VERS 
 
 C A L 
 
 F O R N 
 
 
 #** 
 
 1IFORNIA AGRICULTURAL 
 iperiment Station 
 (tension Service 
 
 CIRCULAR 467 
 
 Revised 
 
Wet face— slope 3 to ] 
 
 Upstream toe 
 
 so' 
 
 ->ft-io'-*f«- 
 
 75' 
 
 Here is a cross section of a typical small earth dam showing the 
 recommended ratios of width to height and naming the main parts. 
 
 Dam building is a very involved undertaking. This circular en- 
 deavors to supply you with practical suggestions which may be 
 of assistance to you. It is up to you to fit them together as they 
 apply to your situation. 
 
 REVISED DECEMBER 1965 
 
 THE AUTHOR: 
 
 Lloyd N. Brown is Extension Soils Specialist Emeritus in the Agricultural 
 Extension Service, Berkeley. 
 
 [2] 
 

 »*- 
 
 About 20,000 small earth dams have been built on California farms in 
 the last 25 years and more are being built all the time. While many of the 
 early dams were built primarily as stock-watering ponds, farmers are also 
 deriving benefits from their reservoirs in the way of irrigation water and 
 even recreational purposes. 
 
 But dams have disadvantages too. They are expensive to build; they 
 require labor and more expense to maintain properly; they may increase 
 the nearby mosquito population. 
 
 So perhaps a dam would be a good investment for you; perhaps not. 
 This circular discusses: 
 
 The laws involved — some are restrictive, some are 
 beneficial, but you should know 
 them. 
 
 Selection of a site — for the dam, the reservoir, the 
 spillway. You'll undoubtedly 
 have to compromise — you might 
 want to give up the idea of 
 building a dam. 
 
 Construction details — there are a lot of things to 
 
 watch if your dam is to be a 
 good one. 
 
 Maintenance practices — for the dam, the spillway, 
 
 the reservoir, the watershed. 
 
 Management practices — for both practical and rec- 
 reational uses of the reser- 
 voir. 
 
 [3] 
 
Before you start you should know the law 
 
 When you build a dam to impound water 
 for your own use, in California, you 
 should be familiar with the regulations 
 of a number of public agencies. They are: 
 
 The State Water Rights Board 
 
 To legally impound water for deferred 
 use, a water right must be secured from 
 this board and anyone proposing to build 
 a dam would do well to write to this 
 agency for information. The address is 
 1140 Resources Building, Ninth and 
 Streets, Sacramento. 
 
 The Department of 
 Water Resources 
 
 By California statutes, the State Di- 
 rector of Water Resources must approve 
 the design and construction or enlarge- 
 ment of all dams over a certain size and/ 
 or storage capacity. The reason for this 
 is to assure that sound design principles 
 and construction procedures are used for 
 larger dams, to minimize failures. This 
 procedure not only protects the public 
 from floods due to dam failure, but makes 
 available to builders of large dams valu- 
 able technical advice. 
 
 This publication, however, is con- 
 cerned primarily with small dams that 
 are exempt from supervision by the De- 
 partment of Water Resources. Such dams 
 are described as follows: 
 
 1. All dams 6 feet or less in height, 
 regardless of storage capacity. 
 
 2. All dams storing 15 acre-feet of 
 water or less, regardless of height. 
 
 3. All dams less than 25 feet high which 
 have a storage capacity of less than 50 
 acre-feet of water. 
 
 (See pages 6 and 7 for directions on 
 measuring the height of a dam and stor- 
 age capacity of a reservoir.) 
 
 For dams exceeding the above limita- 
 tions, construction of a new dam or the 
 enlargement of any dam above these limi- 
 
 [4 
 
 tations MUST NOT BE COMMENCED 
 
 until the owner has applied for and ob- 
 tained from the department written ap- 
 proval of plans and specifications. Other- 
 wise severe penalties are involved. The 
 department's address is P.O. Box 388, 
 Sacramento. 
 
 Enlarging small dams. Occasionally 
 small dams are built with the idea of later 
 enlarging them to a size that will bring 
 them under the supervision of the De- 
 partment of Water Resources. The builder 
 of such a dam would be wise to consult 
 the department in the beginning and use 
 specifications for the larger dam to save 
 expensive alterations later. 
 
 For instance— the department usually 
 requires that the outlet pipe in large 
 dams be laid in a trench and backfilled 
 with concrete, rather than using concrete 
 collars (which are allowed in smaller 
 dams). If specifications for the larger 
 dam were used from the start a consider- 
 able saving could be effected. 
 
 Agricultural Conservation 
 Program — USDA 
 
 If you comply with the regulations of 
 this agency when you build a dam, you 
 may be entitled to financial help toward 
 the cost of construction. The agency has 
 offices in most California counties. 
 
 It is not anticipated that there will be 
 any material differences between the tech- 
 nical recommendations in this publica- 
 tion and the requirements of the ACP. 
 
 State Department of 
 Fish and Game 
 
 The reservoir behind your dam may be 
 big enough to provide good fishing and 
 thus bring you into contact with the regu- 
 lations of the Fish and Game department. 
 
 Two kinds of ponds are recognized: 
 recreational ponds in which the owner, 
 his family, and friends may fish for fun, 
 but license and bag regulations of the 
 
State Fish and Game Department apply ; 
 commercial ponds in which fish are raised 
 for sale to restaurants, fishermen, pond 
 owners, etc. Consult with the Fish and 
 Game Department about regulations that 
 apply to this type of pond. 
 
