
 formulae have been arrived at, especially in France, and 
 are mentioned in various volumes of the Proceedings of 
 the Institution of Civil Engineers. 
 
 In all cases predictions are liable to be more or less 
 upset if rain falls in the tract between the upper and 
 lower gauges. In very dry weather the speed of a 
 flood wave may be somewhat reduced, and the height to 
 which it rises will almost certainly be reduced. 
 
 The full effect of a change will not be felt at the 
 lower station unless the change at the upper station is 
 maintained for a sufficiently long period. A short wave 
 or trough flattens out. Thus in any empirical formula 
 or system of prediction, the time over which the change 
 extends at the upper gauge must be taken into account, 
 
DRAINAGE AND FLOODS 153 
 
 or else there must be several upper gauges and the 
 readings of all of them be taken into account. 
 
 In mountainous districts landslips sometimes occur 
 and block the valley of a stream which then forms a 
 lake. The water gradually rises and eventually flows 
 over the dam and sweeps it away causing a flood, which 
 is of great suddenness and height but decreases very 
 quickly in height as it travels down the valley. In a 
 case which occurred in the Himalayas in 1888 the 
 inhabitants of the valleys, from the dam to the point 
 where the river debouches from the hills, were com- 
 pelled by Government to vacate all habitations below 
 the probable level of the flood, and no loss of life 
 occurred. Similar floods, but on a smaller scale, may 
 be caused by the bursting of ordinary reservoir dams. 
 In some continental rivers ice may obstruct the stream 
 and cause floods. 
 
 5. Prevention of Floods. The extended use of 
 field drains has, in recent years, done much to increase 
 the severity of floods in England and other countries. 
 One method of mitigating or preventing floods is the 
 construction of reservoirs for storing the water. Ke- 
 servoirs locally known as " washes," formed by setting 
 back the embankments, exist on the Fen rivers. One 
 wash, on the Nene, below Peterborough, is 12 miles 
 long and half a mile wide and is filled, in floods, to a 
 depth of 7 feet and holds 1 inch of rainfall over the 
 river basin, and this is found to be sufficient. Reservoir 
 construction is, however, in most cases, impracticable 
 owing to the expense. To store the water which is 
 given by 1 inch of rain in the basin of the Thames, a 
 reservoir would be needed 50 feet deep and covering 
 about 7 square miles. It might cost 7,000,000. 
 
154 RIVER AND CANAL ENGINEERING 
 
 The afforestation or reforestation of river basins 
 (CHAP. II. , Art. 4) is also occasionally undertaken, but 
 is not generally practicable. 1 
 
 The most practicable methods for preventing flooding 
 are lowering the water-level of the stream and con- 
 structing embankments along it. These will be con- 
 sidered in the next two articles. 
 
 6. Lowering the Water-Level -The water-level of 
 
 a given length of stream can be lowered by lowering the 
 bed, widening the channel or straightening the channel. 
 The efficiency of these processes is in the order named. 
 As stated in CHAP. L, Art. 4, the alteration to the channel 
 must in any case be continued to some point downstream 
 of the reach under consideration. Let the channel be 
 supposed to be of "shallow" section with sloping sides. 
 Let W be the mean width, D the depth, and S the slope. 
 Let it be required to lower the water-level by an amount 
 
 equal to ^- This can be effected by lowering the bed 
 o. 
 
 by about 25 per cent, of D, or by increasing the width 
 by about 50 per cent., or by increasing the slope by 
 about 100 per cent. If the bed is lowered, V is not 
 affected, and the mean width is reduced. Increase in 
 W reduces D, and therefore reduces the hydraulic radius 
 and the velocity. Hence the large amount of widening 
 necessary. When S is increased the velocity, if E 
 remains the same, is affected only as ^/S (Hydraulics, 
 CHAP. VI., Art. 2\ but the depth of water is reduced 
 and R therefore reduced. Dressing the sides of a 
 channel, so as to make it smoother, produces the same 
 effect as a slight widening. 
 
 It does not, of course, follow that lowering the bed 
 
 1 See also Note on p. 161. 
 
DRAINAGE AND FLOODS 155 
 
 is always the best plan and straightening the worst. 
 Any one of the processes may be more or less im- 
 practicable because, for instance, of the hardness of the 
 material to be removed, or the expense including 
 compensation of removing obstructions. 
 
 A particular kind of widening consists in digging a 
 new channel and keeping both the new and the old 
 channel open. 
 
 If a channel contains a weir, or a local raised portion 
 of bed forming a kind of submerged weir, or a con- 
 tracted place or narrow bridge, the upstream water-level 
 can be lowered by simply removing or reducing the 
 obstruction. The lowering of the water-level will be 
 greatest at the site of the obstruction, and will be zero 
 at some point far upstream (Hydraulics, CHAP. VII. , 
 Art. 5}. If the raised portion forms a long shoal, its 
 removal supposing its height above the general bed to 
 be the same will have more effect than if it were 
 short. If the height of the raised portion is small 
 compared to the depth of water, or the amount of con- 
 traction small compared to the width of the stream, the 
 removal may have much less effect than might appear 
 (CHAP. L, Art. 4). 
 
 In soft soils one advantage of the straightening 
 system for lowering the water-level is that short-cuts 
 can be dug to a small section, and left to enlarge 
 themselves (CHAP. VII. , Art. 1). 
 
 Another advantage is that after any diversions have 
 enlarged themselves to the size of the rest of the 
 channel or have originally been so excavated the 
 whole channel may scour, and the water-level continue 
 to fall. This, of course, should be allowed for if likely 
 to occur. 
 
156 RIVER AND CANAL ENGINEERING 
 
 The same thing may occur in the case of the removal 
 of a weir, shoal, or contracted piece of channel. The 
 scour will act at first close to the site of the obstruc- 
 tion, but it may work upstream. 
 
 In widening or deepening a channel for the purpose of 
 mitigating floods, it is a good plan to begin work at the 
 downstream end, because the lowering of the water-level 
 will extend upstream beyond the reach in which work 
 is done, and this may facilitate work further upstream. 
 As regards any tendency for a widened reach to silt up 
 again, any such silting is not likely to be great in a 
 short period of time, and need not prevent the carrying 
 on of work in various reaches, if this is convenient. 
 
 7. Flood Embankments. A flood embankment 
 may be close to the edge of the river or it may be set 
 back. If set back it need not follow all the windings 
 of the stream. The setting back of an embankment 
 gives an increased waterway to the stream during floods, 
 and therefore a lower flood-level, but the effect of this 
 is trifling in cases where the depth of the water on the 
 flooded land is small, especially if such land is covered 
 with vegetation, or is otherwise much obstructed. 
 Setting back is generally necessary in cases where the 
 stream is liable to erode the banks to any considerable 
 extent. In such a case the embankment should not be 
 so near to the river as to be in much danger from 
 erosion, but the ground, as already stated (CHAP. IV., 
 Art. 9), generally falls, in going away from the river, 
 so that when an embankment is set well back it is in 
 lower ground, more expensive and more liable to breach. 
 The most suitable alignment is a matter of judgment, 
 and depends largely on the extent to which the river 
 is likely to shift. 
 
DRAINAGE AND FLOODS 157 
 
 Embankments should, where possible, be made in 
 straight or properly curved reaches. A flood embank- 
 ment, at least at its upstream end, should terminate in 
 ground which is above flood-level. The top of an em- 
 bankment should be, in the case of a large river, 2 or 
 3 feet above the high flood-level of the river. It should, 
 of course, be graded parallel to the general high flood-level, 
 but neither the gradient nor the height of the flood is 
 usually known with accuracy (CHAP. II., Arts. 1 and 2). 
 There is generally a record or mark of some high flood, 
 and this is taken provisionally as the flood-level. Or 
 the level is calculated approximately from the flood 
 readings on the nearest river gauge. If experience 
 shows that the embankment is too low, it is raised. 
 The cross-section of an embankment depends on the 
 soil, on the extent of damage which results if a breach 
 occurs, on the funds available, and on the value of the 
 land which the embankment has to occupy. 
 
 Where an affluent enters the river it will probably be 
 necessary to run out branch embankments. Sometimes 
 cross embankments are run from the main embankment 
 to high land. Their object is to localise the damage if 
 a breach occurs. Along the back of the embankment 
 there may be a drain and it can be made to discharge 
 its water, when the river is not in flood, through the 
 embankment by means of sluices or by pipes closed by 
 flap valves which will not allow flood water from the 
 river to pass through. There may be sluices in the 
 embankment for the purpose of irrigating the land at 
 the back. 
 
 The immediate effect of the construction of flood 
 embankments along a river is to raise the water-level, 
 because the floods can no longer spread out over the 
 
158 RIVER AND CANAL ENGINEERING 
 
 country, but this effect will not be great if the sectional 
 area of the flood water was small or its velocity low. 
 The river may or may not tend, after the construc- 
 tion of flood embankments, to raise or lower its bed. 
 It has already been remarked that questions of 
 silting or scouring cannot be answered in a general 
 manner. In the case, however, of floods spilling over 
 a piece of country, the depth of the flood water is 
 generally small and the country more or less obstructed. 
 Some deposit of silt generally occurs. The construction 
 of an embankment reduces the area of the flood water, 
 and thus generally reduces the silting and leaves more 
 silt in the river proper. The depth and velocity in the 
 river are increased. Everything depends on which is 
 increased most. Most likely the stream is of shallow 
 section and the velocity is increased most (CHAP. IV., 
 Art. 6, par. 6), and the increased silt-supporting power 
 may make up for the increased charge of silt. 
 
 Sometimes when a main embankment is set far back, 
 a subsidiary embankment of smaller section is con- 
 structed closer to the stream. This is often objection- 
 able. The smaller embankment is liable to breach, and 
 the water then rises suddenly instead of gradually 
 against the main embankment, which is thus endangered 
 to some extent, especially as it is dry instead of being 
 soaked. 
 
 It is often said that one effect of embanking a reach 
 of a river is to increase the severity of floods further 
 downstream. The importance of this is generally 
 exaggerated. The narrowing of the flood stream in the 
 embanked portion causes the flood to travel more 
 quickly and rise higher in that particular reach. At a 
 place further downstream the same effect is produced, 
 
DRAINAGE AND FLOODS 159 
 
 but in a less degree and only because of the increased 
 velocity and consequent reduction in the flattening out 
 of the flood wave, especially when the rise is soon 
 succeeded by a fall. When there is a gradual rise 
 lasting for a considerable time and this is most likely 
 to cause a high flood there is no rise of the flood -level 
 downstream of the embanked reach, except such as is 
 due to the increase in the discharge of the stream 
 consequent on the absorption and evaporation being less 
 than before, owing to the reduced area of flooding in the 
 embanked reach. In the case of a long-continued rise, 
 such as that just mentioned, it is the reach immediately 
 upstream of the embanked reach which will, to some 
 extent, share in the increased height of the floods. 
 
 An embankment may suitably have side slopes of 4 
 to 1 on the river side and 3 to 1 on the land side, with a 
 top 10 feet wide and 3 feet above high flood-level. On 
 the Irrawaddy the top width is generally 8 feet. For 
 very high and very low embankments it is 10 feet and 
 3 feet respectively. In Holland 1 foot above high flood- 
 level was at one time supposed to be the rule, but in 
 practice it was usually 4 feet. With sandy soil the 
 river ward slope prescribed was 6 to 1. Such flat slopes 
 are not necessary if fascining or stiff soil is used as a 
 protection. On the Khine the top width of embank- 
 ments consisting of gravel and sand has been made 
 about 15 feet, but the side slopes were 1^ to 1 and 1 to 
 1. The embankments had spurs to keep off the current. 
 
 Sand, protected as above, makes a good embankment, 
 and rats do not burrow into it. Of course, if a breach 
 occurs in an embankment consisting mainly of sand, it 
 will enlarge very quickly. In some cases an embank- 
 ment has a core wall of sand or of clay puddle. In 
 
160 RIVER AND CANAL ENGINEERING 
 
 Holland, on sandy soil, a trench 8 feet wide is made and 
 taken down to the clay. 
 
