Since spillways are high structures, the water flowing over them has considerable velocity and kinetic energy due to conversation of potential energy into kinetic energy when it reached the river bed and this has to be destroyed before the water is allowed to flow over the river bed, because otherwise there will be considerable erosion of bed at the toe of the spillway endangering the stability of the structure. The devices provided for destroying the energy of water flowing over the spillways are termed as ‘dissipator's’. The energy dissipation arrangement depends on the consideration of the nature of the device to be employed and the character of the river bed downstream.

SELECTION OF ENERGY DISSIPATORS.

Various consideration’s governing the selection of energy dissipator are discussed below:

1. Type of dam and spillway. In earthen dam a chute spillway can be provided with hydro-lie jump basin with or without baffles or end sills. In an arch dam, free fall spillway with stilling pool of high depths is normally provided. In the case of a dam where water flows, through a tunnel outlet, the energy is best dissipated by jet diffusion. Likewise, for high dams upturned buckets with high tail water depth can be employed.

2. Tail water elevation. The position of tail elevations vis-a-vis height discuss the energy dissipating arrangement to be provided. Five conditions are generally met with (Fig. 20.15).

3. Nature of foundation. Nature of foundation met with dictates the dissipating arrangement. In sound rock upturned bucket or ski jump bucket is preferred with much lesser length of stilling basin and in alluvial soil a long apron with hydraulic jump type stilling basin with baffles and end still is provided.

4. Velocity of flow. Under high velocity flow, cavitation may occur if the baffles in the stilling basin are provided. Jet diffusion or hydraulic jump type stilling basin or roller buckets are preferred under suitable foundation and water course conditions.