 Trout ponds can be successful only 
 where the water temperature does not 
 rise above 70° F for any length of time. 
 They will be stocked free by the state, 
 provided the public is allowed to fish 
 there. The department will provide a list 
 of trout dealers and a transportation per- 
 mit without charge to owners of ponds 
 who do not wish to open them to the 
 public. 
 
 Warm-water ponds are those in 
 which the summer temperature of the 
 water rises above 70° F for several weeks 
 or more, and in which bass, bluegills 
 (also known as bream, sunfish, or perch) , 
 and crappie, will thrive. The state will 
 provide free an initial stocking for such 
 ponds if they are less than 25 surface 
 acres in size but not less than one-quarter 
 acre. For warm-water ponds larger than 
 25 acres, the state will stock them only if 
 they are open to the public for fishing. 
 Since these fish will reproduce in reser- 
 voirs, restocking may not be necessary. 
 
 There is no closed season on warm- 
 water fish in California. 
 
 Before you turn over a shovelful of earth on construction of a dam, 
 KNOW THE LAW. 
 
 Selecting a site may call for a compromise 
 
 It is possible to build a dam across a 
 watercourse almost anywhere, but it may 
 be practical to build one only when the 
 three major elements (the dam, the spill- 
 way, and the reservoir) will function as 
 a satisfactory whole. It is rare indeed 
 where optimum conditions for all three 
 elements occur together — therefore the 
 selection of a site is nearly always a 
 compromise. 
 
 If no satisfactory compromise between 
 these elements can be made it may be 
 impractical to build a dam. 
 
 But in looking for a site for your dam, 
 consider all of the elements separately 
 and collectively. Here are some pointers: 
 
 For the dam 
 
 Do not locate a dam on springs, seeps, 
 or old landslides — all of these indicate 
 unstable soil conditions. Avoid boulders 
 and outcroppings of rocks on the dam- 
 site, as they are difficult to work around. 
 
 A dam should be as short as possible — 
 such as in the narrow part of a canyon. 
 A long dam requires more fill material 
 and is thus more expensive. 
 
 Consider the elevation of the proposed 
 dam and reservoir — is it higher than the 
 area in which the water is to be used? 
 Can you take water out by gravity or will 
 it have to be pumped? Pumping may be 
 expensive. 
 
 Locate the dam as close as possible 
 to a supply of medium-textured soil for 
 construction. Fine-textured clay is likely 
 to crack when drying; coarse-textured 
 sand may not hold water. 
 
 Is the proposed damsite accessible by 
 an existing road? If not, is it practical to 
 build an access road? It may be the cost 
 of such a road would preclude some 
 damsites. 
 
 If, like some people, you are "disaster- 
 minded," you may want to consider the 
 
 site of the dam in relation to dwellings. 
 
 [5] 
 
Would possible failure of the dam endan- 
 ger lives or property below it? 
 
 How is the site for the dam with refer- 
 ence to the proposed spillway and reser- 
 voir? (See below.) 
 
 For the Reservoir 
 
 Will the reservoir hold all the water 
 you will need? (See page 7 for directions 
 for calculating the reservoir volume.) 
 
 Will the watershed above the dam fur- 
 nish enough runoff to fill the reservoir? 
 This is sometimes a guess because rain- 
 fall and runoff will vary from year to 
 year, but if the stream you are damming 
 is a well-established watercourse this will 
 indicate an appreciable amount of runoff. 
 
 Sometimes the water in the dammed 
 stream can be augmented by diverting 
 water into it from an adjacent stream, 
 using a contour grade ditch. Such a ditch 
 can be made by several trips with a plow. 
 It would probably be impractical to make 
 the ditch large enough to carry peak 
 flows, so you should avoid diverting 
 water during periods of high runoff. 
 
 Avoid locating the reservoir on mate- 
 rial that may allow excessive seepage — 
 shale and similar formations are fre- 
 quently very porous and even solid mate- 
 rial such as granite may have cracks that 
 will allow water to escape. The best reser- 
 voir bottom would be a natural layer of 
 fine-textured soil. 
 
 Since trees and shrubs should be re- 
 moved from the reservoir site (for rea- 
 sons given later) , consider how dense this 
 vegetation is. If it is very dense, the cost 
 of clearing might be excessive. 
 
 For the spillway 
 
 A spillway can be located in any of 
 several positions relative to the dam, but 
 in any event a wide, shallow spillway is 
 usually preferable to a deep narrow one 
 (see page 15). 
 
 The spillway bottom for at least part 
 of its length should be through material 
 that will not wash, such as solid rock. In 
 most instances rock can be found which 
 
 can be excavated with a bulldozer. In 
 some cases it may be necessary to use 
 dynamite. Here are four possible locations 
 of a spillway, arranged in order of ease 
 and probable economy of construction. 
 
 If you can locate your dam just up- 
 stream from the confluence of two streams 
 you can usually build the spillway to the 
 undammed stream (see diagram page 
 15). Where possible, it is well to run 
 such spillways along the contour and 
 bring them into the undammed fork well 
 upstream. This will slow down the flow of 
 the spillway water and prevent erosion. 
 
 Occasionally it is possible to construct 
 a spillway through a saddle upstream 
 from the dam, into another watercourse 
 that drains an adjacent basin. 
 
