Abstract:
In current practice, the two primary approaches for spillway design can be short-listed as gated and ungated free-flow spillways.
Gated spillways have been used for the most dam spillways over 1000 m3/s. They were greatly favoured before 1980, but experience has underlined the recurring overhead cost of permanent operators, the need for careful maintenance and the downstream risks associated with gate operation or gate failure. The risk of some or all of the gates jamming during exceptional conditions corresponding to large floods has been emphasized by several• dam accidents. Consequently, in many countries, this solution is used essentially for very large spillways.
Free-flow ungated spillways have been used for more than 80 per cent of the spillways with discharge capacities of less than 1000 m3/s and about 20 per cent of larger ones. Their safety is well acknowledged ; their drawback is the loss of storage or the increased height (and cost) of the dam corresponding to the depth of the overtopping nappe over the sill. It is thus advisable to reduce this nappe depth : this has been achieved, partly by long spillways over concrete dams or side spillways for many earthfill dams, but the nappe depth often remains in the range of 2 or 3 m and may be more than 5m for large design floods. Consequently, the reservoir depth is often reduced by more than 10 per cent and the live storage volume by more than 30 per cent
