A properly designed earth dam should have a slope on the water face of not over 1 in 3. On the rear face the slope may be as steep as 1 in 12. A firm bond to the underlying ground should be obtained by removing from an area at least 6 feet wide, in line with the future crest of the dam, all vegetation and loose soil for a depth of at least 6 inches, and by digging in the area thus cleaned a trench at least 2 feet deep and filling it with new material similar to that

FIGURE 36.-Profile through typical Papago represo

to be used in the dam. On many adobe flats, however, simple plowing will be sufficient. The crest of the dam should not be less than 5 feet above the bottom of the spillway except in a small reservoir, where a height of 3 feet is sufficient. A type of cross section that has proved successful in Wyoming and South Dakota 20 is shown in Figure 37.

An earth dam should be protected by a fence from loose stock, which destroy the smooth slopes and often wear trails deep enough to lower the dam considerably. In a large reservoir waves rapidly erode the dam, especially if the water body lies on the windward side of the dam. Protection against waves can be made by riprapping with rock or brush or by building a wave fence.

A wave fence is constructed of 1 by 10 inch boards 8 feet long, pointed at one end and driven into the embankment at high-water level. The boards are driven in as close together as possible, about 3 feet in the ground, at a slope of about 5 in 1 away from the water.

26 Hermann, F. C., Small reservoirs in Wyoming, Montana, and South Dakota: U. S. Dept. Agr. Office Exper. Sta. Bull. 179, 1907.

The fence should be made rigid by two strings of 1 by 10 inch boards, one nailed near the top and the other near the bottom of the fence. Braces fastened to deadmen should also be installed. Wave fences are very effective and with ordinary repairs will last in this region seven or eight years.

Brush protection of the face of the dam, if properly constructed, is as good as a wave fence, and the large quantities of mesquite, palo verde, and creosote brush available in most localities in southwestern Arizona make this the natural and cheapest thing to use. The brush should be made up in bundles about 1 foot in diameter and of any convenient standard length. At least two layers of bundles should be laid at right angles to each other on the face of the dam, and the whole should be firmly wired to strong stakes driven into the dam. This form of brush protection will probably be more effective than the brush fences that have been used on a number of dams. The wear on an embankment by waves in spite of brush fences, also the damage done by loose stock, can be seen at Compartidero represo (Pl. XXII, B).

FIGURE 38.-Diagram showing the use of logs and brush for diversion dams and spillways

DIVERSION DAMS AND PROTECTION OF SPILLWAY CHANNELS

Diversion dams in rocky channels can easily be constructed of plank or cribbing, or, if the expense is justified, of masonry. In channels where no firm foundation exists diversion dams are usually constructed of brush and rock held in place by strong stakes. Such dams are likely to be washed out by any flood. If the washout occurs during the only flood of the year that is sufficient to fill the reservoir, the consequences are serious.

W. E. Kibbey, formerly of the La Osa Land & Loan Co., has used successfully the structure illustrated in Figure 38. The channel is cleared of loose sand, and the banks are cut in sufficiently to give a good bond. The largest logs obtainable are then laid end to end across the channel at the toe of the dam. Mesquite trees and large branches, each trimmed so that one branch makes a hook at the end, are laid over the logs with the hooks downstream, and the hooks are forced down over the logs as shown in the diagram. The dam is raised by laying successive rows of logs with broken joints and holding each row in place with brush. Each row lies upstream from the

last, so that the face of the dam has a slope that tends to break the force of the flowing water. The crest should slope from the banks to the center so as to divert the water away from the ends of the dam. The upstream side of the dam is covered with earth, and each flood augments this material up to the level of the top of the dam. It is obvious that as water flows over the dam the transverse logs can not be loosened and carried away, because they are held by the hooks on the brush; and if the toe of the dam is undermined, the somewhat flexible structure will settle without, however, shifting in position. If the dam settles so far as to be ineffective, it may be built higher by adding one or two additional layers.

A reservoir that is built without a wasteway in the expectation that it will hold all the flood discharge of a stream may have a structure of the type just described built at one end of the dam. It

FIGURE 39.-Diagram showing construction of a water drop to prevent erosion in channels. The dotted pattern indicates bundles of brush laid at right angles to the others

can be completely covered with dirt and in the damp soil will rot very slowly. By leaving this part of the crest low or by opening it with a shovel a safe emergency spillway will be available for any exceptional flood that may threaten the dam. This device, installed by Mr. Kibbey in the dam of the lake at Buenos Aires, has been in successful use for a number of years.

