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ENGINEERING LI^AI 
 
REPORTS 
 
 ON THE 
 
 CALAVERAS DAM AND SPILLWAY 
 
 I 
 
 By 
 G. A. ELLIOTT, Engineer of Spring Valley Water Company 
 
 and Approved by 
 WM. MULHOLLAND, Consulting Engineer on the Calaveras Dam 
 
 II 
 
 And By 
 ARTHUR P. DAVIS, Director of the U. S. Reclamation Service 
 
 and 
 DANIEL W. MEAD, Consulting Engineer of Madison, Wisconsin 
 
 SPRING VALLEY, WATER COMPANY 
 ** JULY, 1917 
 
ENGINEERING LIBRARY 
 
REPORT ON THE 
 SPILLWAY FOR THE CALAVERAS RESERVOIR. 
 
 Abstract of Report of G. A. ELLIOTT, Engineer of Spring Valley Water Company. 
 
 Several essentials must be kept in mind in the location and construc- 
 tion of a spillway. Its primary purpose, to protect the dam from the 
 possibility of overtopping during floods, renders it imperative to have 
 sufficient capacity to discharge the maximum floods. The inlet and outlet 
 must be so placed that no danger will exist from the entering current 
 washing the slope of the dam or the discharge from cutting away the 
 heel of the dam. It should be located upon as firm a foundation as can 
 be obtained. Last but not least in California, the site must be free from 
 earthquake faults. The Calaveras spillway was located after an investi- 
 gation carried on for three years, during which five sites were explored. 
 It is situated in the sandstone rock at the west end of the dam. 
 
 The drainage area directly tributary to Calaveras reservoir is one hundred 
 square miles. An additional thirty-eight square miles will be included 
 after the construction of the proposed tunnel between the Upper Alameda 
 and Honda streams. The average annual rainfall on this total area is 28 
 inches. The maximum precipitation occurs at Mt. Hamilton, with an 
 annual average of 31.2 inches. Kecords of intensity of rainfall are 
 meager. However, at Mt. Hamilton a total of 9.05 inches of rain 
 occurred on December 21, 1884. An old record of rainfall in the vicinity 
 of the dam gives for March 16, 1899, a precipitation of 3.5 inches in 
 forty hours, including 0.7 inches in five hours. The largest flood of which 
 actual measurement was made occurred in March, 1911, when a maximum 
 flow of 6,840 cubic feet per second, lasting for two hours, was observed. 
 This amounts to 68.4 second feet per square mile of drainage area. In 
 view of the data on hand and with a knowledge of the uncertain 
 maximum conditions of rainfall and runoff which happen in the vicinity, 
 the spillway was originally designed with a capacity of 20,000 cubic feet 
 per second, equivalent to 200 second feet per square mile. In addition 
 to the flow capacity of the spillway channel, there is an available storage 
 of 9,000 acre feet between the lip of the spillway and the crest of 
 the dam. 
 
 The location of the spillway is such that in the vicinity of the inlet 
 end the excavation necessary is not excessive. As the cut proceeds 
 farther from the inlet the bank becomes higher and the excavation per 
 unit of length increases. Under these conditions it was advisable in 
 the interest of economy to decrease the width of the cut as much as 
 possible. The plan as finally worked out calls for an inlet width of 150 
 feet, 15 feet deep. The bottom of the spillway is at elevation 790, or 15 
 
feet below the crest of the dam. From the entrance the width of the 
 channel decreases gradually, reaching a bottom width of IS 1 /^ feet at a 
 point 540 feet from the inlet. From this point the section is constant to 
 the outlet, being 13% feet wide on the bottom, 29 feet wide at the top 
 of the lined section and 15 feet deep. The total length of the channel 
 is 1,100 feet and the elevation of the discharge end is 692 feet, the 
 drop from inlet to outlet being 98 feet. Necessarily the velocity of the 
 water must increase in proportion to the decrease in section of the 
 channel, so that the narrow conduit will carry the same amount as the 
 wider initial section. In order to avoid disturbance of the current, the 
 grade of the bottom of the spillway was so arranged with respect to its 
 width and hydraulic properties that the velocity of the water is con- 
 tinually increased. The upper or throat section is 540 feet long and it is 
 in this section that the incoming water is accelerated up to its maximum 
 velocity of 66 feet per second. After this velocity is attained the channel 
 is maintained at uniform section and grade. 
 