 Many spillways are built around one 
 end of the dam. When this is done, the 
 water must be led into the stream below 
 the dam without washing the dam itself — 
 this may be very difficult in a steep-sided 
 canyon. 
 
 As a last resort it might be necessary 
 to put a concrete spillway over the center 
 of the dam but such structures are very 
 expensive, difficult to maintain, and not 
 infrequently wash out. 
 
 It should also be pointed out that a 
 dam of less than 25 acre-feet storage 
 capacity, built on a major stream, will 
 require such a large spillway that it may 
 make the cost of the project out of line 
 with any benefits to be derived. 
 
 For the watershed 
 
 Check the watershed area for soil ero- 
 sion. Excessive erosion will silt up your 
 reservoir in a relatively short time and 
 perhaps make the whole project economi- 
 cally unsound. 
 
 How to Measure the Storage 
 Capacity of the Reservoir 
 
 To make sure your proposed dam will 
 comply with state regulations, and store 
 enough water to meet your needs, you 
 will want to know the capacity of the 
 
 [6] 
 
reservoir. To get this information, make 
 the following measurements: 
 
 Height of dam. The state defines the 
 vertical height of a dam as the distance 
 between "the level of the natural stream- 
 bed at the downstream toe (of the dam) 
 to the crest (bottom of spillway) of spill- 
 way," or to the crest of any barrier built 
 in the spillway. 
 
 Area of proposed reservoir. Start- 
 ing at the crest of the proposed spillway, 
 use a level (hand-held or surveyor's) and 
 outline the reservoir with lath stakes. 
 Drive stakes at intervals of no more than 
 100 feet. 
 
 Using the line between the stakes at 
 either end of the proposed dam as a base, 
 lay out lines parallel to this base, with 
 one end of each line at an angle in the 
 shoreline (see diagram, below) . 
 
 Lay out as many of these parallel lines 
 as necessary. You will then have a series 
 of strips and one triangle, the areas of 
 which can be computed as follows : 
 
 To get the area of W (in the diagram) 
 measure Lines AB, CD, and Pi. Then: 
 
 Length of AB + Length of CD 
 
 — x Length 
 
 of P 2 = sq. ft. in strip ABDC. 
 
 You can step off the lines shown and obtain 
 a fairly accurate estimate of your reservoir's 
 capacity. 
 
 Do the same for Area X by Lines CD, 
 EF and P 2 . 
 
 Length of CD + Length of EF 
 
 — ^— - x Length 
 
 of F 2 = sq. ft in strip CDFE. 
 
 In the triangle EFG, find the area as 
 follows : 
 
 Length of EF T , , n , . 
 ^— x Length of P 3 = sq. ft. in 
 
 triangle EFG. 
 
 Add figures for the sq. ft. of all strips 
 and the triangle; divide this total by 
 43,560 (the sq. ft. in an acre) to get 
 the number of surface acres in the pro- 
 posed reservoir. 
 
 A widely used formula assumes that 
 the average depth of the reservoir is one 
 third of the maximum depth. So to esti- 
 mate the capacity in acre-feet, multiply 
 the number of surface acres by one third 
 the maximum depth in feet of the pro- 
 posed reservoir. 
 
 Line AB is level with bottom of 
 proposed spillway. 
 
 [7] 
 
Building the dam takes careful preparation 
 
 Whether you build the dam yourself Cut-off trench 
 
 or hire the work done, you should ha« Mos[ watercourses have el ;„ them 
 
 the details of the completed structure well and there are ^ {o be d s{rata 
 
 in mind ; have a carefully planned con- buried [n ^ bankg and bekw ^ s(ream 
 
 struction program by steps Material and bed gince (h ^ ]eak wa(er ^ 
 
 equipment should be available as needed. .. *. u i. 1 diu 
 
 H v strata must be broken up. Bulldoze a 
 
 ex 1 l *l j •* trench across the streambed directly be- 
 
 Stctke out the damsite , , P . . , * 
 
 low the crest 01 the proposed dam. Ihe 
 
 The upstream, or wet face of the dam trench need only be about the width of 
 
 should have a slope of at least 3 feet the dozer blade and should extend both 
 
 horizontally to 1 foot vertically. The ways from the watercourse and to a depth 
 
 downstream, or dry face, should have that will insure interruption of all gravel 
 
 a slope of at least 2 feet horizontally to stra ta and other pervious material. (See 
 
 1 foot vertically. The crest of the dam sketch, page 9.) 
 
 should be 10 feet wide so as to provide Backfill the trench with the same mate- 
 
 a roadway if necessary. rial that will be used for building the 
 
 So assuming an almost level stream- dam. Be sure to get good compaction of 
 
 bed, a dam 25 feet high will occupy 135 this material, 
 feet horizontally. 
 
 Upstream slope 75 ft. Moving the soil 
 
 Downstream slope 50 ft. 
 
 Crest of dam 10 ft. You are now ready to start bringing in 
 
 soil to build the dam and it is assumed 
 
 T° ta l 135 ft. that the site has been chosen with an 
 
 A dam 20 feet high would occupy 110 ample supply of medium -textured soil 
 
 feet horizontally — and so on. nearby. Do not take soil from the reser- 
 voir site if the underlying strata are not 
 
 Prepare the site very carefully watertight. 
 
