What are storm overflows and why are they needed?

Storm overflows act like safety valves on the wastewater network. When the system becomes too full (for example, after heavy rain), storm overflows release the excess. If they didn’t work, sewage would build up in pipes and flood into people’s gardens and homes.

The process is strictly regulated by the Department for Environment, Food and Rural Affairs (Defra) and the Environment Agency.

We've created some videos below to explain more about storm overflows and when they're activated.

Frequently asked questions

Monitoring our use of storm overflows

Every storm overflow has an Event Duration Monitor (EDM) that measures the level of sewage in the system and indicates whether the overflow is active, and for how long. 

All EDMs send us data at least every 15 minutes (and in some cases, every 10 seconds). Using this, together with other data, we can spot issues in the network, and respond as necessary. We also use this data to model our system and identify areas for improvement.

We also use the EDM data on our storm overflow map to show you in near real-time when our storm overflows are active.

How we’re reducing our reliance on storm overflows

We want to reduce the need for storm overflows to operate, which is why we’re investing record amounts into improving, expanding and upgrading our network to better cope with the volumes of wastewater it deals with. We are reducing our reliance on storm overflows by:

• Reducing the amount of rainwater and groundwater entering our system
• Slowing the flow of water through the environment
• Increasing our network’s capacity to store and treat flows

Our monitoring teams constantly review data being sent back to us from our EDMs, sewer level monitors, river water quality monitors, flow and pressure monitors and water samples. This constant data-gathering is fundamental to modelling our system and identifying areas for improvement.