 The velocity of the water when 10,000 second feet is passing through 
 the narrow section is 66 feet per second. At first thought this velocity may 
 appear to be high, but there are precedents which leave no doubt as to its 
 practicability. It is essential that no obstructions be placed in the path 
 of the stream and that the sides and bottom of the channel must be 
 smooth and so shaped as to offer the minimum opportunity for impact. 
 This has been done in the design under discussion by the use of a long 
 (540-foot) gradually converging throat. 
 
 Many examples may be cited of the non-injurious effect of water pass- 
 ing over concrete at high velocities. Among them are the LaGrange 
 Dam on the Tuolumne River, where for six months of the year water 
 with a probable velocity of 70 feet per second has passed with practically 
 no signs of erosion. This is remarkable, as the Tuolumne River in flood 
 stages carries sand and gravel. The culvert through the Pathfinder Dam 
 carried water in 1899, from May to October continuously, at velocities 
 ranging from 75 to 90 feet per second. At the end of this period not only 
 was the concrete lining absolutely uninjured but the form marks were 
 still visible. This is probably the most conclusive evidence that concrete 
 is not injured by erosion without impact. Other cases of concrete chutes 
 used for transmitting water at high velocities are found as follows: 
 
 Location 
 
 Slope 
 
 Velocity 
 
 Boise Project 
 
 6% to 30% 
 
 40 feet per second 
 
 Standley Lake Project 
 
 9% to 39% 
 
 
 Klamath Project 
 
 20% to 30% 
 
 46 " 
 
 Belle Fourche Wasteway 
 
 4,000 sec. ft. 
 
 48 " 
 
 Arrowrock Dam 
 
 100-foot head 
 
 64 " 
 
 2 
 
In none of these cases has there been any trouble due to the velocity. 
 Probably the best summation of experience in this connection is given 
 by Arthur P. Davis, Esq., Chief Engineer of the U. S. Keclamation 
 Service, in "Engineering News," January 4, 1912, in an article entitled 
 "Safe Velocities of Water on Concrete." He says that 
 
 "Where clear water can be made to glide over con- 
 crete without disturbing its velocity or abruptly 
 changing its direction, there is no practical limit to 
 the velocities that can be permitted without harm." 
 
 The exact effect of the discharge from the spillway cannot be prede- 
 termined with sufficient accuracy to warrant additional construction at 
 this time. A transverse ridge of comparatively hard sandstone crossing 
 the line of the spillway channel just before the outlet is reached, pre- 
 sents an effective barrier against the discharged water cutting back along 
 the line of the channel. After the passage of the floods of one season 
 such action as should be taken will be apparent and necessary construc- 
 tion can then be more effectively accomplished. 
 
 Mr. William Mulholland, Chief Engineer, Los Angeles Water Depart- 
 ment, is Consulting Engineer not only on the spillway location and design 
 but also on the construction of the dam. *Mr. Arthur P. Davis, Director 
 of the U. S. Beclamation Service, and. Mr. D. W. Mead, Consulting Engi- 
 neer, both of whom have had long practical experience in the design and 
 construction of hydraulic works, were called in to review the entire 
 spillway plan. Acting on their recommendation it was decided to in- 
 crease the depth of the spillway from 15 feet to 18 feet, thereby securing 
 additional capacity. The joint report of Consulting Engineers, Messrs. 
 Davis and Mead, is presented herewith. 
 
