A spillway is a hydraulic structure built at a dam site for the controlled release of the surplus (or) excess water from Reservoir (or) Levee into a downstream area after it has been filled to its maximum capacity. In the United Kingdom, these are called as "Overflow Channels". Spillways ensure that the water doesn't overflow and damage (or) destroy the dam.
Every reservoir encompasses 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 prevent 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 are often a part of the dam or separate from it.
Spillways can be controlled (or) uncontrolled. A controlled spillway is furnished (provided) with gates which can be raised (or) lowered. Controlled spillways have certain advantages. When a reservoir is full, its water level will be equal to the crest level of the spillway.
If flood enters the reservoir at that point, the water level will start going up and simultaneously water will start flowing out through the spillway. After flood stops, the reservoir level will come down and eventually come back to the normal reservoir level.
Spillways are classified into different types on the basis of the arrangement of the control structure, a conveyance channel and a terminal structure. Within the next article, we will discuss in brief all the different types of spillways with pictures.
Credits: QUORA