 Remove all rocks greater than 6 inches , ^ing in the soil and spread the fill in 
 
 in diameter from the entire damsite. Up- ^ allow layers— not over 6 inches thick, 
 
 root and burn or remove all trees and The equipment passing over these layers 
 
 stumps. Push off or burn all brush, heavy sho T uld result in satisfactory compaction, 
 
 grass and weeds, decaying logs and other If y° u , can ^ mld the dam late in the 
 
 debris. Any of this material that cannot s P nn S' whlle the sml ls stlU dam P ( but 
 
 be burned should be removed down- not wet), you will obtain best results. If 
 
 stream— not upstream into the reservoir the dam 1S built with dry soil it may be 
 
 •. necessary to haul in water and sprinkle — 
 
 5 The soil under the dam should be prac- a costl >' procedure. 
 
 tically free of coarse vegetative matter Slope gauge. To maintain the slopes 
 
 before you start building. you want on both wet and dry faces of 
 
 Next, disk the entire damsite. Mix the the dam you can use a gauge similar to 
 
 soil enough so that there will be no layer the one shown in the illustration (page 
 
 of trash or vegetation that will seep water 10 )- Use a 3-for-l gauge on the wet 
 
 when the dam is filled. This loose soil si( k; a 2-for-l gauge on the dry side, 
 
 will also make a good bond with the fill Reseeding your cuts. If you use soil 
 
 material placed over it when building from a hillside above the dam for your 
 
 the dam. fill, you will probably leave a bare area 
 
 [3] 
 
Trench under proposed crest is about the 
 width of a dozer blade and deep enough 
 to take out pockets of gravel. 
 
 Gravel pockets 
 
 that will erode badly during winter rains. 
 Best protection against this is to push 
 the top, shallow layer of soil and debris 
 to one side, then push it back over the 
 cut after the dam is built. If this does not 
 provide sufficient cover, haul in straw or 
 manure. A little seed and fertilizer will 
 then start growth in the scraped bare area 
 and avoid erosion. 
 
 The outlet pipe 
 
 An outlet pipe can be used for draining 
 water from your reservoir for irrigation, 
 watering stock, making repairs on the 
 dam, locating leaks in the reservoir, dis- 
 posing of floating debris, getting rid of 
 undesirable fish or vegetative matter. 
 
 A siphon over the dam can be used as 
 an alternate to an outlet pipe but priming 
 
 is necessary to start a siphon flowing. 
 
 Priming consists of removing the air 
 with a common suction pump or with a 
 suction pump operated off of and by your 
 auto engine. 
 
 Then, of course, water can always be 
 pumped directly from the reservoir to 
 any place that it is needed, but this may 
 be expensive. 
 
 Assuming that an outlet pipe will be 
 wanted, here are some pointers on in- 
 stallation. 
 
 The pipe should be large enough to de- 
 liver all the water you will need, what- 
 ever your intended use. The minimum 
 pipe size — even for a watering trough — 
 should be 1^2 inches in diameter. Any- 
 thing smaller would be too difficult to 
 clean if it got plugged. 
 
 Trench under proposed crest of dam ex- 
 tends into can/on sides far enough to 
 remove pockets of grovel lodged there. 
 
 [9] 
 
Level wired in place 
 
 Two feet high — 6 feet horizontal gives a 3-1 gauge; 3 feet high — 6 feet horizontal gives a 2-1 
 gauge. 
 
 For irrigation, a 6-inch pipe might be 
 required. In any case, galvanized water- 
 pipe is the most commonly used. 
 
 The intake end of the pipe should be 
 high enough above the upstream toe of 
 the dam so that subsequent settling of 
 silt and debris will not clog it, but low 
 enough so that usable irrigation water 
 is not left in the reservoir. In a shallow 
 
 reservoir the pipe might be almost on 
 the bottom; in a deep reservoir it might 
 be 5 or 6 feet above the bottom. The most 
 favorable condition for laying the outlet 
 pipe is in a trench dug in the fill material 
 used for the dam as described below. 
 
 When to install. Build the dam 2 
 feet above the point you have selected for 
 the position of the outlet pipe, then dig a 
 
 The hillside in the background would have been benefited by seeding. This reservoir is already 
 starting to silt up — note the fan forming at the far end of the dam. 
 
Two ways of operating the valve on an outlet pipe. 
 
 trench about 2 feet deep in the partly 
 constructed dam. Lay the pipe in the 
 trench. 
 
 Concrete collars should be poured 
 around the outlet pipe in the upstream 
 two thirds of its length. These collars 
 should be at least 8 inches thick, extend 
 out from the pipe 18 inches, and be not 
 more than 15 feet apart. After construct- 
 ing the collars, the backfill around both 
 
 pipe and collars should be tamped thor- 
 oughly. 
 
 The outlet gate should be installed on 
 the upper, or reservoir end of the pipe 
 so that in case of pipe failure the water 
 can be shut off while repairs are made. 
 If the gate is on the downstream end, the 
 pipe will be under pressure at all times, 
 making repairs for leaks difficult. 
 
 A cast iron slide gate of the type com- 
 
 The bank of this reservoir was seeded, but note the better growth of covercrop where fertilizer 
 was applied (as pointed out by the man in the background). 
 
*0h> 
 
 This dam uses a siphon instead of an outlet pipe which is proving very satisfactory for the owner. 
 Power from the automobile engine is used to pump air out of the pipe and start the water flowing, 
 but any small, gasoline-driven engine can be used to perform the same function. 
 
 monly used in concrete irrigation pipe 
 systems is satisfactory. The slide gate 
 should be fastened securely to the outlet 
 pipe by means of an ample block of con- 
 crete. It may be set in a vertical position, 
 in which a catwalk will be needed for 
 operation, or in a sloping position so that 
 it can be operated from the crest of the 
 dam. (See diagram.) 
 
