A weir /wɪər/ or low head dam is a barrier across the horizontal width of a river that alters the flow characteristics of the water and usually results in a change in the height of the river level. There are many designs of weir, but commonly water flows freely over the top of the weir crest before cascading down to a lower level.
1 Etymology 2 Function
2.1 Flow measurement 2.2 Control of invasive species 2.3 Watermills 2.4 Flood control and altering river conditions
3.1 Ecology 3.2 Fish migration 3.3 Safety
4 Common types
4.1 Broad-crested 4.2 Compound 4.3 V-notch 4.4 Polynomial
5 See also 6 References 7 Sources 8 External links
Etymology There is no single definition as to what constitutes a weir and one English dictionary simply defines a weir as a small dam, likely originating from Middle English were, Old English wer, derivative of root of werian, meaning "to defend, dam". Function
A broadcrest weir at the Thorp grist mill in Thorp, Washington, USA
Weirs are commonly used to prevent flooding, measure water discharge and help render rivers more navigable by boat. In some locations the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. A dam is usually specifically designed to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics. A common distinction between dams and weirs is that water flows over the top (crest) of a weir or underneath it for at least some of its length. Accordingly, the crest of an overflow spillway on a large dam may therefore be referred to as a weir. Weirs can vary in size both horizontally and vertically, with the smallest being only a few inches in height whilst the largest may be hundreds of metres long and many metres tall. Some common weir purposes are outlined below. Flow measurement Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams/rivers 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. However, this can only be achieved in locations where all water flows over the top of the weir crest (as opposed to around the sides or through conduits/sluices) and at locations where the water that flows over the crest is carried away from the structure. If these conditions are not met, it can make flow measurement complicated, inaccurate or even impossible. The discharge calculation can be summarised as:
Q = C L
displaystyle Q=CLH^ n
Q is the volumetric flow rate of fluid (the discharge) C is the flow coefficient for the structure (on average a figure of 0.62). 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)
However this calculation is a generic relationship and specific
calculations are available for the many different types of weir. Flow
measurement weirs must be well maintained if they are to remain
Control of invasive species
As weirs are a physical barrier they can impede the longitudinal
movement of fish and other animals up and down a river. This can have
a negative effect on fish species that migrate as part of their
breeding cycle (e.g., salmonids), but can also be useful as a method
of preventing invasive species moving upstream. For example, weirs in
19th century weir of porphyry stone on a creek in the Alps. During periods of high river flow this weir would be significantly more substantial.
Because a weir impounds water behind it and alters the flow regime of
the river it can have an effect on the local ecology. Typically the
reduced river velocity upstream can lead to increased siltation
(deposition of fine particles of silt and clay on the river bottom)
that reduces the water oxygen content and smothers invertebrate
habitat and fish spawning sites. The oxygen content typically returns
to normal once water has passed over the weir crest (although it can
be hyper-oxygenated), although increased river velocity can scour the
river bed causing erosion and habitat loss.
Weirs can have a significant effect on fish migration. Any weir
that exceeds either the maximum height a species can jump or creates
flow conditions that cannot be bypassed (e.g., due to excessive water
velocity) effectively limits the maximum point upstream that fish can
migrate. In some cases this can mean that huge lengths of breeding
habitat are lost and over time this can have a significant impact of
In many countries it is now a requirement by law to build fish ladders
into the design of a weir that ensures that fish can bypass the
barrier and access upstream habitat. Unlike dams, weirs do not usually
prevent downstream fish migration (as water flows over the top and
allows fish to bypass the structure), although they can create flow
conditions that injure juvenile fish. Recent studies suggest that
navigation locks have also potential to provide increased access for a
range of biota, including poor swimmers.
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 a
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". If caught in this situation, the
The bridge and weir mechanism at
Two weirs on the
A manually operated needle dam-type weir near Revin on the River Meuse, France
A broad-crest weir in Warkworth, New Zealand
A complicated series of broad-crest and V-notch weirs at
There are many different types of weirs and they can vary from a simple stone structure that are barely noticeable, to elaborate and very large structures that require extensive management and maintenance. Broad-crested A broad-crested weir is a flat-crested structure, where the water passes over a crest that covers much or all of the channel width. This is one of the most common types of weir found worldwide. Compound A compound weir is any weir that comprises several different designs into one structure. They are commonly seen in locations where a river has multiple users who may need to bypass the structure. A common design would be one where a weir is broad-crested for much of its length, but has a section where the weir stops or is 'open' so that small boats and fish can traverse the structure. V-notch A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch (often V-shaped) cut into the panel. At times of normal flow all the water must pass through the notch, simplfiying flow volume calculations, and at times of flood the water level can rise and submerge the weir without any alterations made to the structure. Polynomial A polynomial weir is a weir that has a geometry defined by a polynomial equation of any order n. Most weirs in practice are low-order polynomial weirs. The standard rectangular weir is, for example, a polynomial weir of order zero. The triangular (V-notch) and trapezoidal weirs are of order one. High-order polynomial weirs are providing wider range of Head-Discharge relationships, and hence better control of the flow at outlets of lakes, ponds and reservoirs. See also
Fishing weir Drop structure International Control Dam
^ "the definition of weir". Dictionary.com. Retrieved
^ "Weir". www.etymonline.com. Online Etymology Dictionary. Retrieved
20 May 2017.
^ "Weirs - Flow Rate Measure". www.engineeringtoolbox.com. Retrieved
^ Factors affecting weir flow measurement accuracy
^ Tummers, J. S., Winter, E., Silva, S., O’Brien, P., Jang, M. H.,
& Lucas, M. C. (2016). Evaluating the effectiveness of a Larinier
super active baffle fish pass for European river lamprey Lampetra
fluviatilis before and after modification with wall-mounted studded
tiles. Ecological Engineering, 91, 183-194.
^ Silva, S., Lowry, M., Macaya-Solis, C., Byatt, B., & Lucas, M.
C. (2017). Can navigation locks be used to help migratory fishes with
poor swimming performance pass tidal barrages? A test with lampreys.
Ecological Engineering, 102, 291-302.
^ Michael Robinson, Ph.D. P.E., Robert Houghtalen, Ph.D., P.E.
"Dangerous dams". Rhode Island Canoe/Kayak Association. Rhode Island.
Archived from the original on 2010-08-12. Retrieved
2011-06-26. CS1 maint: Multiple names: authors list (link)
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). Chanson, H. (2007). Hydraulic Performances of Minimum Energy Loss Culverts in Australia, Journal of Performances of Constructed Facilities, ASCE, Vol. 21, No. 4, pp. 264–272 doi:10.1061/(ASCE)0887-3828(2007)21:4(264). Gonzalez, C.A., and Chanson, H. (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. 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.
Clemmens, Albert (2010). Water Measurement with Flumes and Weirs. ISBN 978-1887201544. Akers, Peter (1978). Weirs and Flumes for Flow Measurement. ISBN 978-0471996378.
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Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways (Click "proceed" at the UQ-ITS