Resilience measure: Weir


A weir is a barrier across a river designed to alter the 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. In some places the crest of an overflow spillway on a large dam may also be called 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. Unlike a dam, which has the specific purpose of impounding water, a weirs purpose can be less obvious and they most commonly are used to alter or control the flow characteristics of the river. A particular distinction between dams and weirs is that a weir has water flowing over the top (crest) of the structure along at least some of its length. Some of the most common weir types are labyrinth weir, broad-crested weir, sharp crested weir, piano keys weir, compound weir and V-notch weir.
(Wikipedia, Weir, accessed on Sept. 2016)

Co-benefits and impacts

Weirs are commonly used to control the flow rates of rivers during periods of high discharge. Sluice gates (or in some cases the height of the weir crest) can be altered to increase or decrease the volume of water flowing downstream. Weirs of this purpose are commonly found upstream of towns and villages and can either be automated or manually operated. By slowing the rate at which water moves downstream even slightly a disproportionate effect can be had on the likelihood of flooding. On larger rivers a weir can also alter the flow characteristics of a river to the point that vessels are able to navigate areas previously inaccessible.
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. Weirs can have a significant effect on fish migration. Any weir that exceeds either the maximum vertical 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 fish populations. ven 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".
(Wikipedia, Weir, accessed on Sept. 2016)

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Last modified: Sept. 13, 2016, 8:36 a.m.