 * Mr. Arthur P. Davis was Chief Engineer of the U. S. Reclamation Service for 
 eighi years prior to 1915 and since that date has been Director of the Service. 
 During that time there have been designed and constructed under his supervision, 
 collecting and distributing works sufficient to irrigate almost two, million acres of 
 land. It is not possible to specify here in detail even those features of these irriga- 
 tion projects which are well known. The Elephant Butte dam (300 feet high), the 
 Shoshone dam (328 feet high), the Roosevelt dam (280 feet high) and the Arrowrock 
 dam (354 feet high), may be mentioned in passing as examples of the extent of 
 the work designed and executed under the supervision of Mr. Davis. In addition 
 to the Reclamation Service work, Mr; Davis was a member of the Engineering Com- 
 mission which went to China in 1914 to report on flood control of the Huai River 
 Conservancy, and has been called in consultation by many utility corporations enga-ged 
 in constructing hydraulic works, such for instance as the Pacific Gas and Electric 
 Company in connection with Lake Spaulding dam. 
 
 Mr. Daniel W. Mead, one of the leading engineers in the United States, has had a 
 large experience in the design and construction of water-works and hydro-electric 
 plants. He is the author of several standard engineering books and has made a 
 special study of hydrology, a subject which is important in the design and construction 
 of hydraulic control works. Mr. Mead was a member of the Engineering Commission 
 which was sent to China to study the flood conditions of the Huai River Conservancy. 
 Recently he has been Consulting Engineer for the Miami Conservancy Commission, 
 created after the Dayton flood of 1913, to prepare plans for preventing the repetition 
 of a similar flood. 
 
REPORT ON THE 
 CALAVERAS DAM AND SPILLWAY, 
 
 3y ARTHUR P. DAVIS, Director of the U. S. Reclamation Service, and 
 DANIEL W. MEAD, Consulting Engineer. 
 
 Mr. S. P. Eastman, Jul ^ 6 > 1917 ' 
 
 Vice President and Manager, Spring Valley Water Company, 
 
 San Francisco, California. 
 Dear Sir: 
 
 The undersigned Board of Engineers has made an examination of the 
 conditions at the Calaveras Dam, now under construction for the Spring 
 Valley Water Company near Sunol, California, with especial reference 
 to the character and capacity of the necessary spillway provisions. The 
 structure is about 75% completed and work has proceeded to some extent 
 upon the excavation for a spillway which is to be provided at the west 
 end of the dam, and it is to this spillway structure that most of our 
 attention and study have been directed. 
 
 The dam itself is being built mainly by the hydraulic process, but a 
 large amount of coarse material containing much rock is deposited on 
 the outer edge in heavy retaining dikes and sluiced material is being 
 deposited between. These methods result in placing a mass of tight 
 puddle in the body of the dam, well calculated to resist the passage of 
 water, enclosed within bodies of coarse material adapted to resist the 
 action of water and weather, and also to resist any tendency to slide; 
 and assuming proper bond with foundation, abutments and conduit, it 
 constitutes a substantial and safe construction when provided with an 
 adequate spillway which is essential to such a structure. At the time of 
 our visit to the dam the construction was well advanced, and no 
 opportunity was possible for the examination of the bond between the 
 fill and the conduit, abutments and foundation, but the work in progress 
 which we did see, was being carefully and thoroughly done. 
 
 The partial failure of the Necaxa Dam, which occurred before com- 
 pletion, has thrown some doubt in the minds of laymen upon the safety 
 of hydraulic dam construction. We have given careful consideration to 
 the causes of that accident and to a comparison of the conditions, both 
 natural and structural, at the Calaveras Dam. 
 
 According to the best evidence obtainable, the Necaxa slide was 
 caused by an insufficient retaining dike at the edge of the pond of sluiced 
 material, and the large amount of water and semi-liquid mud burst the 
 thin dike provided. No such conditions, nor any conditions approaching 
 them, are allowed at the Calaveras Dam. The banks retaining the central 
 heart of sluiced material are heavy, massive and composed largely of 
 
rock which has no tendency to slide upon the slopes on which it is 
 deposited. The tests, carefully made by the engineers, show beyond any 
 doubt a progressive consolidation of the sluiced material at the outer 
 edges and the bottom, which is very reassuring as to the generous margin 
 of safety against any such accident either during construction or after 
 the completion of the dam. 
 
 We have carefully examined the abundant data on this subject and 
 are perfectly confident that not the slightest danger of this kind exists 
 in connection with this structure. The embankment is designed along 
 safe and conservative lines, but not more so than advisable in a structure 
 of this importance. 
 
