AskDefine | Define weir

The Collaborative Dictionary

Weir \Weir\ (w[=e]r), Wear \Wear\,n. [OE. wer, AS. wer; akin to G. wehr, AS. werian to defend, protect, hinder, G. wehren, Goth. warjan; and perhaps to E. wary; or cf. Skr. v[.r] to check, hinder. [root]142. Cf. Garret.]
A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, forming a fish pond, or the like. [1913 Webster]
A fence of stakes, brushwood, or the like, set in a stream, tideway, or inlet of the sea, for taking fish. [1913 Webster]
A long notch with a horizontal edge, as in the top of a vertical plate or plank, through which water flows, -- used in measuring the quantity of flowing water. [1913 Webster]

Word Net



1 a low dam built across a stream to raise its level or divert its flow
2 a fence or wattle built across a stream to catch or retain fish

Moby Thesaurus

aboideau, air lock, arch dam, avenue, backstop, bamboo curtain, bank, bar, barrage, barrier, bear-trap dam, beaver dam, blowhole, boom, breakwater, breastwork, brick wall, buffer, bulkhead, bulwark, channel, chute, cofferdam, dam, debouch, defense, dike, ditch, dock gate, door, earthwork, egress, embankment, emunctory, escape, estuary, exhaust, exit, fence, flood-hatch, floodgate, flume, gate, gravity dam, groin, head gate, hydraulic-fill dam, iron curtain, jam, jetty, leaping weir, levee, lock, lock gate, logjam, loophole, milldam, moat, mole, mound, opening, out, outcome, outfall, outgate, outgo, outlet, parapet, penstock, pore, port, portcullis, rampart, roadblock, rock-fill dam, sally port, seawall, shutter dam, sluice, sluice gate, spiracle, spout, stone wall, tap, tide gate, vent, ventage, venthole, vomitory, wall, water gate, way out, wicket dam, work



Old English wer



  1. an adjustable dam placed across a river to regulate the flow of water downstream
  2. a fence placed across a river to catch fish


A weir () (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.
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 complicate or make flow measurement impossible.
There are different types of weirs. 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.
While 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.
While 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

Broad-crested weir

A broad-crested weir is a flat-crested structure with a crest length large 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.

Sharp 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 Weirs

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 Co. and has flow rates engraved along the side of the weir. This is typically used in pipes from 4" to 15" in diameter.


  • 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.
weir in Czech: Jez
weir in German: Wehr (Wasserbau)
weir in Spanish: Presas de tierra
weir in French: Seuil (barrage)
weir in Italian: Briglia
weir in Dutch: Stuw
weir in Japanese: 堰
weir in Polish: Jaz
weir in Portuguese: Barragem de aterro
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