Spillway is a structure, use to pass surplus flood water from reservoir to the downstream side of the dam.
It is sometimes also know surplussing work, as its main job is to handle the surplus water.
It is a sort of safety valve for a dam.
During floods, the reservoir level goes on increasing, as more and more water enters the reservoir.
If spillways had not been there, the level in the reservoir will go on rising even beyond the maximum reservoir level.
This may lead to over topping of the dams.
Additional stresses may be induced in the dam and this may cause failure of the dam.
More rise in water level beyond the maximum flood level will cause submergence of vast additional areas and may cause suffering to the people living on the U/S side.
Spillways act as safety valve for the safety of the dam and also for the adjoining areas.
When water level in the reservoir reaches some specified level, the surplus water automatically starts escaping through the spillways.
As more and more flood water enters the reservoir, there is more and more rise in water level and consequently more and more water starts escaping over the spillways.
At maximum flood level (H.F.L.) of reservoir, the outflow rate over spillways reaches the inflow in the reservoir and thus further rise in the water level.
The spillways should be of sufficient capacity so as to be capable to effectively handle the largest floods, without causing.
Spillways are also design according to the safe capacity of the D/S channel, otherwise D/S areas may get flooded.
The design flood discharge required to be handled by the spillway can be found by flood routing.