 A regular valve that screws onto the 
 pipe is also satisfactory, but such valves 
 are usually quite expensive. The pipe next 
 to the valve should be anchored in a block 
 of concrete for stability. 
 
 A siphon may be used in place of an 
 outlet pipe. Siphons are usually made of 
 sheet metal pipe, coated with asphalt — 
 considerably cheaper than regular water 
 pipe. However, when using a siphon, 
 some method of priming must be devised 
 to remove the air to start the water flow- 
 ing. 
 
 There is usually quite a lot of debris, 
 scum, moss, etc., in a reservoir. If the 
 upper end of the outlet pipe is not pro- 
 tected against this material by a screen 
 it is apt to become stopped up. It is usu- 
 ally a good idea to have a rather large 
 screen so that it will catch a lot of trash 
 before it has to be cleaned. In general 
 
 a satisfactory screen can be built by mak- 
 ing a crate shaped as a cube with about 
 3-foot edges and covering five sides with 
 wire netting with openings of 1-inch or 
 less. If you think the screen may become 
 plugged before the end of the season you 
 may want to provide some means of rais- 
 ing it to the surface for cleaning and 
 restoring it to its former position. 
 
 Crown the dam both ways 
 
 The crest of the dam should be graded 
 lengthwise so that the center is at least 
 6 inches higher than the ends. This will 
 serve the double purpose of keeping rain- 
 water from running from the ends toward 
 the center, then down the faces where it 
 will cause erosion. It will also be helpful 
 in case water overtops the dam and causes 
 washouts — the washouts will occur at 
 either end, where they are relatively easy 
 to repair. A hole in the middle of the 
 dam requires a big repair job. 
 
 Crown the top of the dam sideways as 
 a road so that it will shed water instead 
 of gullying the road tracks. 
 
 When a dam has been filled with water 
 and has gone through one winter it will 
 usually settle — just how much is difficult 
 to predict. It may therefore be desirable 
 to add more soil and regrade the crest of 
 
 [12] 
 

 . .-}••■• 
 
 The crest of this dam has been crowned both ways so that it will shed water. The road across the 
 dam dips through the spillway on the far side. 
 
 the dam after it has been through one 
 winter. 
 
 Protective covering 
 
 The entire downstream face, the top, 
 and the upstream face above the water- 
 line should be protected against erosion 
 by a vegetative cover. This will be espe- 
 cially important during the first winter. 
 
 Annual ryegrass will do the job. Fer- 
 tilize well and use a mulch as described 
 for cuts above the reservoir for rapid 
 growth. 
 
 Avoid grain cereal crops as they will 
 attract squirrels and gophers that will 
 burrow into the dam. 
 
 Final Check for leaks 
 
 After the dam is completed make a final 
 check of the reservoir site for potential 
 leaks in an area at least 100 feet wide, 
 extending upstream from the dam. 
 
 Cover all rock outcrops and areas of 
 sandy or gravelly soil with surface soil. 
 Fill all holes left when trees or brush were 
 removed. 
 
 It is difficult to prescribe definitely how 
 much of this work will be needed or how 
 it will be done, but the idea is to seal all 
 sources of possible leaks. The importance 
 of this operation cannot be over-empha- 
 sized — a leaky dam is a real headache. 
 
 The spillway can be very important 
 
 The location of the spillway was dis- 
 cussed on page 6. Its size will be deter- 
 mined by the volume of water it must 
 accommodate, which in turn is dependent 
 on the number of acres in the drainage 
 area above the dam. 
 
 Size. The depth of the spillway is 
 measured as the difference in elevation 
 between the highest point in the bottom 
 of the spillway to the lowest point on the 
 crest of the dam. A minimum spillway 
 should be 3 feet deep and 10 feet wide. 
 
 [13] 
 
The table below gives the desirable 
 sizes for spillways for different drainage 
 areas. 
 
 Shape. It is usually more economical 
 to make your spillway shallow and wide 
 rather than deep and narrow because 
 the latter type of spillway will probably 
 necessitate a higher (bigger) dam. 
 
 For example — your drainage area 
 takes in 600 acres; from the table con- 
 sider 2 of the 6 alternatives. You find 
 that your spillway can be 5 feet deep and 
 20 feet wide, or it can be 8 feet deep and 
 only 10 feet wide. 
 
 If you can use the 5 x 20 foot spillway 
 you can make your dam 3 feet lower than 
 would be the case with the 8 x 10 foot 
 spillway. The sketch on page 15 gives an 
 idea of the savings you can effect in the 
 movement of fill. 
 
 Remember also — if you intend to use 
 a barrier in your spillway to store water 
 from the late spring runoff, the spillway 
 will have to be of such a size that it will 
 accommodate any permanent structures 
 supporting the removable part of the bar- 
 rier and still fulfill minimum require- 
 
 ments of outlet dimensions (see below — 
 Barriers) . 
 
 Another consideration is that a higher 
 barrier can be built in a deep spillway 
 than in a shallow spillway, thus permit- 
 ting more water storage. Hence this whole 
 question of dam height, spillway size, and 
 water storage can involve quite a lot of 
 figuring to get the most economical setup. 
 When doing this figuring be sure to take 
 into account the limitations on barriers 
 in the spillway as discussed below. 
 
