What is a Weir? And What is Their Purpose?
- Ian
- 4 hours ago
- 4 min read
Weirs are a common sight along rivers in the UK. They’re used as part of water management, regulating upstream water levels, reducing flood flows, and preserving habitats. Their existence dates back centuries, with the most common design being the broad-crested weir.
Many people can recognise a weir but have no idea what their purpose is. In fact, people can walk over them, mistaking them for crossings. But weirs are extremely dangerous and can cause severe injury or fatal drownings.
So, what is a weir? What is the purpose of a weir in a river? And what are the different types of weirs? Find out in this article, we'll discuss:
What is a Weir?
A weir (sometimes called a low-head dam) refers to any control or barrier placed in an open channel to limit water discharge. Weirs almost always span the entire width of the water body. They’re commonly found in:
Rivers
Outlets of lakes
Ponds
Reservoirs
Industrial discharge sites
Drainage control structures
The main logic of a weir is to slow water without blocking the flow entirely. Usually, this is achieved with a raised crest or barrier. Weirs are typically made from stone or concrete. They’re relatively simple, passive structures that require little to no upkeep. However, more rarely, weirs can be raised or altered and may include sluice gates.
What is a River Weir?
A river weir is simply a weir that spans the width of a river. Unlike other forms of weirs, it must deal with a substantial and constant flow of water. Most river weirs are fixed weirs made of concrete, stone, or timber.
The water flows over the crest of the weir rather than freely downstream. This creates a calm, deeper section of water upstream and faster, often turbulent, water downstream.
There are many reasons to install a river weir system. It raises and calms the upstream water level, making it navigable, it regulates flow during dry periods, and it stabilises river levels near town, bridges, or infrastructure.
Types of Weirs
Weirs come in all shapes and sizes. In the UK, the most common type of river weir you’ll see is a broad-crested weir. But there are examples of all types of weirs dotted along the waterways.
Broad-crested weirs have a wide, flat top that water flows over evenly. They’re commonly used to keep water levels steady and control flow in rivers and channels.
Sharp-crested (V-notch) weirs force water through a narrow opening, making them very good for measuring flow accurately, especially at lower water levels.
Ogee weirs use a smooth, curved profile that follows the natural shape of moving water. They’re typically found in high-flow or flood-management settings where large volumes need to pass safely.
Compound weirs combine different shapes in one structure, allowing them to measure both low and high flows reliably as river conditions change.
The weir at Hambleden Lock on the River Thames is an example of a multi-tiered broad-crested weir. Unlike other examples, a walkway exists over the top, giving the public the right of way to cross the river. Elsewhere, the Dobbs Weir in Hertfordshire is a complicated series of broad-crest and V-notch weirs.
The Main Purposes of a Weir
Weirs don’t serve a single purpose. The primary purpose of a weir is flow regulation. The artificial barrier slows the water substantially, raising the water level behind the weir and creating a deep, gentle section of water. Because the top height of the weir is known and all water flows over it, hydrologists and engineers can carefully determine the depth of water behind the weir and also the rate of flow.
This slowing of water has several other benefits.
During periods of heavy rain, weirs prevent water from rushing downstream, reducing flood risks in certain areas. Behind the weir, the calmer pool supports fish, birds, amphibians, and aquatic plants. They serve a similar role as beaver dams in natural systems. Conversely, they also prevent invasive species from moving upstream (or native species such as salmonids).
Finally, the weir allows for sections of the river to be permanently navigable. By controlling water depth, boats can pass safely along a waterway, although they cannot move downstream within a specific passing feature.
Environmental Considerations of Using Weirs
Weirs pose and solve several environmental issues.
As mentioned, they’re a natural barrier to fish. That’s beneficial in the case of invasive species, providing a permanent barrier, but it can also be an impediment to native species that travel upstream for breeding. Specialised ladders or fish passes can be integrated into the weir to allow species like salmon to “jump” upstream.
Weirs also alter sediment transport. Upstream, it leads to increased siltation, reducing water oxygen content and affecting invertebrate habitats and fish spawning sites.
Downstream, the increased velocity may scour sites, causing erosion and habitat loss. Modern weirs attempt to mitigate these ecological challenges with fish ladders and alternative designs.
Where You Commonly Find Weirs
Weirs are found on most waterways in the UK. The most common sites include:
Rivers and streams
Reservoir outflows
Drainage systems
Agricultural land
Flood management schemes
You may also see weirs in industrial and wastewater treatment sites for flow management. They’re an ingenious and low-cost solution to any flow problem.
The Importance of Weirs in Water Management
Weirs aren’t something people think about very often. They’re quiet, understated, and rarely ever break. But without weirs, we couldn’t control, measure, and manage waterflows across the UK. Each day, they reduce flood risks, stabilise river levels, and support safe navigation — they even create habitats for many species.
If you’re planning work near a weir or need expert support with design, installation, or maintenance, it’s smart to call in the experts.
The Rothen Group understands waterway management better than anyone. We deliver safe, compliant, and well-planned water management solutions, including weirs, booms, and trash screens. Get in touch with us to find out more about our weir and waterway services.
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