 Embankments require to be made with great care. 
 The earth should be deposited in layers. In Holland, 
 horses are driven up and down over each layer. In 
 some parts of India the earth for embankments is 
 brought from the borrow pits by scoops drawn by 
 bullocks. The earthwork is of so excellent a character, 
 owing to the earth being trodden down, that no settle- 
 ment has to be allowed for. Where the soil is sand the 
 top and faces of the embankment should be of good stiff 
 soil, if it can be obtained, for a thickness of 9 inches or 
 a foot, or else the face next the river should be protected 
 by fascining (CHAP. VI., Art. 3) for 2 feet above, and 
 several feet below high flood-level. Such protection 
 may be necessary in any case where waves are liable to 
 occur. In Holland embankments are turfed, and trees 
 and shrubs are not allowed to grow. In the Punjab the 
 growth of all kinds of jungle is encouraged. It binds 
 the soil together and protects it from the wash of waves 
 and from winds which blow away sand and dust, and so 
 wear the embankment slowly away. 
 
 In embanking a long reach of a river it is convenient 
 to begin from the upstream end, because otherwise floods 
 may get behind the finished part of the embankment 
 and, becoming impounded in a " pocket " formed by the 
 embankment and high land, rise to an abnormal height 
 and, unless gaps in the embankment have been left or 
 are subsequently made, cause breaches. 
 
 During high floods pegs should be driven in at 
 frequent intervals, to mark the high flood-levels. If 
 a higher flood occurs, the peg is shifted. The levels of 
 the pegs can be observed at leisure. 
 
DRAINAGE AND FLOODS 161 
 
 When a breach occurs in an embankment, the first 
 thing to do is to protect the ends so that the breach 
 shall not lengthen. If the water passing through a 
 breach becomes pocketed, the embankment may have to 
 be cut to let it out. 
 
 Kegarding the stoppage of leakages, see CHAP. IX., 
 Art. 1. Regarding the closure of breaches, see CHAP. 
 VII. , Art. 2. 
 
 For a description of flood embankments along the 
 great shifting rivers of Northern India, see Punjab 
 Rivers and Works. 
 
 Note to Art. 5. Floods can sometimes be mitigated by sinking pits 
 in the flooded area so that the flood water comes in contact with 
 permeable strata and is absorbed by them. 
 
 M 
 
CHAPTER XIII 
 
 RESERVOIRS AND DAMS 
 
 1. Reservoirs. The object of a reservoir is to store 
 water for town supply or for irrigation or other 
 purposes. Reservoirs for the water supply of towns are 
 divided into "impounding reservoirs" and "service 
 reservoirs," the latter being of comparatively small size, 
 and their object being to store, near to the town, a 
 supply sufficient for a short period. Instead of one 
 impounding reservoir there may be several, formed by 
 various dams and one discharging into another. When 
 a reservoir is mentioned without qualification, an im- 
 pounding reservoir is meant. A reservoir is generally 
 ) made by blocking up a valley by means of a dam of 
 ? ( earth or masonry. The site of the dam should be 
 selected at a place where the valley is narrow. The 
 lowest portion or " bottom water " of a reservoir is 
 usually not drawn upon, because it is less pure than 
 the rest, and it has to be left, in dry weather, for the 
 fish. It is not included in calculating the capacity of 
 the reservoir. 
 
 In Great Britain, when the water of a stream is 
 impounded, "compensation water" has to be given 
 back to the stream lower down. This compensation 
 water is generally given in the form of a constant 
 
 162 
 
RESERVOIRS AND DAMS 163 
 
 supply, and amounts to perhaps a quarter of the 
 available supply. It has to be included in calculating 
 the daily supply taken out of the reservoir. The 
 advantage to the stream in having this addition to it 
 during .dry weather is very great. 
 
 It has already been seen (CHAP. IX., Art. 1) that 
 in an earthen bank which has to retain water the 
 leakage generally decreases rapidly and the bank 
 becomes almost impermeable. The same is true of the 
 surface of a valley, in the case of most ordinary soils, 
 provided that it is kept submerged. Any portions 
 which become exposed to the sun and weather are likely 
 to crack and give rise to percolation. Thus a reservoir 
 formed by the construction of a dam resting on the 
 surface of the ground may be more or less water-tight 
 according to circumstances. There are many which are 
 sufficiently water-tight. But in most cases the dam 
 or an impervious core-wall is carried down to an 
 impervious stratum. A masonry dam is carried down 
 to rock. 
 
 In the case of dams of considerable height the soil 
 should be examined by borings. If there is an inclined 
 stratum not well connected with that below it, unequal 
 settlement of the dam may occur ; and this may also 
 happen if there is a thick stratum of clay, owing to 
 its compressibility. 
 
 Except for very high dams those, for instance, more 
 than 110 or 120 feet in height measured from the 
 ground to the water-level an earthen dam is cheaper 
 than a masonry dam. It is also more easily raised and 
 strengthened though this operation has also been 
 effected on masonry dams in case, for instance, of the 
 silting up of the reservoir, a process which is slow in 
 
 M2 
 
164 RIVER AND CANAL ENGINEERING 
 
 England, but not so slow when water containing much 
 silt is dealt with, as in the case of irrigation reservoirs 
 \ in India. Sometimes a dam consists of a wall of 
 masonry or concrete with earth behind it as a support. 
 Whatever kind of dam is used, its construction always 
 demands very great care. Serious disasters, with much 
 loss of life, have occurred owing to failures of dams. 
 
 A reservoir with an earthen dam is provided with 
 a waste weir for the purpose of passing off flood water, 
 which might otherwise overtop the dam and destroy it. 
 Generally the waste weir is a continuation of the line of 
 the dam. Its crest has to be below the high- water level 
 of the reservoir, but not lower than can be helped, and 
 its length has therefore to be considerable. Sometimes it 
 is provided with grooved piers between which planks are 
 placed in the season when floods are not likely to occur. 
 
 In connection with irrigation reservoirs in Western 
 India, it has been pointed out by Strange (Min. Proc. 
 Inst. C.E., vol. cxxxii.) that a long high-level waste 
 weir is best suited to cases in which the replenishment 
 of the reservoir is uncertain, and that in cases where it 
 is nearly certain, the high-level weir prevents the water- 
 level in the reservoir being quickly lowered in the case 
 of an accident or for the purpose of effecting repairs, 
 * impounds the earliest floods, which are most charged 
 with silt, and causes the water area to be a maximum, 
 and therefore gives all floods the maximum time in 
 which to deposit silt. He accordingly suggests that 
 the crests of waste weirs in these reservoirs should be 
 shortened and lowered and provided with falling shutters 
 {this had been done in one reservoir and has since been 
 done in another), and that sluices be added with sills at 
 a still lower level than the lowered crest. These pro- 
 
RESERVOIRS AND DAMS 165 
 
 posals seem to be entirely reasonable, though of course 
 it would be necessary to have skilled supervision over 
 the working of the sluices. Sometimes the waste weir 
 is made in a separate place, being separated from the 
 dam by a saddle. 
 
 A masonry dam may act as its own waste weir, the 
 flood water flowing over the crest and down the rear 
 slope ; but in cases where heavy floods are liable to occur 
 it is usual to provide a separate waste weir by cutting 
 away the side of the gorge either close to the dam or at 
 some other place. 
 
 While a dam is in course of construction arrangements 
 must be made to deal with flood water. Generally the 
 construction of some part of the dam has to be deferred 
 to let the water pass. In the case of a masonry dam 
 it does not much matter what part is thus deferred 
 provided the usual procedure of stepping the work back 
 is followed. In the case of an earthen dam it is best to 
 defer a portion, not in the lowest ground where the dam 
 is highest, but to one side of it, thus allowing the highest 
 part of the dam to be brought up continuously. Tem- 
 porary embankments and weirs can be constructed to 
 cause the water to traverse the desired route without 
 doing damage. Stepping of the earthwork should be 
 avoided as far as possible. If it has to be adopted, the 
 steps should be small. Sometimes the flood water is 
 conveyed away by means of a " by- wash," by an 
 entirely different route. 
 
 In Indian reservoirs the discharge over the waste weir 
 may at times be great. The waste weir is sometimes in 
 the position shown in fig. 57, a e being the weir. In 
 such a case a special hydraulic problem arises. In a 
 case where a stream whose velocity is V issues from a 
 
166 
 
 RIVER AND CANAL ENGINEERING 
 
 reservoir or takes off at right angles from a larger stream 
 there is (Hydraulics, CHAP. IL, Arts. 19 and 20} a fall 
 
 V 2 
 
 in the water of about . The same thing occurs down- 
 
 2<7 
 
 stream of a weir, at least when there is a clear fall which 
 is vertical or nearly so, so that the water after falling 
 has no horizontal velocity. The water has to be started 
 afresh on its course. In the case represented by the 
 figure, the width of the channel is often restricted 
 because of high ground beyond /, and the velocity in 
 
 FIG. 57. 
 
 the channel may be very high. Suppose the channel 
 below ef to be of brickwork with vertical sides, and to 
 have a 20-foot bed, a slope of 1 in 500, and a depth of 
 water of 10 feet. The velocity may be 15 feet per 
 
 V 2 . 
 
 second, and ^- is 3 '49 feet. If the water has a clear fall 
 *9 
 
 over the weir at e, allowance must be made for a depth 
 of water of 13 '49 feet, not 10 feet, in the channel at e. 
 Ordinarily the length a e will be much greater, relatively 
 to ef, than shown in the figure. Suppose that a eis 300 
 feet and that the slope of the floor of the channel is 
 carried on at 1 in 500 from e f up to a, b, c, and d, 
 
RESERVOIRS AND DAMS 167 
 
 following in each case the lines marked on the figure which 
 represent the directions of flow. The length fa will be 
 about 310 feet, and the floor level at a will be about '62 
 feet higher than at ef. The water-levels below the weir 
 will be in each case 13'49 feet above the floor. This 
 should be allowed for in the design. It is true that the 
 stream on first starting into horizontal motion below the 
 weir moves more or less at right angles to it, and has 
 thus a large sectional area and a low velocity ; but it very 
 soon has to turn parallel to the weir and acquire the full 
 velocity of 15 feet per second, and there must be the 
 requisite extra head to give this velocity. If the weir is 
 drowned, the water on passing over it may have a high 
 horizontal velocity, but it will be at right angles to the 
 axis of the channel, and its effect will be wasted in eddies. 
 2. Capacity of Reservoirs. A reservoir depends 
 for its supply on the yield of a particular valley or 
 valleys which form its catchment area, and the capacity 
 of the reservoir or reservoirs can be altered by altering 
 the height or number of the dams. The need for a 
 reservoir is entirely owing to the inequality in the 
 distribution of the rainfall. If the rain fell in equal 
 quantities week by week, the daily fluctuations could 
 probably be equalised by the service reservoirs. The 
 impounding reservoir could be quite small. Actually, a 
 reservoir is needed to " equalise " the flow that is, to 
 give a steady flow for an intermittent one. The smaller 
 the reservoir, the sooner it will go dry in a drought and 
 the sooner it will overflow in wet weather and cause 
 waste of the water. In other words, the larger the 
 reservoir the better it will fulfil its function of equalis- 
 ing the flow and the greater the degree to which the 
 catchment area will be utilised. 
 
168 
 
 RIVER AND CANAL ENGINEERING 
 
 In the British Isles the distribution of the rainfall 
 which is most trying for a reservoir, occurs when the 
 rain is heavy during the winter and very light in 
 summer. Fig. 58 shows a diagram for a reservoir in 
 the driest year, when the rainfall is (CHAP. II. , Art. 1) 
 '63 of the mean annual fall. The distribution of the 
 fall is supposed to be unfavourable as just described. 
 The lower part of the figure shows the water-level at 
 the end of each month, the reservoir being supposed to 
 have vertical sides so that the quantity of water in it is 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 1 1 1 1 
 
 
 
 
 
 im 
 
 FIG. 58. 
 
 X 
 ./> o 
 
 proportional to the depth of water. The upper part of 
 the figure shows the water impounded (available fall 
 multiplied by area of catchment) in full lines, and the 
 consumption in a dotted line. The distance between 
 the two lines in any month is the same as the rise or 
 fall of the reservoir in that month. There is supposed 
 to be no overflow, and the total consumption of water in 
 the year is equal to the quantity impounded in the year, 
 so that the levels of the reservoir water surface on 1st 
 January and 31st December, as shown by the horizontal 
 lines A, B at the left and right of the figure, are the 
 same. Deacon, who has investigated the subject, has 
 
EESERVOIRS AND DAMS 169 
 
 found (Ency. Brit., Tenth Edition, vol. 33, "Water 
 Supply ") that, in order to satisfy the above conditions, 
 the capacity of the reservoir must be 30 per cent, of the 
 water impounded during the year, or about 110 days' 
 consumption. On 1st January the reservoir must be 
 about two-thirds full. At the end of February it is 
 ready to overflow. At the end of August it is just 
 becoming dry. The daily consumption is supposed to 
 be steady throughout the year. 
 