 Slope of spillway bottom. The 
 spillway bottom downstream from the 
 crest should have a slope of at least 6 
 inches per 100 feet so that the water can 
 get away rapidly. 
 
 Where necessary, line the bottoms of 
 spillways with concrete or other material 
 that will prevent erosion. 
 
 Occasionally a spillway is built 
 through earth, on a relatively flat grade 
 similar to an irrigation ditch. If needed 
 this type of spillway can be protected 
 from erosion by vegetation. 
 
 In any event — the water in a spillway 
 located at one end of the dam must be 
 
 Specifications for Spillways 
 
 Drainage 
 
 area 
 in acres 
 
 3 
 
 4 
 
 Depth of 
 5 
 
 spillway in feet 
 6 7 
 
 8 
 
 9 
 
 0- 129 
 
 10 
 
 
 Width of 
 
 spillway 
 
 in feet 
 
 
 
 129- 190 
 
 22 
 
 13 
 
 10 
 
 
 
 
 
 191- 320 
 
 34 
 
 20 
 
 11 
 
 10 
 
 
 
 
 321- 480 
 
 47 
 
 27 
 
 16 
 
 12 
 
 10 
 
 
 
 481- 640 
 
 58 
 
 34 
 
 20 
 
 15 
 
 11 
 
 10 
 
 
 641- 800 
 
 
 40 
 
 25 
 
 19 
 
 13 
 
 11 
 
 10 
 
 801- 960 
 
 
 44 
 
 27 
 
 20 
 
 15 
 
 12 
 
 10 
 
 961-1280 
 
 
 55 
 
 35 
 
 26 
 
 19 
 
 15 
 
 12 
 
 1281-1600 
 
 
 
 42 
 
 31 
 
 23 
 
 19 
 
 15 
 
 1601-1920 
 
 
 
 48 
 
 35 
 
 26 
 
 21 
 
 17 
 
 1921-2240 
 
 
 
 55 
 
 40 
 
 30 
 
 24 
 
 19 
 
 2241-2560 
 
 
 
 60 
 
 42 
 
 32 
 
 26 
 
 21 
 
 These figures taken from ASCS-SCS specifications for construction of earth dams, revised November 1, 1954. 
 
 [14] 
 
Crest of dam having wide, shallow spillway 
 
 Crest of dam having deep, narrow spillway 
 
 Additional fill needed— about 4 
 cubic yards per foot of dam length 
 
 [15] 
 
delivered to the stream below the dam so 
 that it will not erode the dam itself. 
 
 Barriers in the spillway 
 
 Spillway sizes as shown in the table 
 on page 15 are calculated to leave a free- 
 board of about 1.5 feet for any flood that 
 might occur once in 25 years. Thus dams 
 built to the specifications given in this 
 circular are designed to hold water to 
 within 1.5 feet of their crest at least for 
 short periods. 
 
 It would therefore be practical to store 
 water from late spring runoffs in these 
 dams to a height somewhat lower — per- 
 haps 2 or 2.5 feet from the crest — by 
 means of barriers in the spillways. 
 
 But before you build barriers, 
 consider these facts and possibilities: 
 
 The Department of Water Resources 
 defines the height of a dam as the dif- 
 ference in elevation between the down- 
 stream toe and the crest of any barriers 
 in the spillway. Hence it would be illegal 
 to place barriers in the spillway that 
 would violate the height regulation of a 
 dam that is exempt from state supervision 
 and inspection. 
 
 On dams built with ACP assistance, 
 that agency does not allow any structure 
 in the spillway that reduces the area of 
 the spillway below the minimum require- 
 ments. 
 
 Remember also that barriers in the 
 spillway have . . . 
 
 Advantages — you can store addi- 
 tional water from late spring runoff to 
 be used in irrigating or making stock- 
 watering ponds last longer into the sum- 
 mer and fall. 
 
 And disadvantages — the dam must 
 be watched more closely. Barriers must 
 be removed immediately if late spring 
 rains cause excessive runoff. They must 
 always be removed before fall rains start. 
 Piers in the spillway, to support flash- 
 boards, may cause floating debris to clog 
 the spillway. 
 
 If, after considering all of the good 
 and bad features, you decide you want 
 barriers, here are some construction tips: 
 
 Earth-filled sacks make practical and 
 inexpensive barriers. 
 
 Piered drop structures, similar to 
 those used in irrigation canals can also 
 be built in the spillway. While it is be- 
 yond the scope of this circular to give 
 detailed specifications for such struc- 
 tures, a few generalities may be useful 
 (see photo, page 17). 
 
 Space all piers an equal distance apart 
 so that drop boards will all be the same 
 length and can be used interchangeably. 
 They should not be more than 6 feet long. 
 Drop boards can be of 2" x 6" lumber, 
 but 3" x 6" are better because they will 
 
 This dam was not properly crowned lengthwise and the flashboards were not removed in time. 
 That break (on the left) will be expensive to repair. 
 
Here earth-filled sacks are used in the spillway to store additional water from late spring runoffs. 
 Such barriers are inexpensive and easy to remove. 
 
 give less distortion when the water rises 
 behind them. 
 
 The channels into which the ends of 
 the boards fit are best made of channel 
 iron — slots in the concrete piers are 
 usable but will probably result in more 
 leakage. 
 
 The face of the barrier should slope 
 from 30 to 45 degrees downstream. 
 