 We have given close attention to the studies made upon the require- 
 ments of spillway capacity and to the plans for such a spillway. It must 
 have a large discharge capacity and conduct water safely from the top 
 of the reservoir to the canyon well below the dam without danger to the 
 structure and in sufficient quantity to provide beyond any doubt for the 
 maximum flood that can be expected in the drainage area tributary to 
 this structure. The large quantities of water to be discharged will gen- 
 erate very high velocities, and the provisions made in the plans for such 
 velocities are, in our opinion, safe and reliable if the construction is 
 carried out as planned. 
 
 The spillway as designed consists of a wide opening cut through the 
 hill near the west end of the dam, gradually narrowing and descending 
 on slopes carefully calculated to accelerate the velocity of the water in a 
 safe manner, and to carry it a distance of about 1,150 feet, discharging 
 about 110 feet above the bottom of the canyon upon hard sloping rock 
 of the canyon wall. A spillway thus designed and built of sufficient 
 capacity will protect the dam from injury, but may be expected to erode 
 the rock at the point of discharge. Just what effect this will have depends 
 upon the hardness and internal structure of the rock and cannot accu- 
 rately be foreseen, but it will be slow and if any modification of the 
 discharge end is required in the future the hydrologic conditions afford 
 abundant opportunity for such extensions, repairs or alterations as may 
 be required in the future. Some such additions will doubtless be re- 
 quired, but their character and extent can be much more accurately 
 determined when the needs develop than if they were attempted now, and 
 can then be more cheaply performed. 
 
 The plans prepared by your Engineer, Mr. G. A. Elliott, show careful 
 consideration of the severe conditions to be met, and we can make no 
 suggestions for improvement except as to capacity. In location, shape 
 and general plan the spillway meets with our unqualified approval, and if 
 carefully constructed according to the plans will, without any doubt, be a 
 safe structure. In our opinion, however, its capacity should be some- 
 what increased. Although designed to discharge a flood more than three 
 times the magnitude of any yet observed at this point, the disastrous 
 effects of a possible lack of capacity would be so great that we believe 
 
 6 
 
a still wider margin of safety should be provided. This will somewhat 
 increase the cost, but a spillway of much larger capacity can be provided 
 at very moderate cost simply by carrying upward the concrete sides of 
 the spillway channel with corresponding additions for strength and 
 correspondingly increasing the height of the dam without changing, in 
 any respect, the location or general design of the spillway. The dam can 
 be increased in height with perfect security without increasing the 
 width of the base or of any part except near the top, and will therefore 
 add only a small amount to the yardage and to the resulting cost. 
 
 The concrete lining of the spillway channel should be built entirely in 
 the winter, and preferably during the coldest weather which this climate 
 affords, so that a change of temperature will place the concrete in com- 
 pression and have no tendency to open any cracks. To the same end it 
 would be well to build this section in alternate sections between expan- 
 sion joints so that one section would be entirely set and past the stage 
 of expansion due to the increased temperature of the process of setting, 
 in order that it shall have undergone the contraction which follows 
 setting before the adjacent section is built. The entire surface of the 
 spillway must be made as smooth as possible and free from cracks or 
 projections of any kind, so as to give the water the freest and smoothest 
 possible discharge and avoid any chance for the water to take hold of the 
 concrete. The central line of the spillway channel rs entirely free from 
 any horizontal curvature and the vertical curvature is very slight, being 
 only that required for hydraulic reasons to control the velocity, and if 
 constructed as planned will meet the requirement that "where clear 
 water can be made to glide over concrete without disturbing its velocity 
 or abruptly changing its direction, there is no practical limit to the 
 velocities that can be permitted without harm." The above rule has 
 been much discussed and tested by many engineers under a large variety 
 of circumstances, and no exceptions to its validity have been found. 
 
