What is a Overflow weir

An overflow weir is a structure or device used in water management and drainage systems to control and direct excess water once it reaches a predetermined level. Its primary function is to protect infrastructure and surrounding areas from flooding or overloading by allowing surplus water to spill over into a secondary route, storage area or discharge point.

Overflow weirs are widely used in sewers, stormwater systems, treatment facilities, reservoirs and open channels. They serve both hydraulic and safety purposes by providing a passive and reliable means of flow control without the need for mechanical intervention.

How an overflow weir works

An overflow weir operates on a simple principle. When the water level in a structure such as a tank, chamber or channel rises to the height of the weir crest, the water begins to spill over the weir. This discharge is directed away from the main system to avoid flooding or surcharge.

Typical applications include:

  • Stormwater detention basins where excess rainfall exceeds design storage

  • Combined sewer overflows (CSOs) during periods of high rainfall

  • Balancing tanks in wastewater treatment works

  • Overflow chambers in urban drainage networks

  • Environmental flow regulation in rivers or canals

The height and shape of the weir determine the point at which overflow occurs and the rate at which water is released. Flow is typically driven by gravity and requires no moving parts.

Types of overflow weirs

Various types of overflow weirs are used in civil and environmental engineering. Their design depends on the volume of water being managed, the available space and the intended discharge location.

Common types include:

  1. Sharp-crested weir
    Features a thin, flat edge where water flows cleanly over the crest. Often used in flow measurement and controlled discharge applications.

  2. Broad-crested weir
    Has a flat, wide crest allowing a larger volume of water to pass. More stable in variable flow conditions and common in stormwater infrastructure.

  3. Notched or V-notch weir
    Includes a V-shaped opening for more precise flow control, especially at low flows. Common in monitoring stations.

  4. Side weir
    Built along the side of a channel, allowing excess flow to escape laterally. Used in open channel systems where gradual discharge is preferred.

  5. Crested wall or overflow slot
    Integrated into tank or chamber walls to act as an emergency overflow point.

Each type has specific hydraulic characteristics and may include features such as debris screens, flow regulators or energy dissipation zones downstream.

Design considerations

Designing an effective overflow weir requires careful assessment of the hydraulic conditions, risk levels and environmental context. Key factors include:

  • Crest level: Must be set at the desired overflow threshold to prevent premature discharge or flooding

  • Flow rate: The weir must handle expected peak flows without causing backflow or overtopping

  • Discharge location: Water must be directed to a suitable outlet such as a soakaway, detention basin or watercourse

  • Structural integrity: The weir must be robust enough to handle flow forces and potential debris

  • Access and maintenance: Should be easy to inspect, clear and maintain

Modelling tools are often used to simulate flow conditions under different storm events and to optimise the weir design for resilience and performance.

Role in urban drainage and flood management

In urban drainage systems, overflow weirs are crucial components of both minor and major drainage infrastructure. They serve as relief points when pipes and chambers reach full capacity, helping to manage stormwater surges and prevent surface flooding.

Typical examples include:

  • Combined sewer overflows, where weirs divert excess water to watercourses during high rainfall to protect treatment plants

  • Detention basins where weirs regulate outflow rates to prevent downstream overloading

  • Stormwater inlets with overflow slots to redirect water to alternative storage areas

In modern sustainable drainage systems (SuDS), overflow weirs are often paired with features such as swales, filter trenches or retention ponds to allow gradual, controlled water release into the environment.

Overflow weirs in treatment systems

Wastewater treatment facilities also rely on overflow weirs to manage flow distribution and prevent process overload. Applications include:

  • Flow splitting between treatment stages

  • Emergency overflow from storm tanks or balance tanks

  • Flow regulation in sedimentation or settlement tanks

In these settings, overflow weirs must be precisely calibrated to prevent hydraulic shocks and to comply with discharge regulations.

Maintenance and operational considerations

Although overflow weirs are passive devices, they require routine maintenance to ensure continued functionality. This includes:

  • Clearing debris or sediment build-up at the crest or downstream

  • Inspecting structural components for wear, corrosion or damage

  • Monitoring flow patterns and discharge quality

  • Ensuring that any downstream channels or storage areas are functioning properly

Blocked or damaged weirs can cause significant system failures, including flooding, pollution or surcharge events.

Environmental and regulatory implications

In systems where overflow discharges to natural water bodies, such as rivers or streams, the design and operation of overflow weirs must comply with environmental regulations. In the UK, this includes guidance from the Environment Agency and legislation under the Water Industry Act and the Environmental Permitting Regulations.

Discharges from overflow weirs in combined sewers, for example, are permitted only under controlled conditions and must be monitored and reported. Some weirs are fitted with telemetry systems or event duration monitors to track when and how often overflow occurs.

Conclusion

An overflow weir is a simple yet vital feature in many water management and drainage systems. By allowing excess water to discharge safely when capacity is reached, it helps to protect infrastructure, maintain operational control and reduce the risk of flooding.

Whether used in sewers, storm tanks, treatment plants or open channels, overflow weirs offer a passive, reliable and low-maintenance solution for flow regulation. Their effectiveness depends on correct design, proper integration with the wider system and regular inspection to ensure they perform as intended under all conditions.