 It is usually desirable to pour a slab of 
 concrete for the water to fall on, in order 
 to prevent erosion. 
 
 When drop boards are put in place 
 there may be considerable leakage 
 around them. This leakage can be stopped 
 by putting a blanket of trashy soil or 
 manure on the upstream side of the 
 boards. 
 
 A good spillway — cut down to solid rock. Some 
 of the softer part of the rock has been washed 
 away, but has reached stability. 
 
 Flashboards supported by concrete piers can 
 be removed easily if late spring rains cause 
 excessive runoff from the watershed. 
 
Maintenance will pay off in good service 
 
 Keep the spillway clear of floating 
 debris. Even though you have thoroughly 
 cleared off the reservoir site, a certain 
 amount of material may find its way into 
 the spillway from time to time and cause 
 clogging, if not removed promptly. 
 
 A log boom floating about 50 feet from 
 the spillway will keep most floating ma- 
 terial away. 
 
 A wire mesh fence — such as that used 
 for hogs — will also serve to intercept logs 
 and brush but would have to be rather 
 high. 
 
 Pine cones and small pieces of wood 
 that will clear the spillway need not be 
 intercepted, but all material that is caught 
 by boom or fence should be removed in 
 the fall before the rains start. Such ma- 
 terial should be burned or hauled out of 
 the reservoir. 
 
 Wave action may cause problems on 
 dams in areas where the surface of the 
 reservoir is exposed to high winds. A log 
 boom floating close to the dam is usually 
 effective in preventing wave damage. 
 Sometimes it is necessary to put a blanket 
 
 of coarse gravel on the threatened part 
 of the dam. 
 
 Trees and brush should not be allowed 
 to grow on the dam — they have large 
 roots and if they should die, the root 
 channels might cause leaks. 
 
 Guard against rodents building 
 burrows in the dam. If rodents get 
 started, see your local University of Cali- 
 fornia Farm Advisor for information on 
 rodent control by poisoned baits, traps, 
 etc. 
 
 Seeps through the dam. In some in- 
 stances the water, even in a well-designed 
 dam, may cause seepage on the down- 
 stream face of the dam. Again, if water 
 is held too high in the dam, it may cause 
 similar seepage. In either case this con- 
 dition may cause the downstream toe of 
 the dam to slough off. The reason for this 
 is that a large part of the soil in the dam 
 becomes saturated — the upper limit of 
 the zone of saturation extending from the 
 waterline of the reservoir toward the 
 downstream toe. 
 
 When the above condition is noted the 
 
 This dam shows the results of little or no maintenance. Large trees have been allowed to grow 
 on the dam. If these trees should die or blow over the dam might be seriously weakened, due to 
 rotting roots or large holes. 
 
water level in the reservoir should be 
 lowered immediately to a point where the 
 saturated area on the downstream face 
 disappears. Then you can build a down- 
 stream addition to the dam, bringing the 
 additional fill to a point a few feet above 
 the top of the seep area. 
 
 Watch those barriers. If you have 
 barriers in the spillway it is extremely 
 important — for the safety of the dam — 
 that they be removed when the dam is 
 full and runoff from a late spring rain 
 occurs. This has to be done in a matter 
 of minutes — not hours — so it is abso- 
 lutely essential that someone be on the 
 job when such unusual conditions arise. 
 
 Also be sure that barriers are removed 
 before the rains begin in the fall. 
 
 For the spillway 
 
 Keep the spillway from becoming 
 clogged by removing debris promptly. 
 
 Each fall burn or remove all material 
 collected by a log boom or wire fence. 
 
 Sometimes water flowing in a spillway 
 may cause a gully that heads back toward 
 the crest of the spillway. This is usually 
 due to the fact that the underlying rock, 
 such as decomposed granite, was softer 
 
 than anticipated. When such conditions 
 arise it may be necessary to line part of 
 the channel with concrete, put in a drop 
 structure or use other means to prevent 
 heading back. 
 
 For the watershed 
 
 Check the watershed periodically for 
 erosion of soil that will eventually wash 
 down into, and fill your reservoir. Take 
 whatever steps may be needed to check 
 erosion. This is very important if you are 
 to get good service from your reservoir. 
 
 For the reservoir 
 
 If you have followed the suggestions 
 and specifications in this circular for 
 selecting and clearing the site for your 
 reservoir it is unlikely that severe leaks 
 will develop. However, leaks in reservoirs 
 may develop in spite of all the precau- 
 tionary measures you have taken. 
 
 Leaks are usually caused by previ- 
 ously undetected cracks in underlying 
 rocks, or by channels of pervious mate- 
 rial. They are also usually very difficult 
 to locate. 
 
 Most leaks originate in the reservoir 
 bottom, relatively close to the upstream 
 face of the dam. 
 
 Badly tended dam. Spillway on opposite side was not lined and has eroded to the point where 
 the entire dam is threatened. To repair it would be very expensive. 
 
 
 
 * - 
 
 
Here are some suggestions for locating 
 and repairing leaks. 
 
 Watch for stoppage of leaks as the 
 water in the reservoir is lowered in steps. 
 If the leak stops as the water recedes over 
 a definite area, it is fairly certain that 
 the leak occurred in that area. Stakes 
 around the water's edge will help keep 
 track of recession if the natural shore 
 line is not apparent. 
 
 When the area is located, look for out- 
 croppings of rocks, gravel, or sandy soil 
 — any or all of these may be the source 
 of the leak. 
 
