International Commission on Irrigation & Drainage Commission Interationale des Irrigation et du Drainage



Diversion Weirs

Weir is a low obstruction that is built across a river or canal to raise the water level, or in parallel to divert the water.

Two types of weirs are used to control flow; an in-stream weir and a diversion weir. Low flows accumulate behind the in-stream weir, usually set across the flow, and pass through an orifice (opening) in the weir. As flows increase to the threshold flow rate, water levels behind the in-stream weir increase. As flows increase further, the in-stream weir is overtopped and flows carry on down the watercourse.The in-stream weirs are widely used to measure the rate of flow.

At flow rates above the threshold flow rate the water level rises above the level of the diversion weir and water is diverted. The rate of diversion is a function of the level of water in the main stream.

Weirs can be constructed from reinforced concrete or durable hardwood timber. Timber weirs may be used for small structures (less than 1m crest height and 5 m width). Geotextile is used to prevent water leakage through any gaps in the timbers and also to prevent soil loss from beneath the rock.

Concrete weirs may be used for larger weirs; however the structural details will need to be designed by a qualified engineer.

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Dictionary - Diversion Weirs Terms

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