 The maximum flood flow that should be controlled by the proposed 
 spillway to be built for the Calaveras Dam may be estimated both on 
 the basis of runoff of similar streams and on the basis of the runoff 
 which may result from the extreme rainfall that may occur on the 
 drainage area above the dam. The records of the actual runoff and 
 rainfall that have occurred in the past on the Calaveras drainage area 
 do not cover a sufficient period of years to indicate the extremes that 
 are liable to occur in the lapse of time. 
 
 Many illustrations might be given to show that extreme local rainfall 
 or runoff may occur only at long intervals of time, perhaps one hundred 
 to two hundred years apart, and that the extremes for a thousand years 
 or more do not greatly exceed the extremes for one hundred to two 
 hundred years. 
 
 It seems apparent, therefore, that the only safe criterion for either 
 flood flow or rainfall is the extremes that have occurred on areas more or 
 less similar and over a sufficient range of time and geographic extent 
 to make it certain that the extremes have been covered, or failing 
 
RECORDED FLOODS IN CALIFORNIA 
 
 Stream 
 
 Area 
 Drained 
 
 Sec. Ft. per 
 Sq. Mile 
 
 Calaveras River 
 
 395 
 
 176 
 
 San Gabriel 
 
 220 
 
 215 
 
 Arroyo Seco 
 
 38.6 
 
 366 
 
 Arroyo Seco 
 
 30.5 
 
 374 
 
 Verdusro Creek.... 
 
 21.9 
 
 352 
 
 such sufficient data to allow a sufficient factor of safety to eliminate 
 the risk involved in the problem at hand. 
 
 The following floods have been observed in California: 
 
 Date 
 1911 
 1884 
 1914 
 1914 
 1914 
 
 The rainfalls which produced the above floods have been so far 
 exceeded in various parts of California that we do not consider them the 
 most extreme conditions possible. In fact, judging from rainfall records 
 and flood damages, it seems probable that still heavier floods were 
 discharged in 1916. 
 
 Actual flood data are, however, so meager that we must consider also 
 the far more abundant data of rainfall. 
 
 In considering flood flows based on rainfall, it must be remembered 
 that the peak flood in a day may be much more intense than the rainfall 
 that creates it, and that the runoff in 24 hours may be greater than the 
 maximum 24-hour rainfall of a given storm when the storm lasts for 
 several days. 
 
 A few rainfall records in California extend back about 66 years. The 
 official records of the U. S. Signal Service began in 1871. The work was 
 taken over by the Weather Bureau in 1896, since which date the number 
 of stations has been gradually increased. It is evident that the records 
 available are brief and incomplete, and it is hardly possible that they 
 in any place show either the maximum or the minimum that must be 
 expected in the course of time and which may occur at any time when 
 all the factors happen to become favorable to extreme conditions. 
 
 When the number and location of observations in the United States 
 and in California are considered in relation to the broad areas and 
 especially the mountain areas, for which few observations have been 
 taken, it is quite evident that even the extremes shown in the records 
 available are in all probability not the maximum for the United States 
 or for the State. 
 
 The State of California is particularly subject to extreme rainfall 
 conditions on account of: 
 
 1. The prevailing easterly drift of the surface atmos- 
 
 phere. 
 
 2. The proximity of the Pacific Ocean; and 
 
 3. The diversified topography of the State. 
 
 The average annual rainfall varies from 100.14 at 
 Helen Mine (elevation 2,750) to 2 at Stirling; 
 with annual extremes of from 153.54 at Monu- 
 mental to a Trace at Salton. 
 
 8 
 
The annual rainfall in California therefore varies practically frtom the 
 maximum to the minimum of that which has been observed within the 
 United States. In intensity of rainfall, the records of California contain 
 extremes closely approximating the maximum observed in the United 
 States. 
 
 On August 12, 1891, at Campo, San Diego County (Elevation 2,543) 
 11.5 inches of rain fell in 80 minutes and the total storm amounted to 
 16.1 inches. Other records of rainfalls from one to five days' duration 
 are as follows: 
 
 Station Elev. 
 Lick Observatory 4209 
 
 Magalia 2320 
 Mono Ranch 3210 
 Helen Mine 2750 
 Inskip 4975 
 West Branch 3216 
 
 Average 
 Annual 
 
 32.28 
 83.12 
 
 100.14 
 
 100 ? 
 91 ? 
 
