Introduction Spillways


  • 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.

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