Weir

The weir is the small overflow dam that is used for altering the flow characteristic of water or stream. In most cases, weirs act as barriers across the river, which cause the water to pool or group behind the structure, but it allows the water to flow over the top. Generally weirs are used to prevent flooding, measure the discharge and cause river navigable. The crest or top of an overflow spillway on the dam is often called a weir.

Functions of weir

In the small to medium sized streams or in the industrial discharge locations, weir allows hydrologists and engineers a simple method of measuring the volumetric flow rate. All water flows over the weir, the depth of the water behind the weir can be converted to a rate of flow, since the geometry of the top of the weir is known. The calculation depends on the fact that fluid will pass through the critical depth of the flow regime in the environs of the top of the weir. The flow measurement may be complicated or even impossible if the water is not carried away from the weir. The weir can maintain the vertical profile of a stream or channel, and is called grade stabilizer. Weirs are referred as low head barrier dams and are used in the control of persistent sea lamprey in the great lakes.

Types of weirs

There are different types of weirs are available, they are; Broad crested weir, sharp crested weir, combination weir, v-notch weir, and minimum energy loss weir.

Broad crested weir

This is the flat crested structure weir, with the long crest compared to the flow thickness. The stream line will surely become parallel to the crest invert, if the crest id broad and the pressure distribution above the crest is hydrostatic.

Sharp crested weir

It is available in different shapes such as rectangular, V-notch. This type of weir allows the water to flow away from the weir and it is thinner metal plates.

Combination weir

The combination weir combines a v-notch weir with the rectangular weir for the accurate measurement over a wider range of flow rates. This is typically used in pipes ranging from 4” to 15” in diameter.

V-notch weir

This is used to measure small discharge values and it is triangular channel shape. The upper edge of the section is always above the water level so the channel is always triangular simplifying calculations of cross sectional area. The head above the water crest is more sensitive to changes in flow compared to rectangular weirs.

Questions:

• What is weir?
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