 Date 
 Dec. 1884 
 Jan. 1911 
 Jan. 1906 
 Mar. 1906 
 Dec. 1913 
 Dec. 1913 
 Dec. 1910 
 
 9.05* 
 9.19* 
 10.86 
 11.50 
 10.40 
 10.35 
 10.00 
 
 Inches 
 
 14.75 
 13.23 
 13.60 
 
 in 1 to 
 
 15.47 
 14.23 
 14.25 
 
 5 Days ^ 
 
 Nellie 
 
 5350 
 
 44 '? 
 
 Jan. 
 
 1916 
 
 11.24 
 
 17.24 
 
 20.76 
 
 22.61 
 
 23.12 
 
 
 
 
 Jan. 
 
 1916 
 
 10.16 
 
 12.43 
 
 13.40 
 
 15 
 
 .25 
 
 16 
 
 .10 
 
 Rialto 
 
 .... 2250 
 
 69 ? 
 
 Jan. 
 
 1916 
 
 12.95 
 
 15.99 
 
 18.79 
 
 20 
 
 .16 
 
 20 
 
 .27 
 
 Squirrel Inn .... 
 
 .... 5280 
 
 37.22 
 
 Jan. 
 
 1916 
 
 16.81 
 
 22.64 
 
 25.66 
 
 26 
 
 .87 
 
 27 
 
 .82 
 
 Los Gatos 
 
 .... 600 
 
 33.33 
 
 Jan. 
 
 1911 
 
 6.15 
 
 9.95 
 
 13.30 
 
 16 
 
 .15 
 
 16 
 
 .31 
 
 San Leandro 
 
 48 
 
 23.34 
 
 Jan. 
 
 1911 
 
 4.70 
 
 5.76 
 
 7.41 
 
 8 
 
 .31 
 
 8 
 
 .42 
 
 Livermore .. .. 
 
 483 
 
 15.54 
 
 Jan. 
 
 1911 
 
 2.54 
 
 4.33 
 
 6.83 
 
 7 
 
 .53 
 
 7 
 
 .68 
 
 Needles .. 
 
 427 
 
 3.52 
 
 Julv 
 
 1914 
 
 3.75 
 
 
 
 
 
 
 
 The only records in the United States exceeding those given above 
 are the following: 
 
 Alexandria, La ................................. June 15-16 1886 21.4 inches in 24 Hrs. 
 
 Altapass, N. C ................................... July 15-16 1916 22.22 " " 24 
 
 Montell, Texas .................................. Jan. 28-29 1913 20.60 " " 18% " 
 
 Concord, Pa ....................................... Aug. 5 1843 16. " "3 " 
 
 Guinea, Va ..................................... .... Aug. 24 1906 9.30 " " 1 " 
 
 Galveston, Texas .............................. June 4 1871 3.94 " "14 " 
 
 Ft. McPherson, Neb ......................... May 27 1868 1.50 " "5 
 
 As a general law (which is substantiated by conditions in California) 
 most intense rainfall is induced when moist winds are dynamically cooled 
 on mountain slopes adjacent to the sea, and the lower lands and the 
 interior mountains, unless of greater heights than the first range en- 
 countered, receive less average rainfall than the ocean slopes of the 
 first range. Hence stations on the Western slope of the Coast Range 
 and the high western slopes of the Sierras receive the maximum average 
 and maximum annual rainfalls. 
 
 As shown by a great number of records in California, rainfalls of 5 
 inches and more usually occur on the western slopes of the mountains 
 and at altitudes of 1000 feet or more; yet at Los Gatos (elevation 600) 
 on the eastern side of the outer Coast Range, a rainfall of 6.15 inches 
 
 * Precipitation probably occurred in two or more days. 
 ? Average annual rainfall approximate. 
 
occurred in one day in January, 1916. While so far as the records go no 
 rainfall of 10 inches per day, or more, has occurred at elevations of less 
 than 2200 feet, it is evident from the Alexandria, La., record that such 
 an occurrence is not impossible. 
 
