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A weir 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, 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. Weirs are also used in the study of returning salmonids. ==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 summarised as: : Where * ''Q'' is flow rate of fluid * ''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.〔(Factors affecting weir flow measurement accuracy )〕 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, England, UK. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「weir」の詳細全文を読む スポンサード リンク
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