Storm overflows

Storm overflows

Protecting homes from flooding

Our storm overflows play an important and essential role in our sewerage system, as our combined sewers transport sewage from homes and industry as well as carrying surface water run-off from gutters, drains and some highways.

Heavy or prolonged rainfall can rapidly increase the flow in a combined sewer until the amount of water exceeds sewer capacity. Storm overflows act as relief valves, allowing excess stormwater to be released to rivers or the sea. This protects properties from flooding and prevents sewage backing up into streets and homes during heavy storm events.  

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 within the system. Rarely is a pollution incident attributed to a storm overflows operating correctly as there is no significant environmental impact in terms of ammonia, suspended solids and biochemical oxygen demand. 

Flows are further diluted by the receiving watercourses that will also be swollen by the same heavy rain. Many storm overflows are fitted with screens or scumboards that prevent debris entering the watercourse or have attenuation tanks which also improve water quality. 

We currently monitor around 75% of storm overflows and have a programme in place to install monitoring equipment on all storm overflows by 2023.

Through our Coastwatch service we publish information on our website and provide this information to organisations, including Surfers Against Sewage and local authorities, when storm overflows have been in use. This data feeds into the Surfers Against Sewage ‘Safer Seas’ app and is available from our website outside the bathing season.

Our online mapping system shows the location and historical frequency of operation of storm overflows.   

Could things be done differently?

In an ideal world we wouldn’t have a combined sewerage system, but many sewers were laid hundreds of years ago with areas, including town centres, built on top of the network of pipes. To upgrade the system is simply not feasible without demolishing buildings, causing serious disruption, and investing substantial sums of money.

There is risk that with a growing population and climate change affecting weather patterns, the use of storm overflows could increase. That’s why we are calling on government to rule that all new developments cannot connect their surface water drainage to sewers that have sewage in them.