Weir

A weir (IPAEng|wɪɚ) (also known as a lowhead dam) is a small overflow-type dam commonly used to raise the level of a river or stream. Weirs have traditionally been used to create mill ponds in such places. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir. The crest of an overflow spillway on a large dam is often called a "weir".

Weirs are used in conjunction with locks, to render a river navigable and to provide even flow for navigation. In this case, the weir is made significantly longer than the width of the river by forming it in a 'U' shape or running it diagonally, instead of the short perpendicular path. Since the weir is the portion where water is overflowing, a long weir allows a lot more water with a small increase in overflow depth. This is done in order to minimize fluctuation in the depth of the river upstream with changes in the flow rate of the river. Doing so avoids unnecessary complication in designing and using the lock or irrigation diversion devices. [http://www.watercontrol.org/tech/files/Long%20Crested%20Weir%20Design.pdf]

Weirs also give hydrologists and engineers a simple method of measuring the rate of fluid flow 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.

There are different types of weir. It may be a simple metal plate with a V-notch cut into it, or it may be a concrete and steel structure across the bed of a river. A weir which causes a large change of water level behind it, compared to the error inherent in the depth measurement method, will give an accurate indication of the flow rate.

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".

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. The weir may pose a barrier to migrating fish. Fish ladders provide a way for fish to get between the water levels.Mill ponds provide a water mill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.

A walkway over the weir is likely to be useful for the removal of floating debris trapped by the weir, or for working staunches and sluices on it as the rate of flow changes. This is sometimes used as a convenient pedestrian crossing point for the river.

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.

Types of weirs

* Sharp crested weir
* Broad crested weir (or broad-crested weir)
* Crump weir (named after the designer)
* Needle dam
* Proportional weir
* Combination weir
* MF weir
* V-notch weir
* Rectangular weir
* Cipolletti (trapezoidal) weir
* Labyrinth weir

Broad-crested weir

A broad-crested weir is a flat-crested structure, with a long crest compared to the flow thickness (Chanson 2001, Henderson 1966, Sturm 2001). When the crest is “broad”, the streamlines become parallel to the crest invert and the pressure distribution above the crest is hydrostatic.The hydraulic characteristics of broad-crested weirs were studied during the 19th and 20th centuries. Practical experience showed that the weir overflow is affected by the upstream flow conditions and the weir geometry.

harp crested weir

A sharp-crested weir allows the water to fall cleanly away from the weir. Sharp crested weirs are typically 1/4" or thinner metal plates.

Combination weir

For accurate flow measurement over a wider range of flow rates, a combination weir combines a V-notch weir with a rectangular weir. An example is manufactured by Thel-Mar Company and has flow rates engraved along the side of the weir. This is typically used in pipes ranging from 4" to 15" in diameter.

References

* Chanson, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford, UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).
* Henderson, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
* Sturm, T.W. (2001). "Open Channel Hydraulics." McGraw Hill, Boston, USA, Water Resources and Environmental Engineering Series, 493 pages.

* [http://espace.library.uq.edu.au/view/UQ:107673 Carlos Gonzalez and Hubert Chanson (2007) Experimental Measurements of Velocity and Pressure Distribution on a Large Broad-Crested Weir. Flow Measurement and Instrumentation, 18 3-4: 107-113 (DOI 10.1016/j.flowmeasinst.2007.05.005)]