 The drainage area above the Calaveras Dam, which is long and narrow, 
 varies in height from 790 feet at the reservoir to 4209 at Mt. Hamilton. 
 The average annual rainfall on the area seems to be about 28.5 inches, 
 and few records of intense rainfalls are obtainable even at Lick Observa- 
 tory, on Mt. Hamilton, where observations have been made since January, 
 1881. The Lick Observatory rainfall records are, however, somewhat 
 unsatisfactory, inasmuch as the records available do not show intensity 
 and frequently cover only the total rainfall for a number of days. For 
 example, the extensive rainfalls of 9.05 and 9.19 inches shown in the 
 preceding table are believed to have fallen in two or more days instead of 
 in 24 hours. The Lick Observatory records for December, 1884, show a 
 total of 33.84 inches; but this is believed by Professor A. G. McAdie to 
 be a doubtful record, though if true it probably represents some very high 
 24-hour rainfalls during that month. It should be noted that at Liver- 
 more, which has an average annual rainfall of 15.54 inches, and which is 
 better protected from extreme conditions than the Calaveras drainage 
 area, a rainfall of 7.68 inches, which is about half of the average annual, 
 occurred in five days. The Los Gatos record of January, 1911, of 6.15 
 inches in one day and 16.15 inches in four days, might be equalled or even 
 exceeded on the Calaveras drainage area. 
 
 January 18-21, 1914, and January 12-19, 1916, rainfalls of from 8 to 12 
 inches or more occurred over areas near Los Angeles, greater in extent 
 and of similar elevation to the Calaveras drainage area, and on the latter 
 date a similar rainfall extended over similar areas in the neighborhood 
 of San Diego, and apparently other similar excessive rainfalls have 
 occurred in many other portions of the State. 
 
 It seems apparent that on account of the southern location more 
 extreme rainfall conditions are liable to occur more frequently in South- 
 ern California, near the coast in Central California, and on the high 
 Sierras than on the low mountains behind the first mountains of the 
 Coast Eange; but it seems within possibility, although unlikely, that an 
 average rainfall of from 8 to 16 inches or more might occur on the 
 Calaveras drainage area which might result in a runoff of 12 inches in 24 
 hours or an average of 32,000 second feet, with a peak of perhaps 44,000 
 second feet. 
 
 From the above, and other data not discussed in detail, the conclusion 
 is drawn that the Calaveras Dam spillway, with the reservoir storage 
 available above elevation 790, should provide for a peak flood of about 
 44,000 second feet, an average 24-hour flood of about 32,000 second feet, 
 and a maximum discharge of about 26,500 second feet. While it is true 
 that this will give a capacity about seven times that which would have 
 been needed for the flood of March, 1911, it is believed to be warranted 
 
 -10- 
 
by the serious results that would follow a failure to carry safely the 
 extreme flood. 
 
 In view of the above it is our opinion that it would be well to design 
 the slopes of the spillway with a view to the smooth discharge of 20,000 
 second feet, and the height and strength of its walls for a maximum 
 discharge of 26,500 second feet. 
 
 The increase of the drainage area on account of the construction of 
 the Upper Alameda tunnel need not be considered, as this tunnel will not 
 be large enough to sensibly affect the volume of a great flood. 
 
 The question has been raised whether it is desirable to provide perma- 
 nent gates to the large concrete conduit which has been used during 
 construction to convey the waters of the creek past the dam site so as 
 not to interfere with construction, in addition to the regular discharge 
 gates in the inlet tower. This conduit will continue in use to discharge 
 the water admitted to the tower from the reservoir when required for use, 
 but beyond this we believe it is not only unnecessary but undesirable to 
 install any device for drawing large quantities of water from the bottom 
 of the reservoir for other purposes, or to maintain the temporary gates 
 already installed, which are manifestly unsuitable for permanent use. 
 They would serve no useful purpose after the dam is completed, but we 
 advise that they be not permanently closed until that time. 
 
 Eespectfully yours, 
 
 AKTHUE P. DAVIS, 
 DANIEL W. MEAD. 
 
 11 
 
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