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There are various factors to be considered in the selection of a good reservoir site, regardless of size, and a favorable spillway site is highly important. The ideal dam site is usually round at a narrow point in the valley with a reservoir area widening out upstream. A site on a main channel just below the confluence of a tributary allows both channels to form portions of the reservoir. A site in such a location generally does not have the necessary topographic features for a natural spillway nor is it conducive to the construction of a cut spillway which could take care of several years of excess runoff waters without serious erosion. The ideal condition for a natural spillway is one which will have the overflow spill wholly over solid rock but a satisfactory spillway can be established over an area which is heavily grassed and on a grade which will not develop water velocities high enough to cause serious erosion. Good natural spill way conditions are more often found near the upper reaches of stream tributaries where small, two or three acre foot reservoirs can be constructed as a stockwatering supply. Larger and more permanent reservoirs for a yearly supply of water can, in most cases, be found only on the lower reaches of stream tributaries where spillway problems have to be overcome.
moderate cost spillways for small water storage projects
Aaston, M. 1961. MODERATE COST SPILLWAYS FOR SMALL WATER STORAGE PROJECTS. Canadian Agricultural Engineering 3(1):13-16.