Spillway

A spillway is a structure built into or beside a dam or levee that releases surplus water in a controlled way so it cannot damage parts of the structure never designed to carry flow, particularly during floods when a reservoir fills faster than its ordinary outlets can discharge. Spillways are either controlled, using mechanical gates that let an operator hold water at maximum storage year round, or uncontrolled, where the amount of water that spills over depends only on how far the reservoir has risen above the spillway fixed crest. Because water accelerating down a spillway carries enough energy to erode a dam foundation, engineers pair the basic chute, glory-hole, or siphon spillway designs with stepped surfaces, flip buckets, or stilling basins that dissipate that energy before the water rejoins the river.