Storm overflows

Storm overflows automatically operate during heavy rainstorms to protect properties from flooding and prevent sewage from overflowing into streets and homes.

To eliminate all storm overflows immediately would bring huge disruption to our communities as well as a significant increase in water bills, forcing the digging up of huge numbers of roads at once and costing hundreds of millions of pounds.

That’s why we’ve committed to progressively eliminate the discharge of untreated sewage, starting with storm overflows that discharge most frequently and those that have any environmental impact. We're spending £3 million every month on schemes to tackle overflows. Find out more about what we are doing about storm overflows.

What are storm overflows?

Storm overflows are part of an older type of sewer system called a combined sewer system. These sewer systems carry both surface water (run-off from roof gutters, patios, driveways and some highways) and the foul water from homes and industry together in one pipe. The combined sewage is then transported to a water recycling centre to be treated.

During a storm event, heavy or prolonged rainfall can rapidly increase the flow in a combined sewer and cause it to become overwhelmed. Storm overflows are designed to release excess storm water into rivers or the sea when this happens to prevent sewer flooding.

Media reports suggest water companies ‘dump raw sewage’, but this isn’t accurate. This implies we can control when they operate and what is released is highly polluting. However, we have no control over when they operate and the discharge is heavily diluted by rainfall.


Monitoring storm overflows

There are about 15,000 storm overflows in England and 1,300 of them are in our region. Find out where the storm overflows we monitor are located.

We currently monitor around 80% of storm overflows in our region, which shows when they operate and how long for, and have a programme in place to install monitoring equipment on all storm overflows by 2023.

We provide updates when our storm overflows near bathing waters operate. We also give this information to organisations, including Surfers Against Sewage and local authorities.

Visit our Storm Overflow Improvement Dashboard to find out which overflows are currently being addressed (2020-25) as well as future dates for improvements.

Keep informed

Find out the latest on how Wessex Water is improving river and coastal water quality, now and in the future, by signing up to our free quarterly bulletin.

Storm overflow

The impact of storm overflows

As storm overflows should only operate during periods of intense rainfall, any foul water released from them will be very dilute because of the large volumes of rainwater flowing through the sewers, so they have very little environmental impact.

The Environment Agency is responsible for river and bathing water quality and it regulates intermittent discharges from storm overflows through environmental permits. The main polluting load of the contents of a sewer should flow to the treatment centre, allowing very dilute sewage to overflow when the sewer capacity is exceeded.

When storm overflows operate, the dilute sewage contains faecal bacteria, but their operation does not mean a bathing water’s quality is necessarily unfit for swimming. Bacteria generally do not survive long outside host organisms and are especially fragile when exposed to sunlight in seawater. Learn more about factors that affect water quality.

Combined Sewer Overflow

What we are doing about storm overflows

We’re investing £3 million every month to reduce the impact storm overflows have on rivers, the sea and the environment.

How to help manage rainwater runoff

We can all help to reduce the operation of storm overflows and protect the environment by improving how we manage rainwater runoff.

Find out how you can help manage rainwater run off.

Woman filling up a watering can with water from a water butt