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A weir (pronounced /?w??r/) is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure (not unlike a dam), but allows water to flow over the top. Weirs are commonly used to alter the flow regime of the river, prevent flooding, measure discharge and to help render a river navigable.

Contents

[hide] 1 Function

2 Drawbacks

3 Types 3.1 Broad-crested weir

3.2 Sharp crested weir (fayoum weir)

3.3 Combination weir

3.4 V-notch weir

3.5 Minimum Energy Loss weir

4 See also

5 References

6 External links

[edit] Function

A weir on the Humber River near Raymore Park in Toronto, Ontario

The weir at Coburg lake in Victoria (Australia) after heavy rainfall.

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

Q = C L H^n

Where

Q = flow rate

C = constant for structure

L = width of the crest

H = 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.

[edit] Drawbacks

Because a weir will typically increase the oxygen content of the water as it passes over the crest, a weir can have a detrimental effect on the local ecology of a river system. A weir will artificially reduce the upstream water velocity, which can lead to an increase in siltation.

Weirs can also have an effect on local fauna. While a weir is easy for some fish to jump over, other species or certain life stages of the same species may be blocked by weirs due to relatively slow swim speeds or behavioral characteristics. Fish ladders provide a way for fish to get between the water levels.

Even though the water around weirs can often appear relatively calm, they are dangerous places to boat, swim or wade; the circulation patterns on the downstream side can submerge a person indefinitely. This phenomenon is described in the article on whitewater.

The weir can become a point where garbage and other debris accumulate. However, a walkway over the weir is likely to be useful for the removal of floating