A Weir is a structure built across a river or open channel to control water flow.
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A Weir is a smaller structure used to control flow, while a barrage is a larger dam-like structure built for water storage and diversion.
Weirs regulate water flow, prevent flooding, measure flow rates, aid navigation, and control sediment transport.
Weirs work by creating an obstruction in the watercourse, influencing the flow and controlling water levels.
The flow rate over a weir is often calculated using variations of the Francis weir formula, \(\Q = CLH^(3/2)\), where Q is the flow rate, C is the discharge coefficient, L is the length of the weir, and H is the head of water.
The flow rate over a weir is determined by the characteristics of the weir structure and the head of water above it.
Figure 1. The Radyr Weir in Great Britain.
A weir is a small barrier built across a stream or river to raise the water level slightly on the upstream side; essentially a small-scale dam. Weirs allow water to pool behind them, while allowing water to flow steadily over top of the weir.[2] Additionally, the term weir can be used to refer to the crest of a spillway on a large embankment dam.
Weirs can be constructed out of several different materials, depending on their age and purpose. Wood, concrete,[3] or a mixture of rocks, gravel, and boulders can all be used to construct a weir.[4]
In a weir, the surface over which the water flows is known as the crest. The flow of water that moves overtop of this crest is known as the nappe, which is simply the water that makes it overtop the weir. This nappe does not exist with dams, as dams permit no flow of water over the structure. If this nappe falls a significant distance through the air - meaning that the weir increases the elevation of the water prior to the weir - the weir is said to have free discharge. However, if water flows partially underwater as a result of little elevation increase from the weir it is said to be submerged or drowned.[5]
[6]Figure 2. Diagram of two weirs, depicting both free discharge and drowned versions.
Weir vs. Dam
Weirs differ from dams in one major way. First and foremost, in a weir the water from upstream runs over top of the weir itself, constantly overflowing.[2] In a dam, water flows through spillways and penstocks instead of over top of the dam, which could actually cause damage and is known as overtopping.
Benefits and Drawbacks
Weirs can be built to measure water flow rate, to alter the flow of rivers, or to prevent flooding. Additionally, small-size weirs can be used in large hydropower developments as a way to mitigate potential harm that may come about as a result of dam development. Small weirs can help improve fish numbers, stabilize water levels, and stabilize effects of river alteration.[2]
Weirs do have drawbacks, as they can increase sedimentation and pose potential migration barriers to fish. To aid in migration, fish ladders are sometimes incorporated into weirs to aid in fish passage. Sedimentation can occur as the water slows as it goes over the weir, dropping sediment as it slows.[7] As well, as water passes over the top of the weir, the motion can alter the dissolved oxygen levels. If the water is over or under-oxygenated (called hypoxia or anoxia), this can harm the local ecosystem.
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Weirs are particularly dangerous to humans - and are thus commonly called "drowning machines".[8] Particularly, the circulating "backwash" of water behind the weir is easy to get trapped within, and if a swimmer or boater is caught within this cycle escape and rescue is incredibly difficult. Additionally, debris that can be trapped within this cycle of water can pose harm to anyone trapped behind the weir.[9]
In the video below Grady Hillhouse[10] discusses how weirs work. He's built a small scale model of a weir (skip ahead to 5:24 to see the scale model), as well as other versions of weirs that exist.
For Further Reading
For further information please see the related pages below:
References
Geograph. (December 29, 2015).[Online]. Available: http://www.geograph.org.uk/photo/76949
[4]W. Fowler, "What is A Weir | Weir Structure | Weir Design | SANI-TRED", Sanitred.com, 2015. [Online]. Available: https://sanitred.com/waterproofing-weir-dam/ . [Accessed: 09- Feb- 2020].
"Weir Plate Materials", Openchannelflow.com, 2020. [Online]. Available: https://www.openchannelflow.com/blog/weir-plate-materials . [Accessed: 09- Feb- 2020].
Rock Weir Design Guidance - Reclamation: Managing Water in the West. Denver, Colorado: U.S. Department of Interior, Bureau of Reclamation, 2016, pp. 13-14.
Civil Engineering Portal. (December 29, 2015).[Online]. Available: http://www.engineeringcivil.com/weirs.html
Created internally by a member of the Energy Education team
Minnesota Rural Water Association. (December 29, 2015).[Online]. Available: http://www.mrwa.com/WaterWorksMnl/Chapter%2013%20Sedmentation.pdf
"Weir safety: fact sheet", Seqwater.com.au, 2020. [Online]. Available: https://www.seqwater.com.au/sites/default/files/2019-09/Seqwater%20fact%20sheet%20-%20What%20is%20a%20weir.pdf . [Accessed: 09- Feb- 2020].
Pennsylvania Fish and Boat Commission. (December 29, 2015).[Online]. Available: http://fishandboat.com/damlow_info.htm
His web page is here: https://practical.engineering/about accessed February 17th, 2020.
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