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<br />001532
<br />
<br />plane the full width and,length thereof, and no side dumping of materials
<br />should be permitted.
<br />
<br />The materials should at all times contain sufficient moisture to
<br />provide proper compaction. Puddling of material with water should not
<br />be permitted, and no frozen material orlarge stones should be incorpor-
<br />ated into the dam. The upstream face of the dam should be constructed
<br />with a slope no steeper than 2 1/2 to 1, and the downstream face with a
<br />slope no steeper than 2 to 1. The crest of top of the finished dam should
<br />be not less than 8 feet in width. Outlet pipes other than a small diameter
<br />pipe leading to a tank or trough, are usually not needed through stock
<br />pond dams, but if they are permitted and are necessary, they should be
<br />equipped with a control valve attached to the upstream end of the pipe,
<br />together with a suitable mechanism for operating the valve. Any outlet
<br />pipe should be provided with concrete or metal cutoff collars. The pipe
<br />should be placed in a tranch excavated in stable formation, and should be
<br />completely surrounded with well compacted impervious materials.
<br />
<br />For the protection of the dam, the spillway should be designed
<br />with sufficient capacity to discharge the water from the drainage bas in
<br />above the dam at the rate of peak flow from a maximum hour storm of
<br />25-year frequency. The spillway should be located in stable formations
<br />not easily eroded or it should be riprapped or paved. In all cases the
<br />outlet end should be extended well away from the dam. The top of the
<br />smaller dams should not be less than 1 1/2 feet above the elevation of
<br />the maximum high water line expected in the spillway. Larger dams
<br />should have more free board.
<br />
<br />The following simpUHed curves show the estimated runoff in
<br />cubic feet per second to be expected from various drainage'areas and
<br />storms of different intensities. Using these curves and Weather Bureau
<br />records"o,r other data to arrive at the maximum I-hour precipitation
<br />for a 25-year stor,m, the spillway may be designed by anyone of the
<br />several recog,nized channel or broad crested weir formulae.
<br />
<br />The width of the entrance to the spillway should be one-third
<br />wider than that arrived at by using the curves and channel or weir form-
<br />ula. The bottom should have a negative slope from the throat of the funnel
<br />section toward the reservoir at a rate of not less than 2 percent.
<br />
<br />Borrow pits from which materials are taken to construct the dam
<br />should be cl ear ed of all veg eta blematter and no material should be bor-
<br />rowed within a distance of 50 feet from the downstream toe of the dam.
<br />Materials excavated from the spillway, when suitable, can be used in
<br />constructing the dam.
<br />
<br />42
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