 As an instance, suppose the catchment area to be 
 1000 acres, the mean annual fall 60 inches, with a loss 
 from evaporation and absorption of 14 inches. The 
 available rainfall of the year is (see last column of table 
 below) 23 '8 inches, or T983 feet. The water impounded 
 and consumed during the year is 1000 x 43,560 x 1-983 
 x 6-25 = 539,962,000 gallons. The reservoir capacity 
 must be T %ths of this, or 161,988,600 gallons. This is 
 represented by the height C E. If the mean available 
 rainfall in January and February is 6 '3 inches, or 
 '525 feet, the water impounded during those months is 
 1000 x 43,560 x '525 x 6-25 = 143,931,000 gallons, and 
 
 539,962,000 
 the consumption is - fi - = 89,993,667 gallons. 
 
 The difference, 53,937,333 gallons, represents the addition 
 A C, to the reservoir. Similarly, the light summer 
 rainfall causes the depletion A E, and the heavy rainfall 
 in the last four months of the year the addition E B. 
 If the height of the reservoir above A B were less than 
 A C, there would be overflow at the end of February ; and 
 if the depth below A B were less than A E, the reservoir 
 would go dry before the drought ended. If the capacity 
 of the reservoir were increased either at the top or 
 bottom, the cost would be increased and nothing would 
 
170 
 
 RIVER AND CANAL ENGINEERING 
 
 \ 
 
 be gained. It is not meant that the highest and lowest 
 
 O o 
 
 levels of any reservoir designed as above would always, 
 in the driest year, exactly correspond with the points of 
 overflow and going dry, but they would do so nearly. 
 Deacon states that such a reservoir would fail only once 
 in fifty years, and then only for a short time. 
 
 The reservoir considered above does not, as already 
 remarked, fully utilise the yield of the catchment area. 
 In a wetter year there would be overflow and the yield 
 from the reservoir would not be much increased. In 
 order to equalise the flow of the two driest years the 
 capacity of the reservoir must be increased, its yield 
 being also increased, and so on for larger groups of 
 years. By collecting information for large numbers of 
 places in the British Isles, Deacon has prepared 
 diagrams and tables which show the capacities and 
 yields of reservoirs. The following table gives .the 
 figures for the case where the rainfall is 60 inches and 
 the loss by evaporation and absorption 14 inches : 
 
 ^ ^ 
 e 3 .j 
 
 -* 
 
 > <D 
 
 Gallons. 
 166,000,000 
 258,000,000 
 329,000,000 
 390,000,000 
 441,000,000 
 487,000,000 
 
 Gallons. 
 1,475,000 
 1,815,000 
 1,987,000 
 2,103,000 
 2,187,000 
 2,255,000 
 
 113 
 142 
 165 
 190 
 201 
 216 
 
 63 
 
 72 
 '77 
 80 
 82 
 835 
 
 Inches. 
 23-8 
 29-2 
 32'2 
 34-0 
 35-2 
 36-1 
 
 The figures in the fifth column are those given in 
 
RESERVOIRS AND DAMS 
 
 171 
 
 CHAP. II., Art. 1. The figures in the last column show 
 the corresponding available falls, after deducting the loss 
 of 14 inches. It will be seen that, owing to this deduc- 
 tion, the available falls for the shorter periods are reduced 
 in a greater ratio than the figures in the fifth column. 
 
 In arranging for the supply of towns in the British 
 Isles it is usual to design the reservoirs so as to equalise 
 the flow of the three driest consecutive years. Exist- 
 ing reservoirs, o]d and new, usually contain from 140 
 to 170 days' supply, but some contain less. The above 
 table shows that for the assumed fall of 60 inches and 
 loss of 14 inches, the capacity of a reservoir*, to allow 
 for a six-year dry period, has to be 49 per cent, more 
 than for a three-year dry period, while the daily supply 
 from it is only 13 per cent, greater. 
 
 The following statement gives Deacon's figures for 
 mean annual rainfalls ranging from 30 to 1(10 inches. 
 The columns marked R show the reservoir capacities 
 in millions of gallons, and those marked S the daily 
 yields of the reservoirs in thousands of gallons. The 
 figures for other falls can be interpolated. For a fall 
 of, for instance, 50 inches, the figures, whether of R 
 or S, are practically a mean between those for falls of 
 40 and 60 inches. 
 
 Number of 
 
 F-30 
 
 F-40 
 
 F 60 
 
 F-100 
 
 Years whose 
 
 
 
 
 
 Supply is to 
 
 
 
 
 
 
 
 
 
 
 
 
 
 be Equalised. 
 
 R. 
 
 S. 
 
 R. 
 
 S. 
 
 R. 
 
 S. 
 
 R. 
 
 S. 
 
 1 
 
 35 
 
 300 
 
 79 
 
 695 
 
 166 
 
 1475 
 
 345 
 
 3040 
 
 2 
 
 85 
 
 470 
 
 140 
 
 900 
 
 258 
 
 1815 
 
 495 
 
 3600 
 
 3 
 
 120 
 
 560 
 
 190 
 
 1050 
 
 329 
 
 1987 
 
 610 
 
 3900 
 
 4 
 
 150 
 
 620 
 
 230 
 
 1110 
 
 390 
 
 2103 
 
 710 
 
 4100 
 
 5 
 
 175 
 
 650 
 
 260 
 
 1170 
 
 441 
 
 2187 
 
 800 
 
 4230 
 
 6 
 
 195 
 
 680 
 
 290 
 
 1220 
 
 487 
 
 2255 
 
 887 
 
 4320 
 
172 RIYER AND CANAL ENGINEERING 
 
 In all cases the loss is supposed to be 14 inches 
 annually. If it is 15 or 13 inches, the reservoir 
 capacity is less or more by about five, ten, or fifteen 
 million gallons, according as the number of years in 
 column 1 is 1, 3, or 6. Arid the daily yield is less or 
 more by about 50,000 gallons. 
 
 With a low rainfall the advantage of a large reservoir 
 is somewhat increased. The capacity of the six-year 
 reservoir for a fall of 30 inches is 63 per cent, more 
 than that of the three-year reservoir, but the supply is 
 22 per cent, greater. 
 
 The figures given above for reservoir capacities are 
 suitable for the British Isles. They assume that the 
 distribution of the rainfall is the least favourable that 
 is at all likely to occur. Deacon states that the figures 
 do not relieve the engineer of the exercise of judgment. 
 As regards the British Isles, the chief questions on 
 which judgment has to be exercised are whether to 
 equalise the flow of three years or of another number, 
 and how much to allow for loss. As already stated, 
 three years is the period usually taken. The figures are 
 suitable for most places in Europe, but in some places, e.g. 
 on the Mediterranean coast, the distribution of the rain- 
 fall is somewhat less favourable than in the British Isles. 
 In other parts of the world, and notably in or near the 
 tropics, the distribution of the rainfall must be specially 
 studied, and a diagram be prepared on the same 
 principle as in the case of fig. 58. The diagram should 
 be extended to cover the desired number of years. In 
 hot countries loss by evaporation from the surface of 
 the reservoir should be allowed for. In India during 
 the hot dry months this loss may be half an inch in 
 twenty-four hours. 
 
RESERVOIRS AND DAMS 173 
 
 In the article above quoted it is shown that if, as 
 commonly happens, the consumption of water is, in 
 summer, greater than the mean, and in winter less, 
 the conditions are still more trying for the reservoir ; 
 and that, in the case where the summer consumption is 
 13 per cent, greater than the mean, the capacity of the 
 reservoir which impounds the water of the driest year 
 must be 33 per cent., instead of 30 per cent., of the 
 total supply impounded during the year. It would then 
 contain 121 days' instead of 110 days' mean supply. 
 In the table on page 170 the number of days' supply is 
 113. From this it appears that the tables from which 
 extracts have been given are calculated on the basis 
 of a constant consumption. This, however, in the 
 case where the number of years whose supply is 
 equalised is greater than one, makes, owing to the 
 increased size of the reservoir, no practical difference. 
 
 The calculations for the great reservoirs in Radnor- 
 shire for the supply of the city of Birmingham are as 
 follows (Min. Proc. lust. C.E '., vol. clxxx.). The ratio 
 of the mean fall in the three driest years to the mean 
 annual fall was taken as "80 instead of 77. There is 
 some difference of opinion as to the best figure : 
 
 Mean annual fall determined from readings 
 
 of various gauges . . .65 inches 
 
 Mean fall of three driest years . ... .' 52 ,, 
 
 Deduct loss from evaporation and absorption 
 
 and losses during floods . . 15 ,, 
 
 Available rainfall . . 37 
 
 This multiplied by 44,000 acres, the area of the 
 catchment, gives 102 million gallons per day. Of this, 
 27,000,000 gallons is compensation water, leaving 
 
174 
 
 RIVER AND CANAL ENGINEERING 
 
 75,000,000 gallons for Birmingham. Capacity of 
 reservoirs, 17,250,000,000 gallons, or 169 days' supply. 
 3. Earthen Dams. Before an earthen dam is made, 
 any soft soil on the site should be removed and the 
 ground downstream of the site should be drained. A 
 
 FIG. 59. 
 
 few trenches, running parallel to the axis of the dam, 
 can be dug so as to give the dam a hold, though there 
 is never any danger of its being moved horizontally 
 by the thrust of the water. If the ground has a side- 
 long slope it should be benched as shown in fig. 59. 
 The front slope of an earthen dam is generally about 
 
 FIG. 60. 
 
 3 to 1, and the rear slope about 2 to 1. . The top has 
 a width of ^ to ^ the greatest depth of water held up, 
 and is 5 to 10 feet above the highest water-level. The 
 borrow pits from which the earth for the dam is got 
 should not be near enough to it to in any way affect 
 its stability. 
 
 In England, and generally in other countries, an 
 earthen dam has a core- wall (fig. 60) which is carried 
 
RESERVOIRS AND DAMS 175 
 
 down to an impervious stratum, and is keyed into it 
 to a depth of a foot or more in the case of hard rock 
 and several feet in the case of clay. On this core- wall 
 the impermeability of the dam chiefly depends. The 
 core- wall may be of clay puddle, concrete, or masonry. 
 In England it is generally of clay puddle. The core- 
 wall sometimes extends down to a depth of 100 or 200 
 feet. Its top is horizontal and about level with the 
 highest water-level. It is desirable not to make the 
 foundation stepped, but to let it follow the profile of 
 the impervious stratum. The wall is keyed at its ends 
 into the sides of the valley or gorge. A core-wall of 
 concrete or masonry is, in a high dam, necessarily a 
 comparatively thin structure, and it may be subjected 
 to great strains by unequal pressures of the earth which 
 surrounds it. It is therefore to some extent liable to 
 crack. A core-wall of concrete used for the water-works 
 of Boston, U.S.A., is 100 feet high, 8 feet thick at the 
 base, and 4 feet thick at the top. A clay-puddle wall, 
 being plastic and moist, at least during the period 
 immediately succeeding the construction of the work, 
 is not very liable to crack. The top width of a puddle 
 wall may be 4 to 10 feet, and the batter of the sides 
 from 1 in 20 to 1 in 8. The clay used for the wall 
 above the ground-level should contain about 33 per 
 cent, of sand and stones. This diminishes its shrinkage 
 if it dries. It should not be given too much water in 
 mixing. It should be thoroughly mixed and worked up 
 and trodden down. 
 
 The clay puddle and the earth of the dam should be 
 carried up uniformly. The allowance for settlement 
 may be ^ to ^. The earth should be deposited in 
 thin layers, moistened and rammed, and all clods broken. 
 
176 RIVER AND CANAL ENGINEERING 
 
 In India and some other countries, instead of the earth 
 being rammed, cattle or sheep are driven over it 
 repeatedly. This makes earthwork of most excellent 
 quality, and the settlement, if any, is very small. 
 