Erosion in wasteways due to abnormally steep grades often menaces reservoirs. Where the site does not provide a rock spillway the waste water may cut a channel which, beginning in a sharp cliff, works up the spillway with great rapidity. A structure somewhat similar to the one just described has been used as a water drop to prevent such erosion.28 Figure 39 is a cross section of this structure, which was built of brush and old railroad iron. The larger posts were old boiler flues and the smaller stakes old bolts and fishplates. The brush was made up in bundles about a foot thick, with wire binding, and the bundles were wired to one another and to the stakes. 28 Hermann, F. C., op. cit., p. 33, pl. 7, fig. 1.

Where mesquite is plentiful, mesquite posts would probably be cheaper and equally serviceable. By concentrating the fall at one point the grades of the other parts of the channel are reduced sufficiently to prevent erosion.

DÉBRIS-FILLED RESERVOIRS AND ARTIFICIAL SPRINGS

ADVANTAGES

Throughout southwestern Arizona evaporation from water bodies is excessive and the annual loss in open reservoirs is more than 5 feet. This loss is serious in small reservoirs and, together with the use of water by stray stock and wild animals, makes many rock tanks uncertain water supplies. Sand tanks of the same size do not suffer these losses and with proper protection are more sanitary. A small spring that arises from the fill of an old dam which has long been silted up is the principal source of water at the old mining settlement of Warsaw.29 If reservoirs full of clean sand and gravel could be constructed they would also have these advantages, though their water capacity would be less than a fourth of that of ordinary reservoirs. General plans for such reservoirs are proposed in the following paragraphs in the hope that they may be a guide in constructing watering places in localities where other water supplies are difficult to obtain.

METHODS OF CONSTRUCTION

The filling of such a reservoir must be composed of clean rock particles, preferably of nearly the same size, in order that the maximum porosity may be attained. To reduce expense, the dam must be so constructed as to permit the accumulation of the filling by natural processes as far as possible. In southwestern Arizona proper material for filling such reservoirs can be obtained only from streams in the mountains or in the belts of rocky plains around the mountains. In these localities the sand and gravel of the stream beds are clean and coarse and have about 25 per cent of pore space. An ordinary dam thrown across such a stream checks the current so much that not only sand and gravel but mud also is deposited. To obtain débris free of mud for the reservoir, the dam must be built up gradually, the current being allowed to rework the accumulation of each flood, or else a dam must be constructed that will check the current only enough to hold the coarse material and will let the finer material go through.

A dam of a type that may be built up gradually was constructed in California of boulders held in wire baskets. Chicken-yard netting

Bryan, H. M., personal communication.

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of 2-inch mesh and No. 14 gage galvanized-iron wire was used to inclose bundles of boulders, which were 2 feet wide, 1 foot thick, and 8 feet long. The bundles were laid side by side along the dam. In the second layer the bundles were staggered and lapped 4 feet inside the lower edge of the first layer. The downstream slope was thus 1 in 6; the upstream slope was 1 in 2. The dam was 150 feet long and 35 feet high and resisted not only the weight of the débris behind it but a continuous overflow of flood water for a period of five weeks.30 Such a dam built up by adding one to three layers after each flood would be very effective and easy to construct. Its life is, of course, dependent on the time necessary to rust the wire netting and on the wear on the netting by boulders tumbled over the face of the dam. Individual baskets can be replaced, however, and when it becomes necessary a whole new face can be constructed.

In many mountain ranges there are places where the canyons after running on comparatively flat grades contract and then run on steep grades. These narrows are natural dam sites, and many of

FIGURE 40.-Diagram showing rock-fill dam for an artificial reservoir

them are bordered by high cliffs. It seems probable that without much drilling, by taking advantage of open cracks and joints, the cliffs could be blasted down into the narrows. The mass of material thrown down would be of all sizes, but many of the blocks would be too large for the stream to move even in its greatest floods. As many large blocks as possible should be obtained. By shifting the material and relaying it, especially on the downstream face of the dam, the larger fragments could be made to protect the smaller. Floods on striking this dam would flow through, but with diminished velocity. The sand and gravel which they carried would be largely deposited in and behind the dam, while the mud would be carried through. Figure 40 is a cross section of a dam of this type, somewhat more regular in form than is absolutely necessary, though an effort should be made to make the downstream face as flat as possible. A dam of this type merely reproduces the conditions found in many canyons, where great boulders too large for the stream to move block the channel and cause sand and gravel to accumulate behind them.

30 Palmer, L. A., A novel débris dam: Min. and Sci. Press, July 10, 1915, pp. 43-46.