What is a spillway and why is it important in dam design?

A spillway is a structure that safely passes flood water over, around, or through a dam to prevent overtopping, which could cause catastrophic failure of embankment dams. Types include: overflow (ogee) spillway built into dam, side channel spillway, chute spillway for earth dams, shaft/morning glory spillway for limited space. Spillway capacity must exceed maximum expected flood. Spillways include energy dissipators (stilling basins, flip buckets) to prevent erosion downstream.