 In cases where, owing to a fissure in the rock below 
 the bottom of the puddle trench, water comes through 
 under the puddle, it is usual to carry it away in a pipe 
 running vertically in a groove up the side of the trench 
 and then horizontally til] it emerges from the dam. 
 Such water, and any other leakage, can often in Great 
 Britain be used as part of the compensation water. 
 There is, however, a certain chance, when there are 
 water-bearing fissures in the rock below the bottom of 
 the trench, that some percolating stream of water may 
 wash away the puddle, and it is preferable to use a 
 concrete core-wall in such cases, carrying it up to about 
 ground-level and keying it into a much thicker wall of 
 puddle which is carried up to the water-level. 
 
 It has been suggested that the clay puddle or other 
 impervious layer should be placed, not vertically and 
 in the middle of the dam, but lying on the upstream 
 face of the dam, so as to keep out water from the whole 
 dam instead of from only half of it. Objections to this, 
 if clay puddle is used, are that vermin may bore holes 
 in it, and that, with some clays, it would slip. These 
 objections might be overcome to some extent by laying 
 a pitching of concrete blocks over the puddle. Other 
 objections, applying also when masonry or concrete is 
 used, are that the superficial area and cost are increased, 
 and that cracks would occur from settlement of the 
 earth and from changes of temperature when the water 
 in the reservoir was low. A good many cases have 
 occurred in which an impervious layer laid on the 
 
RESERVOIRS AND DAMS 177 
 
 slopes has failed from one cause or another. In France 
 it is usual to rely on such a layer concrete is used 
 and to dispense with a core-wall. The practice of 
 having a vertical wall appears to be the best, and is 
 the most widely adopted. When puddle is used the 
 weight of the mass above it forces it to completely 
 fill the trench, and when once it is in position and 
 covered up it is not at all likely to be damaged. 
 
 The outer portion of a high embankment sometimes 
 slips (fig. 61), and precautions should be taken against 
 this. A slip may occur if the site of the dam has not 
 been carefully selected as to geological formation, or if 
 
 \\\\\\\\\\\\\\\\\\\\\\\\^ 
 
 FIG. 61. 
 
 there is unequal settlement owing to the work having 
 been done at different times. One cause of slips is 
 sudden and partial changes in the degree of saturation, 
 and another cause is excessive saturation. Some clays 
 when wet require extremely flat side slopes, and will not 
 stand even at 5 to 1. The outer parts of the embank- 
 ment are not required for stopping percolation (this 
 will be further considered in the following paragraph), 
 and, though they must be carefully laid and consolidated, 
 they should be of porous material, and the part on the 
 downstream side of the dam must be well drained. A 
 series of surface drains may be arranged and filled with 
 loose stone and gravel. There is also a distinct advantage 
 in using heavy material such as small stone for the lowest 
 portion of the outer parts on both sides of the dam. 
 
 N 
 
178 RIVER AND CANAL ENGINEERING 
 
 When good material cannot be obtained, the side slope 
 on the downstream side of the dam may be flattened. 
 A side slope starting at the top with 3 to 1 and 
 becoming 4 to 1 lower down, and finally 5 to 1 at the 
 base, is a very good form for prevention of slipping and 
 generally for the safety of a dam. The part on the side 
 next the reservoir is not likely to slip. It becomes 
 soaked, but it has the pressure of the water against it 
 and is pitched. In Madras, where reservoirs are very 
 numerous, the slope on the side next the water is 
 generally only 1^ to I. 
 
 The different parts of an earthen dam fulfil two 
 distinct functions. Some parts, which may be called 
 the staunches, have to stop the percolation of water from 
 the reservoirs. Other parts, which may be called the 
 supports, have merely to hold up the staunches. In the 
 British type of dam the portion nearest the core-wall on 
 either side (fig. 60) is generally made of earth specially 
 selected for impermeability. The distance to which it 
 extends from the wall depends partly on the quantity 
 of such earth available. In any case it has to be very 
 carefully made and consolidated, to avoid unequal 
 pressures on the core-wall, or unequal settlement which 
 might cause it to part from the wall. One of its 
 functions is to keep the core- wall moist when the water- 
 level in the reservoir falls. Whether it is also to be 
 considered as a staunch or a support might at first 
 appear to be of no consequence, but it is of importance 
 as affecting the question of drainage. The support on 
 the downstream side of the dam must, as has just been 
 seen, be made of porous material and be well drained ; 
 but obviously a staunch must not be porous, nor can it 
 be penetrated by drains. The question must be decided 
 
RESERVOIRS AND DAMS 179 
 
 in each case according to judgment. In a discussion 
 which took place on the above-mentioned paper by 
 Strange, at the Institution of Civil Engineers, much 
 diversity of opinion was expressed among eminent 
 engineers as to the desirability of draining the down- 
 stream half of the dam, i.e. the part downstream of the 
 core-wall. By some it was urged that drainage is 
 necessary to lessen the chance of the earthwork slipping. 
 Others contended that any drain which penetrates the 
 dam must facilitate the percolation of water from the 
 reservoir. It is clear that some of the speakers regarded 
 the dam downstream of the core-wall as being partly 
 staunch, and some as being wholly support. If for any 
 reason there seems a chance of water leaking through 
 the core-wall, it is desirable to regard the earth-filling 
 next to it as staunch. 
 
 In Western India a kind of puddle is made by mixing 
 three parts of "black cotton soil "with two of sand. 
 The object of the puddle wall is only to prevent water 
 from finding its way along the surface of the ground. It 
 is carried down only to a fairly water-tight stratum and 
 is carried up only to 1 foot above the ground. Above 
 that the mass of the dam is made of black cotton soil as a 
 staunch, with more porous material on both sides of it. 
 
 In order to afford full protection against waves and 
 their splashes, the pitching on the upstream face of a 
 dam should extend up to a height of 5 feet, measured 
 vertically, above the highest water-level. In the case 
 of a dam in which the "fetch" or distance over which 
 the waves have been in process of formation exceeds 
 two miles, the above height should be slightly increased. 1 
 
 1 The height of a wave is supposed to be r4N/fetch, but this allows 
 nothing for splashing. 
 
 N 2 
 
180 RIVER AND CANAL ENGINEERING 
 
 The pitching is usually of stones roughly squared 
 at their outer ends and laid on a layer of broken 
 stones. 
 
 The water from a reservoir is usually drawn off by 
 means of pipes which are laid inside a masonry culvert 
 built under the dam. The pipes can thus be inspected. 
 The culvert is blocked at its upstream end by a thick 
 masonry wall through which the pipes pass. Accidents 
 which have happened in the past have been due to 
 weakness of the culvert or to water finding its way 
 along the outside of the masonry. The culvert can be 
 made of proper strength, and it should have a thick 
 coating of clay puddle which is worked into the clay- 
 puddle core-wall of the dam. If the core-wall is of 
 masonry or concrete, the masonry of the culvert is 
 properly joined to it. In many cases the culvert and 
 pipes are taken through a cutting or tunnel and .not 
 under the dam. 
 
 At the upstream end of the culvert there is a masonry 
 tower access to it is obtained from the top of the dam 
 by a foot-bridge, and from it valves for opening and 
 closing the pipes are worked. If the reservoir is for the 
 water supply of a town, it is arranged, by means of a 
 vertical pipe, that the draw-off can be at various levels 
 so that the surface-water can always be used. In the 
 case of some of the towers at the reservoirs whence 
 Birmingham is now supplied, the vertical pipe consists 
 of a number of steel cylinders with gun-metal faces 
 which are so accurately made that the joint is water- 
 tight when one cylinder merely stands on another. The 
 draw- off is obtained from a given level by lifting a 
 particular number of cylinders. Sometimes the tower 
 is made of reinforced concrete. When it is lofty it 
 
RESERVOIRS AND DAMS 181 
 
 should be strong enough to resist a strong wind, blowing 
 when the reservoir is empty. 
 
 4. Masonry Dams. For heights much exceeding 
 110 or 120 feet a masonry dam may be cheaper than an 
 earthen dam ; and in case a flood occurs while work is in 
 progress the masonry might suffer little injury, while 
 earthwork might be swept away completely. Masonry 
 dams are usually built of random rubble masonry with 
 faces of dressed stone. Such masonry weighs about 
 140 Ibs. per cubic foot, and is ordinarily quite safe when 
 subjected to pressures of 20 tons per square foot, but in 
 a masonry dam a high factor of safety is necessary, and 
 15 tons per square foot may be allowed. In a wall of 
 such masonry with both faces vertical, the pressure, 
 owing to the weight of the wall, will reach the above 
 limit when the wall has attained a height of about 
 220 feet. 
 
 In a masonry dam, although the masonry is always 
 of the best quality, it is a rule to calculate the dimensions 
 so as to give no tension on any part of the masonry. 
 Any crack or opening of a joint, occurring perhaps 
 before the masonry had hardened, would let in water, 
 and its pressure would tend to gradually extend the 
 crack and eventually to overturn the portion above the 
 crack. 
 
 Fig. 62 shows the upper part of a masonry dam. 
 The lines with arrows show the vertical force due to the 
 weight of the masonry above A B, the horizontal force 
 due to the water-pressure on it acting at two-thirds of 
 the depth, and the resultant of these two. In order 
 that there may be no tension on the masonry, the 
 resultant must always fall within the middle third of 
 the thickness of the dam. In order to prevent its 
 
182 
 
 RIVER AND CANAL ENGINEERING 
 
 falling outside the middle third, the downstream face 
 must be splayed out, and the splay will go on increasing 
 somewhat. Suppose, now, that the reservoir is laid dry. 
 It will be found that in the case of a dam more than 
 100 feet high the pressure due to the weight of the 
 wall alone will fall outside the middle third to the 
 upstream side of it, of course of the thickness of the 
 wall, and a slight splay must be given to the upstream 
 
 FIG. 62. 
 
 side. The vertical pressure of the water on this 
 splayed part must be taken into consideration. The 
 limit of pressure, 15 tons per square foot, may eventu- 
 ally be reached owing to the height of the dam, and 
 additional splay may have to be given for this. When 
 the outside splay becomes considerable a further 
 allowance is made for it, because the stress at the edge 
 of a horizontal section is tangential to the face. In 
 order that the tangential stress may not exceed 15 tons 
 per square foot, the vertical stress at the outer edge of 
 a horizontal section of the dam must not exceed about 
 
RESERVOIRS AND DAMS 183 
 
 12 tons. By following the above rules the section of 
 the dam can be calculated, beginning from the top and 
 working downwards. The resulting profile of the dam 
 is somewhat as shown in fig. 63. If a masonry dam is 
 designed on the principles given above that is, so as to 
 be safe as regards crushing and overturning it will be 
 safe as regards shearing or sliding horizontally, but a 
 test calculation can easily be made for this. 
 
 Calculations of the above kind do not, of course, 
 
 FIG. 63. 
 
 enable all the stresses in a solid mass of masonry to be 
 found. Great stresses are caused by expansion and 
 contraction owing to changes in temperature. Others 
 are caused by the connections of the dam with the 
 rock on which it rests and with the sides of the gorge. 
 The method of calculation described above indicates a 
 suitable form for the profile of a dam. The large factor 
 of safety adopted allows for other stresses. The 
 sections of the oldest dams, made in Spain, were some- 
 what as shown in fig. 64, and contained about twice as 
 much material as was necessary. The object of the 
 calculations is to save this needless expenditure. 
 
184 
 
 RIVER AND CANAL ENGINEERING 
 
 Masonry dams designed on the above principles have 
 been constructed for heights ranging up to nearly 
 300 feet, measured from the foundation to the top. 
 The foundation is always on hard rock free from fissures. 
 Generally a foundation trench is cut. The ends of the 
 dam are carried into the rock on the sides of the gorge. 
 They should not, however, if the sides of the gorge 
 are steep, be built in with mortar, but be allowed to 
 expand and contract vertically, a water-tight joint being 
 made by means of asphalt (Ency. Brit., Tenth Edition, 
 vol. 33, " Water Supply "). This obviously reduces the 
 
 FIG. 64. 
 
 straining. A dam should be built in cool weather, so 
 that any stresses to which it will eventually be subjected 
 owing to changes in temperature will be chiefly com- 
 pressive. The upstream face should be as water-tight 
 as possible. There should not, however, be too sudden 
 a change in the character of the masonry from the 
 face work to the inside work. If there are any 
 springs, they must be carefully connected to pipes 
 and carried outside the dam. No water must be 
 permitted to get under or inside the dam, either 
 from springs in the sides of the gorge or from the 
 water in the reservoir. Many existing dams leak 
 slightly where they join the sides of the valley, 
 
RESERVOIRS AND DAMS 185 
 
 and most have developed some vertical cracks normal 
 to the face. 
 
