Weir

A weir or low-head dam is a barrier across the width of a body of water that alters the flow characteristics of water and usually results in a change in the height of the water level. Weirs are used to control the flow of water for rivers, outlets of lakes, ponds, and reservoirs, industrial discharge, and drainage control structures. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. There is no single definition as to what constitutes a weir. Weirs pose a serious danger to boaters and have been involved in several fatal drownings. They are sometimes colloquially referred to as drowning machines.

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.

Etymology

The word likely originated from Middle English were, Old English wer, a derivative of the root of the verb werian, meaning "to defend, dam". The German cognate is Wehr, which means the same as English weir.

Function

Commonly, weirs are used to prevent flooding, measure water discharge, and help render rivers more navigable by boat. In some locations, the terms dam and weir are synonymous.

A common distinction between dams and weirs is that water may flow through both (controlling flow and often turning turbines in the case of a dam), it only flows over a weir. Thus many dams have an accompanying spillway, lower than the crest of the dam, to release water in excess of its capacity to retain or regulate its flow.

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. Some common weir purposes are outlined below.

Flow measurement

When appropriate conditions are met, 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. 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. However, this can only be achieved in locations where all water flows over the top of the weir crest, and none escapes elsewise.

A generic discharge calculation can be summarised as

Q = CLHⁿ

where

Q is the volumetric flow rate of fluid (the discharge),

C is the flow coefficient for the structure (on average a figure of 3.33),

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).

Flow over a V-notch weir

The flow over a V-notch weir (in ft³/s) is given by the Kindsvater–Shen equation:

${\displaystyle Q={\frac {8}{15}}{\sqrt {2g}}\,C_{e}\tan {\frac {\theta }{2}}(h+k)^{\frac {5}{2}},}$

where

Q is the volumetric flow rate of fluid in ft³/s,

g is the acceleration due to gravity in ft/s²,

C_e is the flow correction factor given in Shen 1981, p. B29, Fig. 12,

θ is the angle of the V-notch weir,

h is the height of the fluid above the bottom of the V-notch,

k is the head correction factor given in Shen 1981, p. B20, Fig 4.

Control of invasive species

As weirs are a physical barrier, they can impede the longitudinal movement of fish and other animals up and down a river. This can have a negative effect on fish species that migrate as part of their breeding cycle (e.g., salmonids), but it also can be useful as a method of preventing invasive species moving upstream. For example, weirs in the Great Lakes region have helped to prevent invasive sea lamprey from colonising farther upstream.

Watermills

Mill ponds are created by a weir that impounds water that then flows over the structure. The energy created by the change in height of the water can then be used to power waterwheels and power sawmills, grinding wheels, and other equipment.

Flood control and altering river conditions

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 for 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 the waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme currents or eddies. Many larger weirs will have construction features that allow boats and river users to "shoot the weir" and navigate by passing up or down stream without having to exit the river. Weirs constructed for this purpose are especially common on the River Thames, and most are situated near each of the river's 45 locks.

Issues

Ecology

Because a weir impounds water behind it and alters the flow regime of the river, it can have an effect on the local ecology. Typically, the reduced river velocity upstream can lead to increased siltation (deposition of fine particles of silt and clay on the river bottom) that reduces the water oxygen content and smothers invertebrate habitat and fish spawning sites. The oxygen content typically returns to normal once water has passed over the weir crest (although it can be hyper-oxygenated), although increased river velocity can scour the river bed causing erosion and habitat loss.

Fish migration

Weirs can have a significant effect on fish migration. Any weir that either exceeds the maximum 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 on fish populations.

In many countries, it is now a legal requirement to build fish ladders into the design of a weir that ensure that fish can bypass the barriers and access upstream habitats. Unlike dams, weirs do not usually prevent downstream fish migration (as water flows over the top and allows fish to bypass the structure in that water), although they can create flow conditions that injure juvenile fish. Recent studies suggest that navigation locks have also potential to provide increased access for a range of biota, including poor swimmers.