![]() 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. Some common weir purposes are outlined below. Weirs can vary in size both horizontally and vertically, with the smallest being only a few centimetres in height whilst the largest may be many metres tall and hundreds of metres long. Accordingly, the crest of an overflow spillway on a large dam may therefore be referred to as a weir. Usually, a dam is designed specifically to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics.Ī 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. In some locations, the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. The German cognate is Wehr, today the often as the composite noun Stauwehr which has the same meaning as English weir.Ī broadcrest weir at the Thorp grist mill in Thorp, Washington, USAĬommonly, weirs are used to prevent flooding, measure water discharge, and help render rivers more navigable by boat. ![]() 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". Weir can also refer to the skimmer found in most in-ground swimming pools, which controls the flow of water pulled into the filtering system. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. Weirs are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. Time lapse video of a new tilting weir being installed in the Caldicot and Wentloog LevelsĪ weir / w ɪər/ or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level.
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