What is a Side weir

A side weir is a hydraulic structure installed along the side of an open channel or conduit to divert excess flow from the main watercourse into another channel, sewer or overflow system. Unlike traditional weirs, which span across the full width of a stream or channel, a side weir is positioned parallel to the direction of flow. It allows water to spill over a lateral crest when levels in the main channel exceed a certain threshold.

Side weirs are commonly found in urban drainage systems, combined sewer overflows (CSOs), flood control infrastructure and irrigation networks. Their purpose is to regulate water levels, manage surcharges and protect downstream assets from overloading. When designed and maintained properly, they play a critical role in safeguarding infrastructure, preventing flooding and improving the efficiency of hydraulic systems.

Functions and Purposes

The primary function of a side weir is to relieve flow from the main channel when it exceeds capacity. By diverting excess water, side weirs help:

  • Prevent overtopping or surcharging of channels and sewers

  • Direct stormwater or sewage to designated storage or treatment facilities

  • Maintain safe flow conditions downstream

  • Protect infrastructure, such as bridges, culverts and treatment plants, from damage

Side weirs are particularly useful in combined sewer networks, where both foul and surface water are conveyed in a single conduit. During heavy rainfall, the weir allows a controlled portion of flow to spill into a separate structure such as a detention tank, stormwater overflow or treatment channel, thereby preventing backflow or urban flooding.

Typical Locations and Applications

Side weirs are used in various engineering contexts. Some of the most common applications include:

  • Combined sewer overflows (CSOs): To divert dilute sewage during storm events.

  • Urban drainage networks: To manage stormwater runoff and reduce the risk of localised flooding.

  • Irrigation canals: To redistribute water into secondary channels or fields.

  • Flood control channels: To spill floodwaters into floodplains, reservoirs or detention basins.

  • Treatment plant inlets: To regulate inflow to different treatment trains or overflow pathways.

They may also be installed in culverts or open channels running through populated areas, where unexpected increases in flow could otherwise cause structural or environmental damage.

Types of Side Weirs

Side weirs can be categorised by their shape, crest configuration and method of control. The design depends on site-specific hydraulic conditions and operational objectives.

1. Free Overflow Weirs

These side weirs allow flow to pass over the crest without any mechanical regulation. The spill occurs automatically once water levels in the main channel reach the weir crest. These are typically constructed with a fixed elevation and are common in stormwater systems or CSOs.

2. Controlled Side Weirs

In some cases, side weirs are fitted with gates, valves or actuators to allow manual or automated adjustment of flow diversion. This is useful where flow control must respond to changing environmental or operational conditions.

3. Broad-Crested vs. Sharp-Crested Weirs

  • Broad-crested weirs have a flat or gently sloping crest, allowing stable flow and reduced turbulence. They are well suited to larger, open-channel applications.

  • Sharp-crested weirs have a narrow crest edge, often used in smaller conduits or measurement stations where precise flow control or monitoring is required.

Each type has specific advantages in terms of flow stability, construction complexity and maintenance needs.

Hydraulic Design Considerations

Designing a side weir requires detailed analysis of flow behaviour, channel geometry and site constraints. The main objective is to ensure the weir functions effectively under a range of flow conditions without causing adverse effects upstream or downstream.

Key factors in the design process include:

  • Weir length and height: Determines how much flow will spill at different water levels.

  • Channel velocity: Influences the pressure and angle at which flow approaches the weir.

  • Weir shape and orientation: Affects flow stability and energy dissipation.

  • Tailwater conditions: If the receiving channel is submerged, flow over the weir may be influenced by backpressure.

  • Discharge coefficient: An empirical factor based on weir geometry and approach conditions, used to estimate the rate of flow over the weir.

Computational fluid dynamics (CFD) modelling or physical model testing is often used for complex installations to ensure safe and efficient performance.

Materials and Construction

Side weirs are constructed using durable materials that can withstand continuous water exposure, variable flow rates and potential debris impact.

Common materials include:

  • Reinforced concrete: Offers structural strength and long-term durability. Often used in permanent installations within urban drainage or treatment networks.

  • Masonry or brickwork: Sometimes found in older systems or smaller-scale works.

  • Steel or aluminium: Used for gates or adjustable weir components.

  • Composite or plastic elements: Occasionally used in modular or prefabricated systems, particularly where corrosion resistance is critical.

Construction methods depend on site conditions and whether the weir is cast in place or assembled from prefabricated components.

Maintenance Requirements

Like all hydraulic structures, side weirs require periodic inspection and maintenance to ensure reliable operation. Common maintenance tasks include:

  • Removing debris: Weeds, leaves, litter or sediment can block the weir crest and reduce its effectiveness.

  • Inspecting structural integrity: Checking for cracks, erosion or corrosion in concrete and metal components.

  • Monitoring flow performance: Ensuring that the weir is diverting the correct volume under expected flow conditions.

  • Cleaning adjacent chambers or channels: To prevent downstream blockages or overflows.

Regular maintenance is essential, particularly in combined sewer systems, where blockages can lead to environmental pollution or sewer flooding.

Environmental and Regulatory Aspects

Side weirs, particularly those associated with CSOs, have received increasing regulatory attention due to their role in discharging dilute sewage into the environment. In the UK, the Environment Agency monitors and regulates overflow discharges under environmental permitting rules.

Sewerage undertakers must:

  • Monitor and report the frequency and duration of overflow events.

  • Install event duration monitoring (EDM) equipment where required.

  • Implement long-term plans to reduce overflow frequency through storage, treatment or sewer separation.

  • Ensure that weirs do not cause deterioration in receiving water quality.

New construction and refurbishment projects involving side weirs must consider both hydraulic performance and environmental compliance.

Advantages and Limitations

Advantages

  • Efficient flow diversion during peak events

  • Simple, passive operation (especially free overflow designs)

  • Helps prevent flooding and protects downstream assets

  • Compatible with many channel and sewer configurations

Limitations

  • Potential environmental impact if used to discharge untreated flow

  • Susceptible to blockage by debris

  • Requires regular maintenance and monitoring

  • Fixed weirs cannot adapt to variable flow conditions without control mechanisms

Despite these limitations, side weirs remain an important tool for flow regulation and flood management when designed and operated responsibly.

Integration with Other Systems

Side weirs often form part of a larger network of stormwater or wastewater infrastructure. They are commonly integrated with:

  • Stormwater detention tanks: Where flow is stored temporarily before being treated or released

  • CSO treatment systems: Such as screens, vortex separators or UV disinfection units

  • Flood alleviation schemes: Including attenuation ponds, storage tunnels or bypass channels

  • Monitoring systems: Using sensors to measure flow levels, event duration or pollution loads

Their effectiveness increases when used in combination with upstream source control measures, such as sustainable drainage systems (SuDS), which reduce peak flow volumes before they reach the main channel.

Conclusion

A side weir is a vital hydraulic structure used to divert excess flow from the main channel into secondary systems, helping to manage flooding, reduce pressure on infrastructure and improve the resilience of sewerage and drainage networks. Whether employed in urban sewer systems, flood control schemes or irrigation channels, side weirs must be carefully designed, constructed and maintained to ensure reliable performance and compliance with environmental standards. As climate change and urban development continue to stress existing drainage infrastructure, the role of side weirs in adaptive water management strategies is likely to become even more important.