What is a weir?
A weir is a fixed or movable structure built across a watercourse that holds water back upstream and so controls the water level. It keeps a river deep enough for shipping, or lets a drainage authority retain water in dry spells. The Dutch term is stuw. A weir is not the same as a lock: a lock lets boats or water pass between two levels, while a weir holds water back.
Types
- Fixed weir: a sill that water flows over once the level gets too high.
- Movable weir: with a gate, flap or other moving part, set by hand or remotely. A flap weir pivots on a low horizontal axis; a sluice gate slides up and down.
- Visor weir: a large arched gate that swings up out of the water. In the Netherlands the three weir complexes on the Nederrijn and Lek, at Driel, Amerongen and Hagestein, use visor gates, each paired with a navigation lock.
- Small weirs: in ditches and streams, simple stop-log or flap weirs manage the water level of a local area.
Application
- Do not build in a watercourse without consent. A weir, dam or culvert in a ditch or stream normally needs permission. In England, work on a main river needs a permit from the Environment Agency, and work on an ordinary watercourse needs consent from the lead local flood authority or internal drainage board. In the Netherlands the water board (waterschap) decides.
- Never change a weir setting yourself. It affects the water level for a whole area, including neighbours and farmers.
- Know the water level near your house. Ditch levels influence groundwater nearby. In older buildings on timber piles, the pile heads must stay below the water table, or pile rot sets in.
- Report problems. Tell the responsible authority about unusually high or low levels or blocked weirs.
Common mistake: dropping a plank or small dam into a ditch to keep more water for the garden. It can cause flooding upstream and is usually not allowed without consent.
Related terms
- Canal lock
- Water level management
- Groundwater level
- Pile rot
- Ditch
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