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| | Looking at playing a new video clip clip game, read the take advantage of book. Most dvds have a book you'll can purchase separately. You may want in consider doing this and reading it before the individual play, or even even as you are playing. This way, you can easily get the most for of your game run.<br><br> |
| [[File:Humber Weir.JPG|A weir on the Humber River near [[Raymore Park]] in Toronto, Ontario|thumb|right]]
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| {{Redirect|Weirs|the Luke Vilbert and Jeremy Simmonds album|Weirs (album)}}
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| A '''weir''' {{IPAc-en|ˈ|w|ɪər}} 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 [[dam]]s, pooling water behind them while also allowing it 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.
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| ==Function==
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| [[File:Thorp Gristmill Weir.jpg|thumb|right|A weir at the Thorp grist mill in [[Thorp, Washington]]]]
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| Weirs allow [[Hydrology|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.
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| The discharge can be summarized as
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| :<math>Q = C L H^n</math>
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| Where
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| * ''Q'' is flow rate of fluid
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| * ''C'' is a constant for structure
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| * ''L'' is the width of the crest
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| * ''H'' is the height of head of water over the crest
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| * ''n'' varies with structure (e.g. 3/2 for horizontal weir, 5/2 for v-notch weir)
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| 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.<ref name=" Factors affecting weir flow measurement accuracy ">[http://www.openchannelflow.com/products/weirs/flow-characteristics/ Factors affecting weir flow measurement accuracy]</ref>
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| 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.
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| The crest of an overflow spillway on a large dam is often called a ''weir''.
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| 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).
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| 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.
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| ==Drawbacks==
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| [[File:Grialëces cuecenes .jpg|thumb|19th century weir of porphyry stone on a creek in the Alps]]
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| * 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.
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| * Weirs can also have an effect on local fauna. While a weir is easy for [[Sockeye salmon|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 ladder]]s provide a way for fish to get between the water levels.
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| * 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".<ref>{{cite news
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| | title = Dangerous dams
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| | url = http://www.ricka-flatwater.org/dams.htm
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| | archiveurl = http://web.archive.org/web/20100812233126/http://www.ricka-flatwater.org/dams.htm
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| | archivedate = 2010-08-12
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| | work = Rhode Island Canoe/Kayak Association
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| | location = Rhode Island
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| | date =
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| | accessdate = 2011-06-26
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| | author = Michael Robinson, Ph.D. P.E., Robert Houghtalen, Ph.D., P.E.
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| }}</ref><ref>[http://watercraft.ohiodnr.gov/education-safety/safety-tips-for-every-boater/activity-specific-safety/lowhead-dams Lowhead Dams], [[Ohio DNR]]</ref>
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| * 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.
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| ==Types==
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| [[File:Dorset sn weir 01.jpg|thumb|The bridge and weir mechanism at [[Sturminster Newton]] on the [[River Stour, Dorset]]]]
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| [[File:Revin Meuse weir 20041230- 024.jpg|thumb|right|A manually operated [[needle dam]]-type weir near Revin on the [[Meuse River]], [[France]]]]
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| [[File:WarkworthweirSeuilNewZealand.jpg|thumb|right|A weir in [[Warkworth, New Zealand]]]]
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| [[File:Dobbsweirvdropsjan2006.jpg|thumb|right|V-notches at [[Dobbs Weir]] near [[Hoddesdon]], [[England]]]]
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| 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.
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| ===Labyrinth weir===
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| 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.<ref>Crookston, Brian Mark, "Labyrinth Weirs" (2010). All Graduate Theses and Dissertations. Paper 802.http://digitalcommons.usu.edu/etd/802</ref>
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| ===Broad-crested weir=== | |
| A broad-crested weir is a flat-crested structure, with a long crest compared to the flow thickness.<ref>Chanson 1999, 2004; Henderson 196;, Sturm 2001</ref> 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.
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| ===Sharp crested weir (fayoum weir)===
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| 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 styles, 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.<ref>http://www.openchannelflow.com/products/weirs/flow-characteristics</ref>
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| 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.<ref>http://www.openchannelflow.com/blog/article/weir-plate-materials</ref>
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| ===Compound weir===
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| The sharp crested weirs can be consolidated into three geometrical groups : 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 two or more types - typically a V-notch weir with a rectangular weir.<ref>http://www.openchannelflow.com/blog/article/compound-weirs</ref>
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| An example of the compound weir is manufactured by Thel-Mar Company.<ref>http://thel-mar.com/</ref> 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.
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| ===V-notch weir===
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| 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 [[Theodor Rehbock|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.<ref name=" V-notch weir accuracy ">[http://www.openchannelflow.com/products/weirs/flow-characteristics/ V-notch weir accuracy]</ref>
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| 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.<ref>http://www.openchannelflow.com/blog/article/free-flow-equations-for-v-notch-weirs-of-any-angle</ref>
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| ===Minimum Energy Loss weir===
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| The concept of the Minimum Energy Loss (MEL) structure was developed by Gordon McKay in 1971.<ref name="Chanson2009">{{cite book|author=[[Hubert Chanson|Chanson, H.]] |title= Embankment Overtopping Protections System and Earth Dam Spillways |url= http://espace.library.uq.edu.au/view/UQ:185350 |publisher=in "Dams: Impact, Stability and Design", Nova Science Publishers, Hauppauge NY, USA, Ed. W.P. Hayes and M.C. Barnes, Chapter 4, pp. 101-132 |year=2009 |isbn= in "Dams: Impact, Stability and Design", Nova Science Publishers, Hauppauge NY, USA, Ed. W.P. HAYES and M.C. BARNES, Chapter 4, pp. 101-132}}</ref> 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).
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| 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.
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| ==See also==
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| * [[Fishing weir]]
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| * [[Drop structure]]
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| ==References==
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| ;Notes | |
| {{reflist}}
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| ;Bibliography
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| * [[Hubert Chanson|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).
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| * [[Hubert Chanson|Chanson, H.]] (2007). [http://espace.library.uq.edu.au/view.php?pid=UQ:84668 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)}}.
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| * Gonzalez, C.A., and [[Hubert Chanson|Chanson, H.]] (2007). [http://espace.library.uq.edu.au/view/UQ:107673 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}}.
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| * Henderson, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
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| * McKay, G.R. (1971). "Design of Minimum Energy Culverts." Research Report, Dept of Civil Eng., Univ. of Queensland, Brisbane, Australia, 29 pages & 7 plates.
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| * Sturm, T.W. (2001). "Open Channel Hydraulics." McGraw Hill, Boston, USA, Water Resources and Environmental Engineering Series, 493 pages.
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| ==Further reading==
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| *{{Cite book |last=Clemmens |first=Albert |title=Water Measurement with Flumes and Weirs|year=2010|isbn= 978-1887201544}}
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| *{{Cite book |last=Akers |first=Peter |title=Weirs and Flumes for Flow Measurement|year=1978|isbn= 978-0471996378}}
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| ==External links==
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| {{Commons category|Weirs}}
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| {{wiktionary}}
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| * [http://www.uq.edu.au/~e2hchans/mel_culv.html Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways] (Click "proceed" at the UQ-ITS Advisory webapge)
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| [[Category:Weirs| ]]
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| [[Category:Dams by type]]
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| [[Category:Rivers]]
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| [[Category:Water transport infrastructure]]
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