When the flow is spilled through the spillway, the potential energy possessed by the flow is converted into the kinetic energy. This kinetic energy causes the high velocity at the downstream of the dam and causes the scouring of the downstream materials. If the scouring is allowed to continue further then it may cause the failure of the structure itself. So this excessive kinetic energy should be dissipated so that it could not any damage downstream of the dam.
Method of dissipation of the kinetic energy:
1) Through the hydraulic jump type stilling basin:
Water flowing down the face of the spillway slope enters over the horizontal apron of the basin. If the flow with high velocity (super critical flow, Froude Number, F= V/ (gH0) ^0.5 > 1.0 come across the slow moving flow (sub critical flow, F < 1.0), then there will be formation of the hydraulic jump. At the hydraulic jump flow depth increases rapidly causing gain of potential energy causing decrease of velocity and loss of energy due to the excessive turbulence. If the obstructions were also provided at the stilling basin to decrease the momentum of the flow and also increase turbulence then much of the energy will be dissipated within the short length.
For the formation of hydraulic jump the tail water depth must match with the post-jump depth.
Y2/y1 = ½? (1+ 8 Fr1^2 1)
Pulsation energy dissipation (turbulence) + frictional force dissipation + conversion of kinetic energy to potential energy (because of rise of the water level) = significant loss of kinetic energy will take place.
TWD (tail water depth) > y2, the jump gets submerged and hence little loss of energy takes place.
TWD < y2, the jump gets swept off downstream, in such case the super critical high-velocity flow continues for some distance on the river bed.
2) Ski-jump type energy dissipater:
When the tail water depth is less than the sequent depth of the hydraulic jump, then ski type energy dissipation is considered. In the ski jump type energy dissipation; glow is overthrown from the bucket type structure at the toe of the dam and jet of water is allowed to follow the trajectory at the air and fall much distance far from the dam. When the jet follows trajectory at the air much of the energy is dissipated along the way for gaining the height of the trajectory and also due to the air friction. When the jet plunged in to the pool of the remaining energy will also be dissipated due to the turbulence. This type of dissipater is suitable where foundation rock is of good/sturdy quality at downstream of dam is available and can withstand the erosive action of plunging jet.
3) Roller bucket type energy dissipater:
This type of energy dissipater is employed where the tail water condition is not favourable for adopting hydraulic jump type basin. The roller bucket is a spoon type structure at the toe of the spillway. When the high velocity sheet of water slides down the spillway it gets arrested by the tail water. This gives rise toe surface roller as well as bottom roller action and eddy turbulence, accompanied by the energy dissipation. This has relatively shorter structure as compared to the shorter structure as compared to hydraulic type stilling basin. For successful roller action the tail water depth has to be slightly greater than needed by the hydraulic type stilling basin. Thus, it could minimize the cost of stilling basin and also applicable when it is not possible to plunge the flow at the downstream side due to vulnerability of excessive scouring.