A Spillway at Sunrise
Fig 1: Water over a spillway (Photo: ref.)

A Spillway is a structure used to regulate the flow of water in conjunction with a dam, levee, or other water contol device. Specifically, it is a structure which conveys water over or through a dam when a reservoir is full.

Spillways, a subset of weirs, are structures consisting of an obstruction across an open channel or body of water that are designed to control the release of water. Spillways are usually concrete and attempt to reduce water separation by taking a form that matches the underside of the nappe. Broad-crested weirs can function as spillways except their form doesn't match the underside of the nappe (like ogee spillways).

Finding Flow

The equation for flow over spillways is similar to that of a rectangular weir:

Q = CLH^{3/2}


  • L = effective length of the crest (ft)
  • C = discharge coefficient which ranges from 3.0 to 4.1 and depends on the ratio of design head to height of spillway. For ogee spillways, C varies from 3.3 to 3.98 (use 3.97 for a first approximation). For broad-crested weirs, C varies from 2.63 to 3.33 (use 3.33 for a first approximation).




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