When they fill up, we use pressure relief valves built into the network – known as storm overflows – to stop homes and businesses from flooding. These overflows release excess water through outfalls into rivers and the sea. Storm overflows are part of the design of the sewers and are regulated by the Environment Agency. They're used in areas where the sewers were built to carry both wastewater from homes and businesses, and rainwater from roofs, gardens and roads.
Storm overflows release excess water that is overwhelming the sewerage system during periods of rain to prevent both flooding and excess water backing up into homes and business. These releases are typically heavily diluted with rainwater and in most cases are consented releases permitted by the Environment Agency.
A dry spill is a storm overflow release that happens when there has not been any rainfall. They can happen for a number of reasons including;
- Groundwater infiltration: When water from the ground squeezes its way into the system through underground public or private pipework.
- Misconnections: There is a legal right to connect to the sewerage system, but no stipulations to say you must connect correctly. Misconnections in private pipework place extra surface water volumes into the networks contributing to the overwhelm.
- Drainage time: Things like size of the area and complexity of its drainage and sewerage systems can affect drainage time, with some areas taking days to fully drain after rainfall.
- Equipment and infrastructure issues: An equipment malfunction or infrastructure issue such as a blockage caused by flushed wet wipes could result in a dry spill. Unflushables reduce the capacity of the sewer network, while adding unforeseen, hard to detect, and unpermitted surface water pressures on the drainage system.
We are fully transparent, and every release is reported to the Environment Agency.
These contributing factors are all issues we are tackling within the Clean Rivers and Seas Task Force, and with £35 million in funding secured to undertake multiple projects including improvements and enhancements to our infrastructure in order to Slow The Flow of water into the system, and digitalisation of processes such as sewer level monitors to help us identify changes before they cause problems, we are well on our way to putting an end to dry spills. Read more about our work on storm overflow reduction in our Pathfinder Update.
What they are, why they happen, how they affect bathing water and what we’re doing about them.
You can download our Detailed Storm overflows guide or view the FAQs below.