Weirs

For other uses, see Weir (disambiguation).
"Weirs" redirects here. For the Luke Vilbert and Jeremy Simmonds album, see Weirs (album).

A weir /ˈwɪər/ is a barrier across a river designed to alter its flow characteristics. In most cases, weirs take the form of obstructions smaller than most conventional dams, which cause water to pool behind them, while allowing water to flow steadily over their tops. Weirs are commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help render rivers navigable.

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

Q = C L H^n

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)

When used for flow measurement it is important that the weir crest be kept free of rust or nicks. Roughness of any form will cause the weir to discharge more water than indicated by the standard discharge equations or tables. Air must also freely circulate under the nappe as discharge errors of as much as 25% may occur if the nappe is not adequately ventilated.[1]

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.

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 can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called an hydraulic jump— can submerge a person indefinitely. This phenomenon is so well known to canoeists, kayakers, and others who spend time on rivers that they even have a rueful name for weirs: "drowning machines".[2][3]
  • The weir can become a point where garbage and other debris accumulates. However, 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 also sometimes used as a convenient pedestrian crossing point for the river.

Types



There are several different types of weirs. A weir 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 that causes a large change of water level behind it, as compared to the error inherent in the depth measurement method, will give an accurate indication of the flow rate. Some weirs are used as bridges for people to walk along.

Labyrinth weir

A labyrinth weir uses a trapezoidal-shaped weir wall geometry (plan view) to increase the weir length. They are versatile structures and can be modified to fit many applications.[4]

Broad-crested weir

A broad-crested weir is a flat-crested structure, with a long crest compared to the flow thickness.[5] 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.

Sharp crested weir (fayoum weir)

A sharp-crested weir allows the water to fall cleanly away from the weir. Sharp crested weirs are typically 1/4" [6.35 mm] or thinner metal plates. Sharp crested weirs come in many different shapes and style, such as Rectangular (with and without end contractions), V-notch and Cipolletti weirs. Under controlled conditions, sharp crested weirs can exhibit accuracies as good as +/-2%, although under field conditions accuracies greater than +/-5% should not be expected.[6]

The crest of a sharp crested weir should be no thicker than 1/8" [3.1496 mm] to ensure that the nappe springs clear of the weir's crest. Where the weir plate is thicker than 1/8" [3.1496 mm], the downstream face of the weir must be beveled.[7]

Compound weir

The sharp crested weirs can be considered into three groups according to the geometry of weir: a) the rectangular weir, b) the V or triangular notch, and c) special notches, such as trapezoidal, circular, or parabolic weirs. For accurate flow measurement over a wider range of flow rates, a compound weir combines a one weir type with another - typically a V-notch weir with a rectangular weir.[8]

An example of the compound weir is manufactured by Thel-Mar Company.[9] These weirs are a combination of a V-notch weir and a rectangular weir and are available for insertion in pipes from 6" to 15" - with secondary adapters available for larger pipe sizes. The weirs are intended to measure no more than 35% of the pipe's open channel flow capacity.

V-notch weir

The V-notch weir is a triangular channel section, used to measure small discharge values. The upper edge of the section is always above the water level, and so the channel is always triangular simplifying calculation of the cross-sectional area. V-notch weirs are preferred for low discharges as the head above the weir crest is more sensitive to changes in flow compared to rectangular weirs, for example, the Rehbock weir. Under laboratory conditions, V-notch weirs typically achieve accuracies of 2% to 5%, while field condition accuracies from 5% to 15% may be expected.[10]

V-notch weirs are sized between 22-1/2° and 120°, with 22-1/2°, 30°, 45°, 60°, 90°, and 120° the common size increments - although free-flow discharge equations can be developed from one universal equation for V-notch weirs from 25-120° in size.[11]

Minimum Energy Loss weir

The concept of the Minimum Energy Loss (MEL) structure was developed by Gordon McKay in 1971.[12] The first MEL structure was the Redcliffe storm waterway system, also called Humpybong Creek drainage outfall, completed in 1960 in the Redcliffe Peninsula in Queensland, Australia. It consisted of a MEL weir acting as a streamlined drop inlet followed by a 137 m long culvert discharging into the Pacific Ocean. The weir was designed to prevent beach sand being washed in and choking the culvert, as well as to prevent salt intrusion in Humpybong Creek without afflux. The structure is still in use and passed floods greater than the design flow in several instances without flooding (McKay 1970, Chanson 2007).

The concept of the Minimum Energy Loss (MEL) weir was developed to pass large floods with minimum energy loss and afflux, and nearly-constant total head along the waterway. The flow in the approach channel is contracted through a streamlined chute and the channel width is minimum at the chute toe, just before impinging into the downstream natural channel. The inlet and chute are streamlined to avoid significant form losses and the flow may be critical from the inlet lip to the chute toe at design flow. MEL weirs were designed specifically for situations where the river catchment is characterized by torrential rainfalls and by very small bed slope. The first major MEL weir was the Clermont weir (Qld, Australia 1963), if the small control weir at the entrance of Redcliffe culvert is not counted. The largest, Chinchilla weir (Qld, Australia 1973), is listed as a "large dam" by the International Commission on Large Dams.

See also

References

Notes
Bibliography
  • 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).
  • 10.1061/(ASCE)0887-3828(2007)21:4(264).
  • Gonzalez, C.A., and 10.1016/j.flowmeasinst.2007.05.005.
  • Henderson, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
  • McKay, G.R. (1971). "Design of Minimum Energy Culverts." Research Report, Dept of Civil Eng., Univ. of Queensland, Brisbane, Australia, 29 pages & 7 plates.
  • Sturm, T.W. (2001). "Open Channel Hydraulics." McGraw Hill, Boston, USA, Water Resources and Environmental Engineering Series, 493 pages.

Further reading

External links

  • Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways (Click "proceed" at the UQ-ITS Advisory webapge)
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