Water from Small Dams

9.2 Washed-out dam wallsThere are several reasons for a dam wall being washed-out, but the most common reasonis that the spillways became blocked, or were made too small and therefore unable todischarge floodwater sufficiently quickly. The water level in a reservoir will thereforerise and flow over the dam wall at its lowest point and thereby create a wash-out.Recommendations to prevent wash-outs of dam walls:• Spillways must always be designed and built to their required width and depth.• Any obstruction, including trees and bushes brought into a reservoir by floods, whichblock the spillway, should be cleared immediately.• Dam walls must always be constructed and maintained, with their crests being at least10% higher at the middle (convex) so that a wash-out will not take place at the middleof a dam wall, where it will require the greatest repair work, but at the end of a damwall where it is easier to repair.• The height of dam walls built without crawler compaction must be increased 30% tocompensate for the settling of soil when the reservoir is flooded.• The freeboard must be l.5 metres on new dams. The freeboard may be reduced to 1.2metres after a reservoir has been flooded several times and the soil in the dam wallhas settled completely.The wash-out of the Kimuu dam wall, which is described in this handbook, wasdue to heaviest rainfall in 72 hours for more than 50 years, and blockage of thespillways by trees that were uprooted by the rains.56