What is a flood spillway?
A flood spillway is a deliberately lowered section of a dyke, embankment or weir over which water is allowed to flow once the water level rises too far. It is the relief valve of the flood defence: a controlled inundation at a chosen point is preferable to an uncontrolled breach somewhere else.
How it works
At normal levels the spillway does nothing. Once the water rises above the crest level, the surplus flows over the lowered section into the area behind. That area — sometimes called a green river or washland — is set up for it: usually farmland or nature, without buildings that would suffer damage.
The governing design decisions are:
- The crest level, which determines the water level at which the spillway starts to operate.
- The length of the lowered section. Too short, and water flows at high velocity over a narrow strip, causing scour. Too long, and more water enters the area than intended.
- The revetment on the slope. The spillway is where water passes over the defence with force; grass alone is often not enough, so spillways are protected with stone pitching, concrete or open stone asphalt.
- A shallow inner slope, so the water runs quietly into the hinterland and does not erode the bank.
An extended, uniformly lowered length of embankment is described as a broad crest.
Application
The principle is old and appears in river valleys across Europe. In current practice the same thinking underlies flood storage polders, retention areas and controlled washlands along major rivers.
At a smaller scale you meet the same principle at a weir with a fixed overflow crest in a stream or drainage ditch, where the water level is regulated by the height of the crest the water flows over.
When a spillway is brought into use, residents and farmers in the area behind it are warned in advance — this is a controlled, announced inundation.
Related terms
- Flood defence
- Embankment
- Culvert
- Pumping station
- Culverting
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