|Table of contents|
|1 Crore+ students have signed up on EduRev. Have you?|
"Spillways are structures constructed to provide safe release of flood waters from a dam to a downstream area".
Every reservoir has a certain capacity to store water. If the reservoir is full and flood water enters it, the reservoir level will go up and may eventually result in over-topping of the dam. To avoid this situation, the flood has to be passed on to the downstream and this is done by providing a spillway which draws water from the top of the reservoir. A spillway can be a part of the dam or separate from it.
Spillways can be controlled or uncontrolled. A controlled spillway is provided with gates which can be raised or lowered. Controlled spillways have certain advantages. When a reservoir is full, its water level will be the same as the crest level of the spillway.
If a flood enters the reservoir at this time, the water level will start going up and simultaneously water will start flowing out through the spillway. The rise in water level in the reservoir will continue for some time and so will the discharge over the spillway. After reaching a maximum, the reservoir level will come down and eventually come back to the normal reservoir level.
The top of the dam will have to be higher than the maximum reservoir level corresponding to the design flood for the spillway, while the effective storage available is only up to the normal reservoir level. The storage available between the maximum reservoir level and the normal reservoir level is called the surcharge storage and is only a temporary storage in uncontrolled spillways. Thus for a given height of the dam, part of the storage - the surcharge storage is not being utilized. In a controlled spillway, water can be stored even above the spillway crest level by keeping the gates closed. The gates can be opened when a flood has to be passed. Thus controlled spillways allow more storage for the same height of the dam.
There are different types of spillways that can be provided depending on the suitability of site and other parameters. Generally a spillway consists of a control structure, a conveyance channel and a terminal structure, but the former two may be combined in the same for certain types. The more common types are briefly described below: