Water from Small Dams

The sketch shows that a freeboard 1.5 m high has been added to the MAX WL, andthat the dam wall has been raised above the freeboard with a convex (upward) crestto cater for settling of the soil in a new dam wall.Convex crestThe settling of soil in a new dam wall depends mainly on the method of compactingthe soil when building the dam wall. Therefore the crest of an earth dam shouldalways be highest at the middle and lowest at the ends (convex). This is to avoid awashout of the middle section of the dam wall in case the spillway is blocked, orcannot cope with the peak discharge in a heavy storm. Should a washout happen, it iseasier to repair the end of a dam wall instead of repairing the deep middle section.Soil used for building dams is excavated and transported to the site by baskets,wheelbarrows, ox-carts, donkey carts or tractor trailers, and off-loaded on the damwall. During the process, soil is broken into small pieces with voids filled by air.No matter whether the soil is compacted manually or by animals, tractors or evencrawlers, the height of a newly built dam wall will settle when the reservoir is filledwith water for the first time. This settling occurs because the soil, made pliable andheavy by water, will press air out of the voids in the soil.Many new earth dams have been washed away by floods over-topping their crestswhich had become too low (concave) at their middle due to soil having settled morethan expected. Dam walls must therefore always be made higher (convex) at theirmiddle section.The height of a convex crest above the freeboard depends on the height of the damwall measured from the floor of the valley to the freeboard as follows:10 % of the height of the dam wall when compacted dry by machinery.20 % of the height of the dam wall when compacted dry by people and livestock.30 % of the height of the dam wall when not compacted at all.The width of the crest of a dam wall should be sufficient to allow traffic to use thecrest as a road spanning across a valley. The minimum width of a crest should be 2 mfor walking and 3 m for vehicles, if they want to pass over the dam wall.35