Weir

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.

Commonly, weirs are used to prevent flooding, measure water discharge, and help render rivers more navigable by boat.

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.

For example, weirs in the Great Lakes region have helped to prevent invasive sea lamprey from colonising farther upstream.

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.

On larger rivers, a weir can also alter the flow characteristics of the waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme currents or eddies.

In some cases this can mean that huge lengths of breeding habitat are lost, and over time this can have a significant impact on fish populations.

Recent studies suggest that navigation locks have also potential to provide increased access for a range of biota, including poor swimmers.

[7] 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.

Hopefully, conditions will be such that the current will push the victim along the bed of the river until swept beyond the boil line and released by the hydraulic.

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.

A weir on the Humber River near Raymore Park in Toronto, Ontario, Canada
A weir on the Yass River , New South Wales, Australia, directly upstream from a shared pedestrian-bicycle river crossing
A weir on the Tikkurilankoski rapids in Vantaa , Finland
Time-lapse video of a new tilting weir being installed in the Caldicot and Wentloog Levels
The broad crested weir at the Thorp grist mill in Thorp, Washington , US
A sluice gate-based weir at Bray Lock on the River Thames , facing downstream. In the background is the smaller secondary "overspill" weir. Two small boats are also visible held against the overspill weir, having been washed against it during a particularly high discharge as a result of meltwater and subsequent rainfall following the 2018 winter cold wave.
During periods of high river flow, this nineteenth century weir of porphyry stone on a creek in the Alps would have significantly more water flowing over it.
Rescue boats designed for rescue and recovery from low head dams.