AskDefine | Define spillway

Dictionary Definition

spillway n : a channel that carries excess water over or around a dam or other obstruction [syn: spill, wasteweir]

User Contributed Dictionary



  1. A path designed to take away overflow safely.

Extensive Definition

A spillway is a structure used to provide for the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. In contrast, an intake is a structure used to release water on a regular basis for water supply, hydroelectricity generation, etc. Floodgates and fuse plugs may be designed into spillways to regulate water flow and dam height. Other uses of the term "spillway" include bypasses of dams or outlets of a channels used during highwater, and outlet channels carved through natural dams such as moraines.


A spillway is located at the top of the reservoir pool. Dams may also have bottom outlets with valves or gates which may be operated to release flood flow, and a few dams lack overflow spillways and rely entirely on bottom outlets.
There are two types of spillways: controlled and uncontrolled.
A controlled spillway has mechanical structures or gates to regulate the rate of flow. This design allows nearly the full height of the dam to be used for water storage year-round, and flood waters can be released as required by opening one or more gates.
An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water within the reservoir. All of the storage volume in the reservoir above the spillway crest can be used only for the temporary storage of floodwater, and cannot be used as water supply storage because it is normally empty.

Inverted-bell spillways

Some spillways are designed like an inverted bell so that water can enter all around the perimeter. These uncontrolled spillway devices are also called morning glory, plughole, glory hole, or bell-mouth spillways. In areas where the surface of the reservoir may freeze, bell-mouth spillways are normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound.

Design considerations

The largest flood that needs be considered in the evaluation of a given project, regardless of whether a spillway is provided; i.e., a given project should have structures capable of safely passing the appropriate spillway design flood (SDF). A 100-year recurrence interval is the flood magnitude expected to be exceeded on the average of once in 100 years. It may also be expressed as an exceedance frequency with a one per cent chance of being exceeded in any given year.


Spillway gates may operate suddenly without warning, under remote control. Trespassers within the spillway run the risk of drowning. Spillways are usually fenced and equipped with locked gates to prevent casual trespass within the structure. Warning signs, sirens, and other measures may be in place to warn users of the downstream area of sudden release of water. Operating protocols may require "cracking" a gate to release a small amount of water to warn persons downstream. Sudden closure of a spillway gate may result in stranding of fish.

External links

Robert-Bourassa Reservoir, Quebec, Canada, fittingly called the "Giant's Staircase".
spillway in German: Hochwasserentlastung
spillway in Estonian: Ülevool
spillway in Spanish: Rebosadero
spillway in French: Déversoir
spillway in Dutch: Noodoverlaat
spillway in Portuguese: Vertedouro
spillway in Thai: ทางน้ำล้น

Synonyms, Antonyms and Related Words

aqueduct, arroyo, bed, canal, creek bed, culvert, donga, dry bed, flume, gulch, gully, gullyhole, headrace, irrigation ditch, nullah, race, river bed, riverway, runnel, sluice, spillbox, stream bed, streamway, swash, swash channel, tailrace, wadi, water carrier, water channel, water furrow, water gap, water gate, watercourse, waterway, waterworks
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