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

They 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.

Most towns and cities in the UK have combined sewer systems and consequently all have storm overflows to protect properties from flooding during heavy rainfall. There are about 15,000 storm overflows in England – 1,300 of them are in Wessex Water’s area. Find out where the storm overflows we currently monitor are located through are Drainage and Wastewater Management Planning portal.

No, we have been building separate sewer systems in the UK since the 1960s. This modern system has one pipe for foul water from homes and industry, and a separate pipe for surface water. The foul water is transported to the water recycling centres for treatment and the surface water is discharged to a watercourse, the sea or into permeable ground.

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. You can use our Drainage and Wastewater Management Planning portal to check the performance of the sites we currently monitor.

No, we provide data all year round for every bathing water in our area affected by storm overflows. Statistics for our region appear much higher than in other parts of the country because other companies only provide this information during the bathing water season (May to September) or for only a subset of their area.

Through our Coast and rivers watch 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.

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. A pollution incident is rarely attributed to a storm overflow that has operated correctly as there is no significant environmental impact in terms of ammonia, suspended solids and biochemical oxygen demand.

Storm overflows were designed to have little environmental impact when operating in heavy rainfall. The Environment Agency 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 does not survive long outside host organisms and are especially fragile when exposed to sunlight in seawater. Dispersion and dilution factors are also vital in determining the public health impact of water where overflow operation has occurred. Find out about the factors that affect river water quality on our wild swimming page.

Storm overflow discharges do have the potential to affect water quality near beaches. Local councils are responsible for beach management and they receive our notifications directly. They may decide to close a beach, but this is very rarely necessary when a storm overflow has operated correctly. There are many other factors which may lead to the council’s decision to close a beach.

It’s important for open water swimming enthusiasts to know about the many different factors that can affect river, lake and sea water quality. There are various sources of microbiological contamination, including agricultural and urban run-off, privately owned sewage treatment systems such as septic tanks and faeces from farm animals, rodents, wildlife and pets.

Permitted discharges from storm overflows can also have an influence on water quality, as can treated waste water from the water recycling centres. Because most sewage is organic, we use organic processes to treat it, which involves bacteria breaking down waste. The treated waste water must then comply with environmental standards before being returned to rivers and the sea. These standards are designed to protect the environment rather than public health, which is why rivers are not designated bathing waters.

Government advice on swimming in rivers can be found here.

Generally, storm overflows have an extremely small impact on rivers. Looking at publicly available information on the Government’s Catchment Data Explorer, you can see that storm overflows have a very small environmental impact, contributing to less than 4% of environmental river water quality problems.

We don’t currently know the real time impact storm overflows have on water quality. It is possible that a storm overflow that discharges for a couple of hours could discharge less harmful sewage than a storm overflow that discharges for just 30 minutes. It all depends on the contents of the combined sewer at the time of heavy rainfall.

Various coastal waters and some inland lakes that are popular locations for swimming are designated as bathing waters. The Environment Agency monitor more than 400 designated bathing waters in England between May and September. Each is given an annual classification of Excellent, Good, Sufficient or Poor. The Environment Agency also makes daily pollution forecasts for a number of bathing waters where water quality may be temporarily reduced for various reasons, such as heavy rainfall. Use Swimfo to search up designated bathing waters by name or location.

In an ideal world we wouldn’t have a combined sewer 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 and causing serious disruption. It’s estimated that it would cost hundreds of billions of pounds to remove storm overflows across the UK.

Since 2000, we have invested £181 million to upgrade more than 582 storm overflows. From 2020 to 2025, we plan to spend a further £150 million on storm overflows to reduce their impact on the environment.

In this time, we have committed to deliver 54 investigations into frequently spilling storm overflows and make improvements to 13 of them. We are also planning to increase the level of flow that 13 of our water recycling centres can treat, increase storm tank capacity at 18 water recycling centres and make improvements to two sites to improve bathing water quality. These measures will reduce the frequency of discharges from improved sites.

We invest more money than we receive through customer bills – see graph below. This investment, approved by Ofwat, comes at risk. The profits made by companies are partly used to finance investment programmes and partly to provide a return to investors for this risk.

It is estimated it would cost around £10 billion to eliminate overflows in our region. To help fund this, customers would see around a 50% increase in bills over 10 years.

The outcome of eliminating storm overflows wouldn’t guarantee an improvement in river water quality or public health benefit for wild swimming as other factors would remain. Meanwhile, there would be increased carbon consequences, due to the need to construct large tanks and install energy-intensive UV processes.

Property developers still have the right to connect surface water drainage from new houses to combined sewerage systems. Sewerage companies lack the powers to discharge surface water that has been disconnected either locally to garden infiltration systems or soakaways or directly to a watercourse. Sewerage legislation requires a rethink to resolve many of the issues sewer overflows are causing.

We can all do our bit to limit the impact of storm overflows. Here are three ways you can help to avoid putting unnecessary stress on our sewer network:

1. Only flush the three Ps: poo, paper and pee. Sanitary items, such as wet wipes, don’t break down properly in our sewers and cause blockages.
2. Don’t put FOGs (fats, oils and grease) down the sink. Instead, allow them to cool and scrape them into the bin so that they don’t solidify in your pipes.
3. Install a water butt to collect rainwater. This water can be used to maintain your garden during summer and will also reduce the volume of water going down the drain.