What is a Stormwater overflow

A stormwater overflow is a controlled structure within a combined sewer system that permits excess flow to discharge directly into a nearby watercourse or receiving environment during periods of heavy rainfall. It typically consists of a weir or overflow chamber that diverts part of the flow once the sewer reaches its hydraulic capacity. Stormwater overflows are designed to protect properties, streets and wastewater treatment works from flooding by acting as pressure relief points in the drainage network.

While these overflows are an engineered solution to manage stormwater surges, they also present environmental challenges due to the nature of the water being discharged — which can contain a mix of rainwater and untreated sewage. As such, their operation, monitoring and mitigation are key issues in urban drainage management.

Why Stormwater Overflows Are Necessary

In many urban areas, particularly those with combined sewer systems, stormwater and foul sewage are conveyed in the same pipe. During dry conditions, the system typically carries only domestic and industrial wastewater. However, during storms or snowmelt, a sudden increase in water volume can exceed the sewer’s design capacity.

Stormwater overflows serve the following purposes:

  • Prevent sewer flooding: By allowing excess water to exit the system in a controlled manner

  • Protect wastewater treatment plants: Limiting the volume of storm-influenced inflow prevents hydraulic overload

  • Maintain public hygiene and safety: Avoiding sewer backups into homes, businesses and streets

  • Control discharge location: Directing overflow to predefined outfalls reduces uncontrolled spillage

These structures are particularly common in older cities, where separating stormwater and foul sewers is either prohibitively expensive or technically unfeasible.

How a Stormwater Overflow Works

A typical stormwater overflow includes the following elements:

  • Main flow channel: Where dry weather flow and moderate storm flow are conveyed

  • Weir or spill crest: Positioned at a set height to allow only excess flow to spill over

  • Overflow chamber or tunnel: Collects diverted flow

  • Outfall pipe: Discharges the overflow to a river, stream, or sea

  • Telemetry and monitoring equipment (in modern systems): Records when, where and how often the overflow is used

The weir acts as a hydraulic threshold. When the flow level rises due to stormwater input, water overtops the weir and enters the overflow path. If the storm is brief or moderate, only a small amount may be released. In more intense events, discharges can continue for hours or longer.

Types of Stormwater Overflow Structures

Stormwater overflow structures are adapted to the layout and hydraulic profile of the sewer network. Common types include:

1. Combined Sewer Overflows (CSOs)

These are the most widely known type. Located in combined systems, CSOs discharge both rainwater and diluted sewage when flow exceeds system capacity.

2. Storm Relief Outfalls

These are dedicated overflow points designed only for stormwater, used in separate surface water systems to prevent ponding or overloading of open channels.

3. Intercepting Chambers with Bypass Weirs

Often used near treatment works, these chambers divert excess flow directly to the overflow route if the downstream network cannot accept the full volume.

Each type is designed to minimise public health and environmental risks while ensuring reliable hydraulic performance during extreme weather events.

Environmental Impacts of Stormwater Overflows

Despite their intended purpose, stormwater overflows can have significant environmental consequences, especially when used frequently or in sensitive locations. The main concerns include:

  • Water pollution: Discharges can contain faecal bacteria, nutrients, hydrocarbons, litter and microplastics

  • Oxygen depletion: Organic matter in the discharge consumes oxygen, affecting aquatic life

  • Algal blooms: Nutrient-rich overflows can lead to eutrophication in lakes and slow-moving rivers

  • Public health risks: Bathing waters and recreational areas can be contaminated

  • Loss of biodiversity: Sensitive species may be displaced or harmed by changes in water quality

The impact depends on the frequency, duration, volume and composition of the overflow, as well as the characteristics of the receiving water body.

Monitoring and Regulation in the UK

Stormwater overflows in the UK are subject to regulation by the Environment Agency (EA) in England, Natural Resources Wales (NRW), SEPA in Scotland and NIEA in Northern Ireland. Water companies are legally required to:

  • Monitor the operation of storm overflows using Event Duration Monitors (EDMs)

  • Report on the number of overflow events annually

  • Comply with Environmental Permits that set limits on frequency and impact

  • Take steps to reduce overflow frequency in high-priority areas, such as bathing waters and Sites of Special Scientific Interest (SSSIs)

In 2022, more than 90 percent of storm overflows in England were equipped with EDMs, and public access to this data is increasing.

Reducing the Need for Stormwater Overflows

To address the environmental impact of overflows, drainage authorities and water companies are pursuing various mitigation strategies:

Infrastructure Upgrades

  • Increasing sewer capacity: Replacing or upsizing undersized pipes

  • Storm tanks and offline storage: Providing temporary containment of peak flows

  • Real-time control systems: Using sensors and valves to optimise network performance dynamically

Green and Sustainable Approaches

  • Sustainable Drainage Systems (SuDS): Retain and infiltrate rainfall close to source

  • Permeable paving and green roofs: Reduce volume and rate of runoff

  • Rainwater harvesting: Captures roof runoff for reuse, reducing system load

These interventions aim to reduce inflow to combined systems and lower the likelihood of overflows being triggered.

Controversy and Public Perception

Stormwater overflows have become a point of growing public concern, particularly in the context of environmental protection and climate resilience. Media coverage of sewage discharges into rivers and beaches has led to:

  • Greater scrutiny of water companies

  • Increased regulatory pressure to invest in upgrades

  • Community-led water quality monitoring

  • Calls for legislative change to eliminate frequent discharges

The Storm Overflows Discharge Reduction Plan introduced by DEFRA outlines long-term targets for reducing overflows, including:

  • Eliminating overflows into sensitive sites by 2035

  • Reducing all storm overflow discharges to an average of no more than 10 per year by 2050

This policy shift signals a significant evolution in how overflow systems will be managed in the coming decades.

Maintenance and Operational Challenges

Operating a stormwater overflow system requires:

  • Regular inspection of weirs and chambers

  • Removal of debris and sediment

  • Calibration and maintenance of monitoring equipment

  • Response plans for pollution incidents

Failures in overflow performance may result from blockages, structural damage or inaccurate sensor data. As such, both physical and digital infrastructure must be maintained to high standards.

Integration into Urban Drainage Planning

Stormwater overflows are often one component of a broader urban drainage strategy. Effective planning involves:

  • Hydraulic modelling of catchment areas

  • Designing systems with sufficient resilience for future climate scenarios

  • Integrating SuDS to manage flows at source

  • Public engagement and awareness of how the system functions

Where retrofitting is needed, engineering interventions must balance cost, land use, community impact and environmental gain.

Conclusion

Stormwater overflows play a vital safety role in combined sewer systems, allowing excess water to be discharged during storms to prevent more severe flooding or treatment failure. However, their use has clear environmental trade-offs, particularly when they occur frequently or in ecologically sensitive areas.

With the increasing availability of monitoring data, stricter regulatory oversight and public demand for cleaner waterways, the long-term strategy in the UK is focused on reducing dependency on stormwater overflows through investment in infrastructure, adoption of nature-based solutions, and better system management.

As climate change brings more intense rainfall and urban growth places greater pressure on existing networks, stormwater overflow systems must evolve to meet higher expectations of performance, transparency and sustainability.