 Out of some hundred high masonry dams which have 
 been erected, only three are known to have failed. Of 
 these, the Puentes dam was partly founded on piles ; and 
 in two, the Habra and Bouzey dams, the rule of the 
 middle third was not attended to. Another dam, not 
 so high, the Austin dam, in Texas, U.S.A., failed seven 
 years after construction. It was 65 feet high and 
 founded on limestone, the width of the base being 
 66 feet. Springs in the bed and sides of the gorge had, 
 during the construction of the dam, given much trouble, 
 and had, after its completion, forced their way through 
 the underlying rock. At the time of failure 1 1 feet of 
 water was passing over the dam, which sheared in two 
 places, a length of 440 feet of it being pushed forward 
 for 40 or 50 feet without overturning, but subsequently 
 breaking up. The dam was founded in a trench cut in 
 the rock. The rock on the downstream side of the 
 foundation trench appears to have been worn away 
 by the water, so that there was no longer a trench 
 (Scientific American, 28th April 1900). The above, 
 however, does not seem to be sufficient to account for 
 failure. The horizontal water-pressure on a 1-foot 
 length of the dam would be 180,000 Ibs. and the 
 weight of masonry to be moved perhaps 320,000 Ibs. 
 It seems probable that water from upstream found its 
 way under the dam and exercised a lifting force on 
 it and so caused it to slide. 
 
 If a masonry dam, instead of being straight, is made 
 curved on plan, with its convexity upstream, it acts as 
 an arch, and its thickness can, in the case of a fairly 
 narrow gorge, be greatly reduced. This type of dam is 
 
186 RIVER AND CANAL ENGINEERING 
 
 a suitable one to use when the sides of the gorge are 
 of firm and solid rock and there is no doubt about 
 their being able to stand the thrust without yielding. 
 Several dams of very considerable size have recently 
 been built in this way. The thickness of the upper 
 part of the dam and the ratio of the versed-sine of the 
 arch to the span can be decided on by the methods 
 used for arches in general. The lower part of the dam 
 is made thicker. The lowest part cannot act as an 
 arch, because it is attached to the foundation. It is, 
 however, assisted by the portion above it, which acts as 
 an arch, and thus need not be so thick as in a " gravity " 
 section. The Bear Valley dam, which is 64 feet high, 
 is only 3 feet thick at the top. The thickness increases 
 gradually to 8^ feet at 48 feet from the top. The chord 
 of the curve is 250 feet and the radius of curvature 
 335 feet. If the gorge is wide, the thickness of the 
 arch comes out so great that nothing is saved by 
 adopting the curved form. But in such a case, and in 
 any case, a dam can be made slightly curved so as to 
 offer a greatly increased resistance to overturning. It 
 need not act as an arch, and can be prevented from so 
 acting, in order that excessive stresses may be avoided, 
 by letting the ends of the dam, after they have entered 
 the grooves cut in the sides of the gorge, stop short of 
 the ends of the grooves. 
 
 During the last few years much attention has been 
 given to the investigation of the stresses to which a 
 masonry dam is subjected. Some investigations have 
 been theoretical and others practical, models of india- 
 rubber and other substances having been used for 
 experiment. The investigations show that generally 
 the stresses in a model of a dam are very much the 
 
RESERVOIRS AND DAMS 
 
 187 
 
 same as would be expected, but that there is a tensile 
 stress, previously overlooked, near the point M (fig. 65), 
 where the dam rests on its foundation. The tension is 
 on the foundation, on the line M N, and is due to the 
 horizontal thrust of the water. It is natural that in 
 an elastic model this stress should manifest itself by 
 deformation. In the case of an actual dam resting on 
 rock, matters are different ; but this tensile stress deserves 
 consideration. For the present let it be supposed that 
 
 FIG. 65. 
 
 there is no trench, the dam merely standing on the rock. 
 Suppose that the rock has only the thickness M R. 
 There is tension in M N, and probably compression in 
 N K. It is assumed that, along the base M P, there is 
 perfect union between the dam and the rock. The 
 tension to which the rock is occasionally subjected 
 owing to changes of temperature may exceed any 
 tension due to the water-pressure, but it is conceivable 
 that the tension occurring from both causes might cause 
 a crack at M N, and that this might extend to E. This 
 implies a minute sliding of the dam and of the rock 
 
188 RIVER AND CANAL ENGINEERING 
 
 below it, movement taking place on the plane R Q. 
 The thrust of the water is now resisted by the rock 
 downstream of P Q. The dam, with the rock M R Q P 
 adhering to it, tends to rotate about the point P. The 
 tendency to rotate will be enhanced if water enters M R, 
 and still more if it enters R Q. No rotation can, however, 
 take place unless the rock at M R is splintered away. 
 The rock would also have to fracture at P Q. It has 
 been suggested that the upstream face of the dam be 
 made curved as shown by the dotted line. This would 
 shift the chief tension to mn, and the dam, with the 
 rock beneath it and the weight of the water above the 
 curved portion, would obviously offer an increased resist- 
 ance to rotation about P. The cost of the dam would of 
 course be increased. The danger of a crack forming at 
 M N seems to exist only when there is a thin upper 
 stratum of rock not firmly connected to rock below. 
 When this condition is believed to exist, a masonry 
 dam, if built at all, should have the upstream face curved 
 as above described. In the case of any existing dam of 
 great height, when the above condition is suspected to 
 exist, the reservoir might be laid dry, and if any crack 
 at M N is discovered a curved portion could be added ; 
 but in this case the union between the new and the old 
 work would be imperfect, and the curve should start from 
 high up on the upstream face of the dam. It has been 
 suggested that asphalt or some impervious material be 
 laid on the rock to prevent water from entering any 
 crack. It would, however, not only have to be laid 
 upstream of the dam, but to extend under part of the 
 dam, and thus weaken it to some extent. 
 
 In the case (fig. 63) in which the dam is founded 
 in a deep trench, the building up of the upstream 
 
RESERVOIRS AND DAMS 189 
 
 triangular space and uniting the material both to the 
 dam and to the side of the trench, might be of some use, 
 but a crack might form in it. It would be desirable to 
 add a curved portion, as above described, on the top of the 
 rock if sound, or to remove the unsound rock and widen 
 the trench and then add the curved portion. Adding 
 material to the downstream triangular space, and uniting 
 it well, would also increase the resistance of the dam 
 to overturning, not so much because of the additional 
 weight, as because of the raising of the point about 
 which the dam would have to revolve in overturning. 
 
 Several recent dams have been built of cyclopean 
 concrete, blocks of rock as heavy as 10 tons being 
 sometimes used in the work. Such blocks are laid on 
 one of their flat faces. In the U.S.A. some reservoirs 
 have been made with walls of reinforced concrete, backed 
 by earth embankments (Min. Proc. Inst. C.E., vol. 
 clxxxix.), and also of cyclopean masonry reinforced 
 with steel rods. Another kind of dam which has been 
 used in the U.S.A. is the rock-fill dam with a core- 
 corresponding to the puddle wall in an earthen dam 
 of steel plates riveted together and made water-tight 
 and inserted into the rock at each side. In the case of 
 the East Canyon Creek Reservoir. Morgan, Utah, the 
 dam is 1 1 feet high. The steel plates vary in thickness 
 from f-inch at the bottom to ^-inch at the top, and are 
 embedded in asphaltum concrete and rest on a concrete 
 base. The dry-stone work of the dam is hand-packed on 
 both faces, and also on both sides of the core. The rest 
 is thrown in. The upstream face is 1 to 1, and the down- 
 stream face 2 to 1. The waste weir is at one end of the 
 dam and is continued by a flume, so that the water falls 
 clear of the dam. The outlet is a tunnel in the rock. 
 
CHAPTER XIV 
 
 TIDAL WATERS AND WORKS 
 
 1. Tides. The tides or "tidal waves" are caused by 
 the attraction of the moon and the sun. The phenomena 
 are complex, and a full discussion of their causes need 
 not be given here. When the tide rises it is said to 
 "flow," and it is called the flood tide ; when it falls it 
 is called the ebb tide. The period between one tide 
 and the next, e.g. from high water to high water, is about 
 twelve hours, twenty-five minutes. At a spring tide 
 the range of the tide is greater than usual ; at a neap 
 tide less. Where there are channels, as, for instance, the 
 seas which surround the British Isles, the tidal waves 
 run up them as the tide rises in the neighbouring ocean, 
 and run back as it falls. At some places, as Southampton, 
 the tide comes in from two directions, and there is a 
 double tide. The times and levels of high and low 
 water at various places have been ascertained by 
 observation, and are recorded. The levels are, however, 
 liable to be affected by winds. A wind blowing towards 
 the shore raises the level of both high and low water ; a 
 wind blowing off shore lowers both levels. A severe 
 storm in the North Sea has caused a double tide at 
 London Docks, by accelerating the North Sea tidal 
 wave. 
 
 190 
 
TIDAL WATERS AND WORKS 191 
 
 In a funnel-shaped estuary, especially if it faces the 
 direction of the tidal wave in the sea, the tide in going 
 up the channel increases in velocity, and the momentum 
 of the water causes it to rise higher and higher as the 
 width decreases. At the upper end of the Bristol 
 Channel the range of the tide is double the range in the 
 sea outside the channel. The Bay of Fundy is another 
 place where a similar phenomenon occurs. When a 
 river or estuary is shallow and the range of the tide is 
 great, so that its rise is rapid, the flood tide in some 
 cases advances in the form of a wave or " bore," causing a 
 sudden rise in the water-level and a sudden reversal of 
 the flow of the stream. A bore is most pronounced at 
 spring tides. That of the Severn is well known. 
 
 In the case of a tide running along a coast or up an 
 estuary, the water of the flood tide, after it has ceased 
 to rise, continues for a short time, owing to its 
 momentum, to flow in the same direction as before. 
 The same thing happens when the ebb tide ceases to 
 fall. The tide also acquires special velocity, just as a 
 river does, round any projecting headland. 
 
 The rise and fall of the tide are least rapid near the 
 turns of the tide. If the time from the beginning to 
 the end of the flow be divided into six equal parts, the 
 proportional rise of the water will be approximately as 
 follows. And similarly with the fall during the ebb. 
 
 Time 1 234 56 
 
 Rise of water '067 '25 '5 75 '933 1 
 
 Tidal waters are frequently charged, more or less, 
 with silt, obtained from the shore or from shallows near 
 it, either by currents or tidal waves sweeping along it, 
 or by the action of ordinary waves. Tidal waters flow- 
 
L92 KIYER AND CANAL ENGINEERING 
 
 ing up and down the lower portions of rivers render 
 them to an enormous degree more capable of carrying 
 navigation and, especially if they become enlarged and 
 form estuaries, more capable of being altered by training 
 works. 
 
 A tide-gauge is constructed on the same principle as 
 a self-registering stream-gauge. The rise and fall of 
 the water are reduced, by mechanism, to a convenient 
 range, and are recorded on a band carried on a drum, 
 which is caused to revolve by clockwork. Another kind, 
 which depends on the use of an inverted syphon filled 
 
 FIG. 66. 
 
 with air and a syphon of mercury, is described in Min. 
 Proc. Inst. C.E., vol. clxiv. 
 
 2. Tidal Rivers. Let AB (fig. 66) be the surface 
 of the lower part or mouth of a river, supposed to be of 
 uniform width, and let B be the mean sea-level. As 
 the tide rises to D the water of the river is headed up 
 and assumes the line A D. When the tide falls to F 
 there is a draw, the river surface taking the line A F. 
 If the rise of the tide B H is so great that the discharge 
 of the river cannot keep pace with it, so as to fill up the 
 whole space between A and H to the level of H, there 
 will be a flow of sea water from H to some point M, and 
 of river water from A to M. The point M will be 
 
TIDAL WATERS AND WORKS 193 
 
 lower than A and H. If the tide now turns and the 
 water-level H begins to fall, there will still be a flow 
 along HM. For a brief period it will be due to 
 momentum, but it will continue until, by the rise of 
 the water-level at M and the fall at H, the surface has 
 assumed the form indicated by the dotted line ANJ. 
 While this is happening, the point corresponding to M 
 where the concave curve of the upland water meets 
 the convex curve of the tidal water rises higher and 
 shifts seaward. The character of the two curves remains 
 the same, but they become flatter and the surface N J 
 nearly level. 
 