 Cover leak-producing areas with a 
 blanket of clay, 3 or 4 inches thick. If 
 the leak is found to occur below the low- 
 water stage of the reservoir a similar 
 blanket of clay will have to be spread 
 over the entire bottom close to the dam. 
 
 Clay for this work can be found on 
 most farms. The clay should be worked 
 up to very small clods — the finer the 
 better — before it is applied to the reser- 
 voir bottom. 
 
 Since a 4-inch layer of clay on one 
 acre would weigh about 650 tons, it is 
 easy to see how important it is to locate 
 small areas for treatment. This also adds 
 importance to the final inspection and 
 
 treatment of the reservoir for possible 
 leaks before it is filled the first time. 
 
 Bank, shoreline and 
 water-loving weeds 
 
 About a dozen plants, including willows 
 and cottonwoods, may prove to be pests. 
 Under some conditions it may be prac- 
 tical to control this vegetation by hand 
 or by cultivation. 
 
 Control measures for cattails, tules, 
 floating and submerged weeds, and algae 
 are described in the booklet mentioned 
 in the box on this page. 
 
 Controlling marginal vegetation by 
 sprays is difficult because any given prod- 
 uct may not be uniformly effective in 
 different areas — the variations possibly 
 being due to varying chemical constitu- 
 ents of the water. 
 
 No recommendations are given here 
 for specific sprays or chemicals to con- 
 trol weeds because new products are 
 being developed continually. Your Uni- 
 versity of California Farm Advisor office 
 can provide you with weed control in- 
 formation in the form of a booklet that 
 is revised each year. 
 
 Management of the reservoir 
 
 Trout ponds 
 
 These can be established in reservoirs 
 where the temperature of the water does 
 not rise above 70° F for any length of 
 time. If the owner permits the public to 
 fish in his trout pond, the Fish and Game 
 Department will provide fish for stock- 
 ing the pond (trout will not spawn in 
 reservoirs so have to be replaced con- 
 tinually) . 
 
 If the owner of a trout pond does not 
 open it to public fishing, the state will 
 not provide fish but will furnish a list 
 of trout dealers and a transportation per- 
 mit at no charge. 
 
 [20 
 
 In any case, fishing is allowed only 
 during trout season. 
 
 Warm water ponds 
 
 Theoretically, the most productive 
 planting of warm-water fish is a mixture 
 of bass and bluegills — botli species re- 
 produce rapidly and both make good eat- 
 ing. One of the main sources of bass 
 food is young bluegills; carp and catfish 
 tend to eliminate other species and take 
 over the reservoir. 
 
 After an initial stocking, allow warm- 
 water fish to spawn once before doing 
 any fishing — spawning will be evidenced 
 
 ] 
 

 
 Water for sprinkler irrigation of otherwise poor pastureland can often be taken from a dam but 
 pumping is usually required. 
 
 by the presence of small fish. Most reser- 
 voirs are underfished — when large num- 
 bers of fish are present they do not attain 
 the size they would if fewer were left in 
 the pond. 
 
 Food for fish is quite often produced 
 in abundance by plankton-like material 
 in the reservoir. It is evidenced by 
 brownish coloring in the water. 
 
 While opinions differ on whether reser- 
 voirs need fertilizer to help produce fish 
 food, local experience indicates that it is 
 seldom necessary. It should be avoided, 
 if possible, because addition of fertilizer 
 to the reservoir will encourage undesir- 
 able marginal growth which will have 
 to be destroyed. 
 
 Muddy water in the reservoir may 
 be due to the presence of a large number 
 
 of carp or catfish which feed on the bot- 
 tom and stir up the mud. This can only 
 be avoided by eliminating the carp or 
 catfish. 
 
 If the muddy condition is not due to 
 carp or catfish, but winter runoff, it can 
 be alleviated by adding finely ground, 
 agricultural gypsum to the water — a 
 rather expensive process. The gypsum 
 costs about $15 per ton and it may take 
 as much as half a ton per acre-foot of 
 water to clear up muddiness. However, 
 much smaller amounts may do the job 
 so it is suggested that the gypsum be 
 applied at the rate of about 200 pounds 
 per acre-foot, allowing a few days to ob- 
 serve results before adding more. 
 
 Spread the gypsum uniformly over the 
 surface by shoveling from a boat. 
 
 Owners of this lake 
 not only enjoy fish- 
 ing but have set up 
 a picnic area on a 
 pleasant part of the 
 shore. 
 
 % 
 
: ■- " 
 
 2fe 
 
 Here a small pump takes water out of a dam through the spillway. This saves lifting the water 
 over the crest of the dam and is practical in some instances. 
 
 Some do's and don'ts in dam construction 
 
 Here are random photos of farm dams cover all good points of dam construction 
 
 in two California counties — some good, in a booklet this size it is hoped that these 
 
 some with problems. Since it is obviously photos will bring to mind a few ideas 
 
 impossible to warn against all hazards or that may prove valuable. 
 
 This pond occupies land formerly made useless by a winding creekbed. The photo was taken 
 from end of dam, looking upstream. 
 
 [22] 
 
Here the dam itself held but the soil on the bottom of the pond was too porous and the water all 
 seeped out. Small pool in the background is all that is left of a good-sized pond. 
 
 Front and back views of the concrete-lined spillway for the dam pictured at the left. Spillway 
 is in the center of the dam and empties into the old creekbed. 
 
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