Spillway

 Spillway

The spillway is a structure constructed at a dam site, for effectively dispose of the surplus water from upstream to downstream. Just after the reservoir gets filled up, up to the normal pool level, water started flowing over the top of the spillway crest (which is generally kept at normal pool level). A spillway can be located either within the body of the dam, or at one end of it or entirely away from it, independently in a saddle. It act as a safety valve for dam and is essential in every dam storage project. If the flood overtops the earthen dam eventually the dams fails .Once the dam fails , catastrophic flood of extreme magnitude occurs causing damages to property , dwelling houses and roads , line of communication human and wild life etc. To save such catastrophy , spill way is the only device in each and every dam reservoir project.

Function and design consideration of spill way

The basic purpose of spill way is to provide a mean of control inflow and providing conveyance from reservoir to tail water for all flood discharges up to the spill way design flood (SDF) .The spill way can be used to provide control regulation for floods either in combination with floods control, sluices or outlets works, or in some cases, as the only flood control facility.

A terminal structure to provide energy dissipation is usually provided at the downstream end of the spillway. The degree of energy dissipation provided is dependent up on the anticipated use of the spill way and the extent of the damage that will occurs if the terminal structure capacity is exceeded .

Following points should be considered while designing spill way or essential requirements of the spill way are :

1) Spill way should be design properly so as to dispose of the excess water without causing any damages to the dam or any appurtenant.

2) Spill way should hydraulically (less flow separation, smooth flow at entrance )and structurally (stable, strength for different loading condition) adequate.

3)  Design discharge for spillway should be appropriate ( 1000 years return period  is taken for design of large dam).

4) As the flow velocity is very high through the passage of the spill way ,its surface should have sufficient erosion and cavitation resistance capacity.

5) There should be sufficient provision of appropriate dissipation of high kinetic energy possessed by flow at downstream otherwise there will be severe bad erosion.