 Thus the time of high tide at M is later than at H. 
 It is later for each point passed in going up the river 
 from H towards A. Eventually a point A is reached 
 where there is no tide, that is, no rise or fall. Far 
 below this point, between A and B, there is a point 
 above which there is no upward current but only a 
 slackening of the downstream flow. At H a diagram 
 showing the rise and fall of the tide is symmetrical, at N 
 the rises and falls are less than at H, and the periods 
 of their occurrence later. In going up the river the 
 duration of the flood tide decreases and that of the 
 ebb tide increases. The flood tide attains its greatest 
 velocity soon after its commencement, the ebb tide 
 towards its close. The distances to which the tidal 
 influence extend are of course greater the greater the 
 range of the tide and the flatter the slope of the river. 
 The discharge of the river of course varies. The greater 
 the discharge the more the rise of the river tends to 
 keep pace with that of the tide and the less the distance 
 to which the tidal influence extends. On a longitudinal 
 section of the river, the high-water line will be shown as 
 
 o 
 
194 
 
 RIVER AND CANAL ENGINEERING 
 
 A N H. This is merely done for convenience. It is 
 never high water at all points simultaneously. To show 
 the actual state of affairs at various stages of the tide, 
 series of lines must be drawn as in fig. 67, where the firm 
 lines show the flood, and the dotted lines the ebb tide. 
 
 The flow in the tidal reach of a river is the same as if 
 the water was alternately headed up by a movable weir 
 and then allowed to flow freely and be drawn down. 
 If the water carries silt, the tendency for deposit to 
 occur is (CHAP. V., Art. 2) no greater than if there was 
 no heading up or drawing down. The tendency depends 
 
 FIG. 67. 
 
 chiefly on whether there is, on the average, any reduction 
 in velocity or increase in depth as compared with the 
 non-tidal upstream reach, and whether the water in 
 that reach is fully charged with silt. If both the answers 
 are in the negative, no deposit due to river silt is likely 
 to occur in the tidal reach. 
 
 If the sea water is charged with silt, it will of 
 course carry silt into the river as it flows up, but the 
 whole volume of water which enters has to flow out again. 
 On the whole, the tendency for silt to be deposited in 
 the river is due only to the period of " slack tide" near 
 the time when the flow ceases. The tendency is seldom 
 marked. 
 
TIDAL WATERS AND WORKS 195 
 
 If the sea water carries silt and the river water is 
 clear, the latter assists of course in removing any deposit 
 that is, it tends to keep the channel clear. 
 
 If the river channel is soft and if the sea water carries 
 no silt, it may, in passing up and down the river, become 
 charged with silt and return to sea still carrying it. It 
 thus has a scouring effect on the channel, and may deepen 
 or widen it. If, owing, for instance, to the flattening of 
 the bed slope in its lower reaches, the river tends to 
 deposit its own silt in its tidal reach, the sea water may 
 prevent this deposit. Thus, as regards silting in the 
 tidal reach of the river, the tidal water of the sea has 
 little prejudicial effect if ib is silt-laden, and a beneficial 
 effect if it is not. Silt is likely to deposit in the tidal 
 reach of a river of uniform width, only in a case in 
 which the river water carries much silt, and the slope 
 is flat or cross-section great compared to that of the 
 upper reach. 
 
 Sea water is heavier than fresh water by about 2*4 
 per cent., and this, to some extent, prevents their mixing. 
 At all stages of the flood tide the tendency at the point 
 where the fresh water meets the salt water is for the 
 fresh water to accumulate towards the surface and the 
 sea water towards the bottom. When the tide begins 
 to flow up the river there may be a low-level salt water 
 current moving landward arid a high-level fresh water 
 current moving seaward, but this is quite a temporary 
 state of affairs. The surface slope is landward, and the 
 water moving seaward is not moving in obedience to 
 the surface slope. It is only moving as a result of 
 momentum previously acquired. The low-level current 
 may have some extra velocity and extra scouring power, 
 
 but this cannot be much, because the mean landward 
 
 o2 
 
196 RIVER AND CANAL ENGINEERING 
 
 velocity of the whole stream must, owing to the internal 
 resistances caused by the two currents, be less than it 
 would be if there were not two currents. Moreover, 
 the state of affairs is temporary. The two kinds of 
 water mix eventually, arid their temporary separation 
 has no considerable effect on the general tendency of 
 the river in the tidal reach to scour or to silt. 
 
 A body of water included at any moment between 
 any two cross-sections of the tidal portion of a river 
 may not reach the sea during the next ebb tide. In 
 this case it will flow back up the channel with the next 
 flood tide, and so be kept moving up and down, getting 
 nearer, however, to the sea at each tide. 
 
 De Franchimont has shown (Min. Proc. Inst. C.E., 
 vol. clx.) how a diagrammatic route-guide can be pre- 
 pared for any tidal river to show pilots or captains of 
 vessels the best times for starting on voyages up or 
 down the river, and for passing each point on it. 
 
 3. Works in Tidal Rivers. If any works are 
 required in the tidal portion of a river, the principles to 
 be followed in designing them are the same as if the 
 river was non- tidal. All that has been said in CHAP. 
 VIII. , Arts. 1 to 3, applies to them. The river may be 
 straightened or trained or dredged. Generally training 
 and dredging are combined. Any dredging in the 
 portion of the river nearest the sea will not, of course, 
 alter the water levels near the mouth, but it will alter 
 them further up. The tide will come up in greater 
 volume and will rise higher and extend further up. 
 The ebb will be facilitated, and the low-water level will 
 be lowered. If any narrowing of the channel near its 
 mouth is effected by training walls for the purpose of 
 lowering the bed, the effect on the volume of tidal water 
 
TIDAL WATERS AND WORKS 197 
 
 entering the river must be taken into consideration. If 
 the narrowing is confined to a reach near the mouth, and 
 if the resulting deepening is not sufficient to counteract 
 the effect of the narrowing, the volume of tidal water 
 reaching the unn arrowed portions of the channel will be 
 reduced, and this may be injurious. Its scouring action 
 may be insufficient. The proper course may be to 
 continue the narrowing upstream. If this is done, then 
 it is obvious that the width of channel in which deep 
 water is to be maintained at high water, or which is to 
 be kept free from deposit, is reduced in about the same 
 proportion as the volume of tidal water is reduced, and 
 no harm is likely to result. 
 
 Any weir or similar structure which abruptly stops 
 the flow of the tide up a river checks it of course for a 
 long distance back, perhaps to the mouth. Old London 
 Bridge used to obstruct the tide, and its removal in- 
 creased the range of the tide, and was beneficial. 
 
 Tidal rivers generally widen out to some extent near 
 their mouths, and are thus rather estuaries than rivers. 
 The works in such rivers are more fully discussed in 
 Art. 5. 
 
 4. Tidal Estuaries. If, instead of a river of uniform 
 width, there is an estuary whose width increases steadily 
 towards the sea so that it is funnel-shaped, the conditions 
 described in Art. 2 are modified. An estuary is formed 
 first by the waves of the sea, which wear away the angles 
 at the mouth of the river and allow the tide to enter in 
 greater volume, and then by the flow and ebb of the 
 tides. The slope of the bed of the estuary is usually 
 much flatter than that of the river, and the water 
 surface is as shown in fig. 67. The tidal movements 
 extend further upstream than in the case of a river, not 
 
198 RIVER AND CANAL ENGINEERING 
 
 only because of the greater difficulty experienced by 
 the upland water in filling up the wide channel of the 
 estuary, but because of the momentum of the tidal 
 water driving its way up the funnel-shaped channel 
 (Art. 1). The capacity of the estuary is of course much 
 greater than is required for the discharge of the upland 
 water alone. If the sea-level remained always at one 
 height and if the upland water contained silt, it would 
 tend to deposit in the estuary and would certainly 
 deposit in it to some extent. The action of the sea 
 water is the same as described in Art. 2, scouring if it 
 is clear when entering, of less account if it is not clear. 
 Owing to the funnel shape of the estuary, the tide rises 
 higher at its upper end than if the estuary were re- 
 placed by a river channel, and the tide also extends 
 further up. This may partly or wholly compensate for 
 the greater tendency of silt to deposit in an estuary as 
 compared with a river channel. 
 
 The ebb tide in an estuary does not always follow 
 exactly the same course as the flood tide. Of course 
 the lowest parts of the estuary are filled first and 
 emptied last, but the channels are not all continuous. 
 A channel open at its lower end may have a dead end 
 at its upper termination, and vice versa. Also, at sharp 
 bends in the channels, the momentum of the water may 
 cause differences in the paths traversed by the flowing 
 and ebbing currents. Wherever there is a deep channel 
 the water from the adjacent sandbanks tends, towards 
 the close of the ebb, to flow cross-wise into the channel, 
 and in doing this it to some extent washes down the 
 banks into the channel. 
 
 5. Works in Tidal Estuaries. Estuaries, when 
 shallow, offer great facilities for training. It used at 
 
TIDAL WATERS AND WORKS 199 
 
 one time to be said that any change which reduces the 
 volume of tidal flow must be injurious. It would be 
 injurious to restrict the mouth of the estuary, unless it 
 were exceptionally wide, and leave the rest untouched. 
 If the whole estuary is narrowed, and a suitable funnel 
 shape preserved, the width to be kept open is, relatively 
 to the size of the mouth, no greater than before, and the 
 tide may flow up as far as before, and rise to as high a 
 level. The narrowing, if properly arranged, will improve 
 the shape of the estuary and cause an increased scour. 
 The effect of the upland water is also greater in the 
 narrower channel. Improvements to estuaries are not, 
 however, restricted to training. There is always one or 
 more deep channels, and the best of these can be selected 
 and improved by dredging. The channel should be one 
 along which both the flood tide and the ebb tide will 
 run. The above remarks as to training do not apply to 
 a case in which there is a bar outside the mouth of the 
 estuary. Training might check the scour at the bar. 
 Bars are treated of in CHAP. XV. 
 
 If an estuary is not funnel-shaped, if, for instance, it 
 widens out very rapidly, the tidal flow is much less 
 effective in keeping the channel open. In this case, 
 training works, which would give the necessary funnel 
 shape, are indicated rather than dredging. If an estuary 
 is narrow at the entrance, the flow is much less power- 
 ful, unless the narrow part is of greater depth, but even 
 then the force of the tide is reduced owing to the change 
 in the shape of the channel. 
 
 The bed of an estuary may be of such soft or sandy 
 material that a dredged channel would be likely to be 
 quickly filled up again by the slipping in of material 
 at the sides (Art. 4). In such a case an untrained 
 
200 RIVER AND CANAL ENGINEERING 
 
 channel can only be kept open to its full depth by 
 constant dredging, and probably the best course is to 
 construct a trained channel, although it may be more 
 expensive than in the case of a harder channel, because 
 of the depth to which the foundations of the walls must 
 be sunk into the soft bed. Also, if the bed of the 
 estuary is constantly shifting, a dredged channel alone 
 will not succeed, and training must be resorted to. 
 Again, the bed may be of such hard material that 
 training walls would not cause it to scour. In this case 
 a channel should be dredged and need not be trained. 
 
 FIG. 68. 
 
 For the great body of intermediate cases in which the 
 deep channel can be formed either by dredging or 
 training, both methods can be adopted. A common 
 plan is to train the upper part and to dredge the lower 
 part where the estuary is wider and the training walls 
 would be more exposed to the waves. 
 
 When an estuary is thus partly trained, the deepening 
 due to the training does not extend far beyond the 
 point where the walls terminate. The deposit of 
 material along the sides of the estuary may, however, 
 extend some distance further down in places where the 
 tide can no longer have free play. This occurred in the 
 Seine estuary (fig. 68). The authorities of Havre, 
 
TIDAL WATERS AND WORKS 201 
 
 which ]ies at one side of the estuary not far from its 
 mouth, feared that if the training walls were brought 
 further down, the deposits might extend to their 
 neighbourhood. The reduction in the capacity of the 
 estuary, due to the deposits, caused it to become filled 
 up more quickly, and the time of high water at Havre 
 was advanced. The dotted lines show a good arrange- 
 ment of training walls proposed by Harcourt. 
 
 There is no doubt that it is always feasible to carry 
 training walls right through an estuary, or at least 
 down to a point where deep water is reached, and if a 
 proper funnel shape is given to the channel the reduction 
 of the tidal flow and silting up of the spaces behind the 
 walls need not cause any trouble. Training the com- 
 plete estuary was carried out in the case of the Tees, 
 where, however, the estuary was not of great length, and 
 was not of a good shape for keeping itself open. Any 
 affluents entering the estuary can be provided with 
 separate trained channels. Difficulty may, however, 
 arise if there are towns which would be shut off from 
 the estuary by the silt banks. 
 
