Spillway

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A spillway is a structure used to provide for the controlled release of flood 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 may be designed into spillways to regulate water flow and dam height.

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.

Some spillways are designed like an inverted bell so that water can enter all around the perimeter (also termed a morning glory design). 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. This type of Spillway is also known as a Glory hole.