IV UC-NRLF B 3 ENGIN. 2 I LIBRARY I- It o o -^ I r o O 00 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 photomount pamphlet Binder Gaylord Bros Makers Stockton PUT. H. 21, Inc. B27M67TC LJb THE UNIVERSITY OF CALIFORNIA LIBRARY