 G-enerally the line selected for the trained or dredged 
 channel should, though it must be as short and direct 
 as possible, coincide as nearly as possible with that 
 which the water naturally tends to keep open. This 
 may be toward one side of the estuary or the other, 
 according to the direction from which the tidal wave 
 approaches. In the case of the Dee, the best line was 
 not adopted, attention having been chiefly given to the 
 question of silting up the spaces outside the walls and 
 so reclaiming land, a matter which should always be 
 treated as of quite secondary importance. Training 
 walls in estuaries are generally built only up to half- 
 
202 RIVER AND CANAL ENGINEERING 
 
 tide level. Were it Dot for the expense they might be 
 built up to high-water level. In the Seine estuary the 
 walls were made of blocks of chalk. 
 
 Whether a trained channel will keep itself open or 
 will need periodical dredging depends, of course, on the 
 amount of silt in the water and on its velocity and 
 depth. The question must be worked out and calculated 
 as in the case of a non-tidal river. 
 
 The estuary of the Mersey differs from most others. 
 Towards the mouth, near Liverpool, it is narrow and it 
 widens out further inland. The tides, running through 
 the narrow portion, to fill up the large inland basin 
 and to empty it again, keep the narrow part scoured to 
 a great depth. It was proposed to train the wide 
 portion for the Manchester Ship Canal. The training 
 would, no doubt, have succeeded, but, owing to the 
 silting up of the greater part of the estuary, the scouring 
 near Liverpool would have been very greatly reduced 
 and serious damage done to that port. 
 
CHAPTEE XV 
 
 RIVER BARS 
 
 1. Deltaic Rivers. When a river flows into a tide- 
 less sea its silt deposits and forms a shoal or bar. 
 This shoal may in time extend and rise up to the water- 
 level. The current of the river makes its way through 
 it in various directions, and in this way a delta is formed 
 and constantly extends seawards. This flattens the 
 slope of the lower portions of the river, and causes 
 raising of the bed in the reaches upstream, and this 
 again may cause the water to break out further 
 upstream and form fresh channels to the sea. The bars 
 at the mouths of deltaic rivers are generally formed 
 with great rapidity, and they are apt to form a complete 
 hindrance to navigation. They are sometimes partly 
 scoured away by floods in the river, but in this case the 
 scoured material rnay deposit on the outer slopes of the 
 bar. If a river which carries silt has no delta, it is 
 probably because there is a littoral current, which 
 prevents the silt from depositing. On the other hand, 
 if a river brings down very heavy sediment, a delta 
 may be formed even when tidal flow is not wholly 
 absent. This occurs in the case of the Ganges. 
 
 The bars at the mouths of deltaic rivers cannot usually 
 be kept down by dredging except at great expense. 
 
 203 
 
204 RIVER AND CANAL ENGINEERING 
 
 The usual method of dealing with them is to run out 
 two parallel jetties, in continuation of the river banks, 
 so as to bring the mouth of the river out to the bar. 
 The river then scours a channel through the bar and, if 
 the walls are not too far apart, the depth will probably 
 become as great as in the river and sufficient for naviga- 
 tion. The river, however, tends to at once form a new 
 bar further out. The rapidity with which the new bar 
 forms will be greater or less as the specific gravity of 
 the materials carried by the river is greater or less, and 
 as the strength of any littoral current is less or greater. 
 Clay is spread far out while sand quickly sinks. All 
 deposits are, however, swept away if there is a strong 
 littoral current. The steeper the slope of the bed of the 
 sea away from the bar the longer the new deposit will 
 take in forming a fresh bar. Also the less the discharge 
 of the river the less the deposit will be. The branch of 
 a deltaic river selected for improvement by having the 
 bar at its mouth removed, should be one which has a 
 small discharge and whose mouth is in a position where 
 there is a strong littoral current. In the case of the 
 
 o 
 
 Khone, the branch selected was the eastern one, whose 
 mouth was not exposed to any littoral current. More- 
 over, the other branches of the river were closed, and 
 this increased the discharge of the branch which was left 
 open. The work did not succeed. In other cases, the 
 parallel jetty method has succeeded, and notably in the 
 case of the Mississippi. In this case willow mattresses 
 weighted with stones were used. The question of keep- 
 ing down the discharge does not, however, appear to 
 have always received sufficient attention. In the case 
 of the Mississippi the "South pass" was selected for 
 improvement. In order to remove a shoal its upper end 
 
RIVER BARS 205 
 
 was narrowed and its discharge reduced. The upper 
 ends of the other " passes " were then obstructed so as 
 to restore the discharges of all the passes to their former 
 amounts. The wisdom of this step is questionable. It 
 is desirable to keep down the discharge of the branch 
 which is to be improved to the lowest limit consistent 
 with free navigation. 
 
 If the width of the river near its mouth is greater 
 than is desirable for the width between the jetties, the 
 latter are sometimes made to converge though their 
 outer ends are made parallel. 
 
 In the case of the Mississippi the jetties were made 
 with a slight curve to the right. It would seem 
 desirable always to make the jetties with quite a con- 
 siderable curve. The jetty which was convex to the 
 channel could then probably be shortened. In a case 
 where there is a littoral current, say to the right, the 
 curve of the jetties could be to the right, so that the 
 stream on issuing would tend to merge into the current 
 and assist it. 
 
 2. Other Riv6rs. It often happens that the materials 
 sand, gravel, and shingle of which a sea beach is 
 composed shift gradually along the shore. This is 
 known as " littoral drift." It is by some supposed to be 
 due to the action of the tides, and by others to the action 
 of waves, the drift taking place in the direction of the 
 prevailing winds, excluding those which are off shore. 
 The latter cause is the more probable. 
 
 Most rivers have bars at their mouths. In the case 
 of deltaic rivers the bar, as already stated, is caused 
 by the heavy silt carried by the river, though it may 
 be assisted by littoral drift. In the case of rion-deltaic 
 rivers flowing into tideless seas, the quantity of silt is 
 
206 RIYER AND CANAL ENGINEERING 
 
 not enough to form a bar, and the same is generally 
 true in the case of tidal rivers where the volume of tidal 
 water is usually much greater than that of the upland 
 water. In both these classes of rivers the formation 
 of the bars is due chiefly to littoral drift or to sediment 
 brought in by the sea water. The bar, as in the case 
 of deltaic rivers, may be partly scoured away by a flood 
 in the river, and the scoured material may deposit on 
 the seaward slope of the bar. Generally, the navigation 
 channel across a bar of this kind can be kept sufficiently 
 deep by dredging, but sometimes jetties, like those 
 mentioned in the preceding article, have been constructed, 
 and in this case there is the great advantage that the 
 bar is not liable to form further out. If littoral drift 
 tends to accumulate, the jetties, or at least the one on 
 the side whence the drift comes, can be lengthened. 
 This was done, as mentioned by Harcourt (Rivers and 
 Canals, CHAP. IX.), in the case of the rivers Chicago, 
 Buffalo, and Oswego, which flow into the Great Lakes 
 of America. The same writer states that the jetties at 
 the Swine mouth of the tideless rivei* Oder were made 
 to curve to the left, the convex or left-hand jetty being 
 the shorter, but that this exposed the mouth to littoral 
 drift coming from the left. The river, upstream of the 
 jetties, had a slight curve towards the left, but this 
 could have been corrected or, at all events, the jetties 
 made to curve to the right. 
 
 A case (fig. 69) where parallel jetties were recently 
 constructed in a tidal sea is that of the mouth of the 
 Richmond River, New South Wales (Min. Proc. Inst. 
 C.E., vol. clx.). 
 
 In the case of a bar at the mouth of an estuary, 
 parallel jetties would be too far apart. In such cases 
 
RIVER BARS 
 
 207 
 
 converging breakwaters (fig. 70) are sometimes made, 
 especially if the tidal capacity of the estuary is small. 
 The entrance is generally 1000 to 2500 feet wide. If 
 
 made narrow, it would reduce the tidal flow too much. 
 The space inside the breakwaters adds to the tidal 
 capacity, and thus induces scour at the bar. The case 
 is similar to that of the Mersey estuary (CHAP. XIV., 
 Art. 5), the breakwaters assisting scour at the bar, 
 
208 RIVER AND CANAL ENGINEERING 
 
 though perhaps slightly interfering with the tidal flow 
 in the estuary. 
 
 Converging breakwaters also tend to stop littoral 
 drift, and the space inside them acts as a harbour of 
 
 FIG. 70. 
 
 refuge in storms and as a sheltered place where dredgers 
 can work (Rivers and Canals, CHAP. XL). They have 
 to be heavily built and are very expensive, and they 
 are generally adopted only when there is an important 
 seaport, and when they can be put to all the uses above 
 indicated. 
 
APPENDIX A 
 
 Fallacies in the Hydraulics of Streams (CHAP. I., Art 4, and 
 CHAP. VI., Art. 2). In an inundation canal in India the supply 
 during floods was excessive. Orders were given that a flume 
 be made at the head, as shown in fig. 71. The sides were to 
 be revetted, as shown in fig. 19 (CHAP. VI., Art. 3) ; the length, 
 excluding the splayed parts, was to be 200 feet, and the floor 
 
 Fia. 71. 
 
 was to be a mattress well staked or pegged down. The order 
 stated that "by this means we cannot get into the canal 
 much more than its true capacity." With 9 feet of water, 
 a surface fall of 4 inches in 300 feet would give a velocity of 
 some 6*5 feet per second, and a further fall of about 8 inches 
 would be required at the head of the flume to impress this 
 velocity on the water. The flume would reduce the depth of 
 water in the canal by 1 foot, i.e. from 9 feet to 8 feet. This 
 would not be in anything like the proportion desired. More- 
 over the flume, unless the bed was extremely well protected, 
 
210 RIVER AND CANAL ENGINEERING 
 
 would be destroyed. The above is a case of exaggerating the 
 effect of an " obstruction." 
 
 Again, on a branch canal it was observed that " wherever 
 cattle crossings exist there is a deep silt deposit which practi- 
 cally blocks the branch." The deposit exists because the sides 
 of the channel are worn down. A wide place always tends 
 to shoal (CHAP. IV., Art. 9). If the deposit obstructed the 
 flow of water there would be a rush of water past it, and it 
 could not exist. 
 
 The Gagera branch of the Lower Chenab Canal the left- 
 hand branch in fig. 72 was found to silt. It was pro- 
 
 FIG. 72. 
 
 posed to make a divide wall (fig. 72) extending up to 
 full supply level. The idea is unintelligible. The silt does 
 not travel by itself but is carried or rolled by the water. 
 As long as water entered the Gagera branch, silt would 
 go with it. The authorities, who had apparently accepted 
 the proposal, altered the estimate when they received it, 
 and ordered the wall to be made as shown dotted and of 
 only half the height. This was done. The idea seems to 
 have been that the wall would act as a sill and stop rolling 
 silt. This is intelligible, but see CHAP. IV., Art. 2, last para- 
 graph. Moreover, there was a large gap, A B, in the wall. 
 The work is said to have proved useless, and proposals have 
 been made to continue the wall from A to B. In this form it 
 is conceivable that it may be of use. 
 
 In a river, the rises and falls at different places are not, of 
 
APPENDIX A 211 
 
 course, the same, even when they are long continued. In the 
 river Chenab, at the railway bridge at Shershah, the rise from 
 low water to high flood is generally a foot or two more than 
 the rise at a point 25 miles upstream. It has been suggested 
 that the railway embankments, which run across the flooded 
 area, cause a heading up of the stream. If this were the case, 
 to any appreciable extent, there would be a " rapid " through 
 the bridge, which, if it did not destroy the bridge, would at 
 least be visible and audible. 
 
 The exaggerated ideas which often prevail regarding the 
 tendency of a river, when in flood, to scour out a new channel, 
 have been mentioned in CHAP. IV., Art. 8. Spring, in his 
 paper on river control, admits, when mentioning Dera Ghazi 
 Khan, that there was little danger, but in mentioning the 
 Chenab Bridge at Shershah he quotes, without disputing it, an 
 opinion of the opposite kind (Government of India Technical 
 Paper, No. 153, " River Training and Control on the Guide 
 Bank System "). 
 
