Weir - Function

Function

Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams or in industrial discharge locations. Since the geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. The calculation relies on the fact that fluid will pass through the critical depth of the flow regime in the vicinity of the crest of the weir. If water is not carried away from the weir, it can make flow measurement complicated or even impossible.

The discharge can be summarized as

Where

• Q is flow rate
• C is a constant for structure
• L is the width of the crest
• H is the height of head of water over the crest
• n varies with structure (e.g. 3/2 for horizontal weir, 5/2 for v-notch weir)

A weir may be used to maintain the vertical profile of a stream or channel, and is then commonly referred to as a grade stabilizer such as the weir in Duffield, Derbyshire.

The crest of an overflow spillway on a large dam is often called a weir.

Weirs, referred to as low head barrier dams in this context, are used in the control of invasive sea lamprey in the Great Lakes. They serve as a barrier to prevent recolonization by lamprey above the weir, reducing the area required to be treated with lampricide, and providing a convenient point to measure water flow (to calculate amount of chemical to be applied).

Mill ponds provide a watermill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.