 For some other fallacies, see Hydraulics, CHAP. VII., Arts. 
 
APPENDIX B 
 
 Pitching and Bed Protection (CHAP. VI., Art. 3, and CHAP. 
 X., Art. 2). Any scour upstream of a weir is merely due to 
 the eddies formed upstream of the crest (Hydraulics, CHAP. 
 II., Art. 7), and is not serious. And, similarly, as to scour 
 upstream of a pier. A hole formed alongside a pier or 
 obstruction, if there is no floor, may work upstream. The 
 chief use of a floor extending far upstream is to flatten the 
 hydraulic gradient (CHAP. X., Art. 3}. 
 
 For pitching of the sides, monolithic concrete is not very 
 suitable, because it may settle unequally and crack. For 
 heavy pitching, concrete blocks can be used. They can rest 
 on a layer of 3 to 6 inches of rammed ballast or gravel. The 
 toe wall, as shown in fig. 13, page 65, is sometimes dispensed 
 with, the pitching being merely continued to a suitable depth 
 below the bed, and the bottom edge being at right angles to 
 the slope instead of horizontal. The portion below the bed 
 may be of concrete. 
 
 212 
 
INDEX 
 
 Abrupt changes in streams, 28. 
 Alterations in a channel, upstream 
 
 effect, 4. 
 
 Aqueduct, Kali Nadi, 149. 
 Available rainfall, 9. 
 
 Bank protection, 60. 
 
 artificial weeds, 70. , 
 
 berms, 70. 
 
 bushing, 68. 
 
 fascining, 66. 
 
 heavy stone pitching with apron, 
 
 71. 
 
 on the Adige, 67. 
 
 reinforced concrete, 70. 
 
 rolls of wire-netting, 70, 140. 
 
 staking, 69. 
 
 trees, 68. 
 
 twig revetment, 68. 
 
 Villa system, 69. 
 
 Banks, 92. 
 
 Bell's guide, 137. 
 
 continuous lining of, 64. 
 
 dimensions of, 93. 
 
 guide, 137. 
 
 side slopes of, 92. 
 
 Barrage of the Nile, Assiut, 117. 
 Bars, river, 203. 
 Bed, protection of, 58. 
 Villa system, 59. 
 Bell's guide banks, 137. 
 Bends, effect of, 44. 
 
 short cuts of, 44. 
 
 Bengal Dooars Railway, bridge and 
 
 floods, 139. 
 
 Bifurcation of a channel, 53. 
 Birmingham water supply, 173. 
 Borrow-pits in bed of channel, 53. 
 Breakwaters, converging, 207. 
 Bridges, 132. 
 
 foundations or floor, 132. 
 
 on Indian rivers, 134. 
 
 piers and abutments, 132. 
 
 protection of, 137. 
 
 at Wazirabad, 134. 
 
 British Rainfall Organisation, 9. 
 
 Canalisation of rivers, 84. 
 Canals, 92. 
 
 fall, 113. 
 
 headworks, 54. 
 
 navigation, 93. 
 
 rapid, 113. 
 
 Ship, 95. 
 
 Catchment area, "yield," 9. 
 Channel, alterations in, 4. 
 Chanoine falling shutters, 121. 
 Chenab River at Shershah, 210. 
 Choice of types of work, 3. 
 Closures of streams, Colorado River, 
 80. 
 
 cradle for, 78. 
 
 Tista River, 81. 
 
 Collection of information concerning 
 
 streams, 18. 
 
 Colorado Kiver, closure of, 80. 
 Conduits, 92, 100. 
 Cradle for closing streams, 78. 
 Culverts, 135. 
 
 flooding of, 136. 
 
 Dams, culvert of, 180. 
 
 design of masonry, 181. 
 
 earthen, 174. 
 
 masonry 181. 
 
 construction of, 185. 
 
 design, 181. 
 
 failures of, 185. 
 
 stresses in, 185. 
 
 pitching of, 179. 
 
 reservoir, 162. 
 
 Sidhnai Canal, 117. 
 
 tower for culvert, 180. 
 Dee estuary, 201. 
 
 Dera Ghazi Khan, Indus at, 71. 
 Discharge curves, 23. 
 
 observations, 21. 
 
 tables, 24. 
 
 Divide wall, Gagera branch canal, 
 
 209. 
 
 Drainage, 141. 
 
 Dredging and excavating, 84, 88. 
 Drift, littoral, 205. 
 
 213 
 
214 
 
 RIVER AND CANAL ENGINEERING 
 
 Eddies, scouring power of, 28. 
 Embankments, 156. 
 
 design of, 157. 
 
 Holland, 159. 
 
 Irrawaddy, 159. 
 
 Rhine, 159. 
 
 slips in, 177. 
 Estuaries, Dee, 201. 
 
 Mersey, 202. 
 
 Seine, 200. 
 
 tidal, 197. 
 
 Fall, canal, 113. 
 Fallacies in hydraulics, 5. 
 Falling shutters, Chanoine, 121. 
 
 b'ouracres, 121. 
 
 Khanki, 124. 
 
 Thenard's, 121. 
 
 Flood discharge, estimating, 148. 
 Floods, 141, 146. 
 
 Bengal Dooars Railway, 139. 
 
 formulae for, 147. 
 
 prediction, 150. 
 
 prevention, 153. 
 Flowing stream, closure of, 75. 
 Ford in a river, 43. 
 
 Forests and vegetation, influence of, on 
 
 rainfall, 14. 
 Formulae for floods, 147. 
 
 Groynes or spurs, 58, 60, 61. 
 
 on the Indus, 53. 
 Guide banks, 137. 
 
 Headworks of a canal, 54. 
 Holland, embankments, 159. 
 Hurdle dykes, 79. 
 Hydraulics of open streams, 4. 
 fallacies in, 209. 
 
 Important works, precautions at, 130. 
 
 Indian rivers and bridges, 134. 
 
 Indus, groynes on, 53. 
 
 Information concerning streams, col- 
 lection of, 18. 
 
 Inundation canal, flume in, 209. 
 
 Irrawaddy, embankments, 159. 
 
 Irrigation channels in embankment, 
 52. 
 
 Irwell, weir on, 124. 
 
 Jetties, in continuation of river banks, 
 204. 
 
 Mississippi, 205. 
 
 Richmond River, New South Wales, 
 
 206. 
 
 Kali Nadi, aqueduct, 149. 
 Khanki, falling shutters at, 124. 
 
 Leakage, stoppage of, 78. 
 Lockage, 98. 
 Locks, 96. 
 
 in flights, 98. 
 
 Mersey estuary, 202. 
 Mississippi jetties, 48. 
 
 Narora weir, 109. 
 
 Navigation canals, 93. 
 
 Needles, regulator, 117. 
 
 New South Wales, available rainfall, 12. 
 
 Obstruction, effect of, 5. 
 Obstructions in streams, 28. 
 Okla weir, 112. 
 Open streams, hydraulics of, 4. 
 
 Perishable materials, use of, 4. 
 Permanent regime of streams, 29. 
 Pitching, 64. 
 Prediction of floods, 150. 
 Prevention of floods, 1 53. 
 Protection of banks (see Bank protec- 
 tion). 
 Rainfall, 6. 
 
 available, 9, 26. 
 
 New South Wales, 9. 
 
 Sudbury River, Massachusetts, 
 
 12. 
 various countries, 11. 
 
 British Organisation, 9. 
 
 "catchment area," " basin," 9. 
 
 distribution of, 7. 
 
 driest year, 7. 
 
 evaporation, 9. 
 
 heavy falls in short periods, 15. 
 
 influence of cultivation, 15. 
 of forests and vegetation, 14. 
 
 local figures, 8. 
 
 measurement of, 13. 
 
 observations, period of, 7. 
 
 statistics, 6. 
 
 variation of, 6. 
 Rain-gauge, 8, 13. 
 Rapid, canal, 113. 
 Regulators, 118. 
 Reservoirs, 162. ^ 
 
 capacity, 167. 
 
 compensation water, 162. 
 
 leakage, 163. 
 
 waste weir, 164. 
 Resume of the subject, 1. 
 Rhine, embankments, 159. 
 Richmond river, jetties, 206. 
 
 weir at, 119. 
 River bars, 203. 
 
 training groynes, 85. 
 walls, 85. 
 
INDEX 
 
 215 
 
 Rivers, deltaic, 203. 
 
 floods in, 146. 
 
 non-deltaic, 205. 
 
 tidal, 192. 
 
 training and canalisation, 84, 89. 
 Run-off in small streams, 143. 
 
 Salt water, .effect of, 29. 
 
 Sand separator, 34. 
 
 Sandbanks, 47. 
 
 Scour (see Silt and scour). 
 
 Seine estuary, 200. 
 
 Set of stream, effect of, 5. 
 
 Ship canals, 95. 
 
 Shutters, self-acting, Switzerland and 
 
 Bavaria, 127. 
 Silting and scouring, 27. 
 Silt and scour, action at bends, 42. 
 
 on sides of channel, 40. 
 
 effect ' of regulator or movable 
 
 weir, 49. 
 
 in the Sirhind Canal, 32, 54. 
 
 increasing or reducing, 48. 
 
 materials carried in suspension, 3. 
 
 methods of investigation, 33. 
 
 practical formulae and figures, 37. 
 
 production of, 48. 
 
 rolled materials, 29. 
 
 sand separator, 34. 
 
 scrapers or harrows, 48. 
 
 clay and sand, 36. 
 
 deposit, production of, 51. 
 
 in river Sutlej, 35. 
 
 in river Tay, 35. 
 
 quantity and distribution of, 35. 
 
 upstream of weirs, 103. 
 Sidhnai Canal, dam, 117. 
 
 Sirhind Canal, silt and scour in, 32, 54. 
 
 Slips in embankments, 177. 
 
 Sluicegates, Stoney's, 119. 
 
 Sluices, 102. 
 
 Small streams, floods in, 141. 
 
 Soundings, 21. 
 
 Spurs or groynes, 58, 60, 61. 
 
 Stream gauges, 19. 
 
 diversions of, 73. 
 
 general tendencies of, 45. 
 
 information concerning intermittent, 
 
 25. 
 small, 24, 25. 
 
 overflow, 46. 
 
 shifting, 20, 47. 
 
 Sudbury River, Massachusetts, avail- 
 able rainfall, 12. 
 
 Surface slope observations, 22. 
 Survey of a stream, 21. 
 Sutlej River, silt in, 35. 
 Syphons, 132, 135. 
 
 Tay River, silt in, 35. 
 Teddington, weir at, 119. 
 Tidal estuaries, works in, 198. 
 
 river, diagrammatic route-guide, 
 
 196. 
 
 rivers, 192. 
 
 works in, 196. 
 
 Tide-gauges, 192. 
 Tides, 190. 
 
 Tista River, closure of, 81. 
 Training of rivers, 84. 
 Types of work, choice of, 3. 
 
 Upper Jhelum Canal, syphons to carry 
 torrents, 144. 
 
 Velocities which enable a stream to 
 
 scour, 40. 
 
 Villa system of bank protection, 69. 
 bed protection, 59. 
 
 Waste weir, hydraulic problem, 165. 
 Water supply, Birmingham, 173. 
 Waterway, area of, in regulators, 129. 
 Wave, travel of, down a stream, 152. 
 Waves, effect of, 29. 
 Wazirabad, bridge at, 134. 
 Weeds, artificial, 70. 
 
 growth of, 29. 
 Weirs, 102. 
 
 adjustable, 126. 
 
 Bear Trap, 123. 
 
 drum, 127. 
 
 frame, 120. 
 
 general design of, 105. 
 
 Narora, 109. 
 
 oblique, 103. 
 
 Okla, 112. 
 
 on sandy or porous soil, 106. 
 
 Richmond, 119. 
 
 silt deposit, upstream of, 103. 
 
 types of, 111. 
 
 waste, 164. 
 
 with sluices, 115. 
 Wire-netting for bank protection, 70, 
 
 140. 
 Works, design and execution of, 3. 
 
 Zifta Regulator, 109. 
 
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SCIENTIFIC BOOKS. 37 
 
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SCIENTIFIC BOOKS. 39 
 
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SCIENTIFIC BOOKS. 41 
 
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SCIENTIFIC BOOKS. 43 
 
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SCIENTIFIC BOOKS. 45 
 
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SCIENTIFIC BOOKS. 47 
 
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