What are storm overflows?

During heavy rain, local sewer networks can struggle to cope with the amount of water entering pipes and storage tanks.

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.


Dry Spills

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.

How big is Southern Water’s wastewater network?

We operate 367 wastewater treatment sites, more than 3,000 pumping stations and a network of almost 40,000km of sewers.

We pay for our wastewater to be treated as part of our water/wastewater bill, why aren’t you doing just that?

Every day we treat nearly 1,400 million litres of wastewater, to some of the highest regulatory standards in the world. 95% of all wastewater is returned to the environment safely, maintaining river levels and providing a stable ecological environment for aquatic life. 5% is released, usually during heavy or prolonged rainfall and is diluted storm water. We are working hard to reduce that figure.

Why don’t you start investing to improve your assets and stop paying your shareholders instead?

We are, we’re investing £2bn between 2020 and 2025, with most going to improving our environmental performance and our assets. We haven’t paid our shareholders any dividends since 2017. Instead, all profits are being invested back in the business.

What is a combined sewer?

This is a system that contains both foul water from homes or businesses and rainwater runoff, treated together at a wastewater treatment site. Foul water from homes or businesses includes water from toilets, sinks and washing machines. Rainwater runoff comes from roofs, driveways and roads. There are over 100,000km of combined sewers still in existence in the UK.

What are storm overflows?

During heavy rain, local sewer networks can struggle to cope with the amount of water entering pipes and storage tanks. When they fill up, we use storm overflows to stop homes, businesses and roads from flooding. These overflows release excess water through outfalls into rivers and the sea. Storm overflows are part of the network’s design and are regulated by the Environment Agency. They are used in areas where the sewers were built to carry both foul water from homes and businesses, and rainwater from roofs, driveways and roads.

How many storm overflows are there?

There are around 15,000 storm overflows in England and approximately 1,000 in our region. How often they operate and release to the environment varies widely, ranging from
infrequent (less than 10 spills per annum) to frequent (greater than 100 spills per annum).

Where do storm overflows release?

They release into rivers and the sea. To see the location of our coastal outfalls, please visit Beachbuoy.

How do I know when there has been any storm release activity?

We show all our coastal releases on Beachbuoy, our near real-time storm overflow activity tool. Beachbuoy also informs the user if a release
affects bathing waters, taking into consideration the location of the outfall, duration of the release and the tidal conditions at the time. It’s worth
noting that releases shown on Beachbuoy can occur several days after rainfall, due to the amount of time taken for the water to pass through our network and arrive at the final treatment works

Where can I find data on historical storm overflows?

We publish our flow and spills data annually. You can also view recent release data on Beachbuoy. You’ll also see where the outfall pipes are which impact each bathing water.

What is the difference between a ‘storm’ and an ‘emergency’ overflow?

A storm overflow is permitted by the regulator and occurs when the system becomes overwhelmed with excess water. In rare incidences, an emergency overflow is triggered when there has been a technical fault or a blockage in the system. Both storm and emergency coastal overflows are shown on Beachbuoy.

Are you dumping raw sewage?

Most storm releases are heavily diluted wastewater – up to 95% is rainwater. Storm overflows are not manually operated, they work automatically to release excess water, for example after heavy rain has filled the sewers. These releases are permitted by law and we report all spills to the Environment Agency. Our industry is heavily regulated by the Environment Agency, which sets the permits on storm overflows.

What are the Govt. targets on reducing storm overflows?

Defra published their Storm Overflows Discharge Reduction Plan in August 2022, which sets targets for the water industry to eliminate storm overflows by 2050 (except for unusual heavy rainfall). We welcome this plan and are already leading the way with some of the targets outlined. For example, we are already hitting the average number of spills per outfall per year, that other water companies are aiming to achieve by 2025. We’re therefore confident that we’ll not only meet Govt. targets, but that we’ll likely exceed them.

What would happen if storm overflows were banned today?

During heavy or prolonged rainfall, the network would become overwhelmed in several areas – or catchments as we call them – with nowhere for the wastewater to go, but back up into people’s homes and onto roads. This would cause major flooding and pollution for the community.

What are you doing to reduce storm overflows?

We are taking several steps to significantly reduce storm overflows by 2030. As well as working on our own assets and reducing blockages in the network, we have a dedicated task force working in ‘Pathfinder’ areas to tackle the root causes of storm overflows. We recently published a Pathfinder Update which outlines our progress and future plans. For the task force’s latest news, updates and community studies, please visit our Storm Overflow pages.

How can we prevent storm overflows?

Preventing water from entering the combined sewer system during heavy rainfall, is the most sustainable and cost-effective way to reduce storm overflows going forwards.

There are currently three main ways to reduce storm overflows:

  1. Source control (removing and slowing the low of rainwater) – for example using rainwater harvesting, permeable paving, green
    roofs, soakaways (including tree pits), rain gardens (swales) and planters.
  2. Optimisation of existing infrastructure – adjusting connected systems and interfaces, using different mechanical and electrical equipment (e.g. pumps), making improvements in pumping station and storm tank use and control, and using smart network control with increased digitalisation.
  3. Building bigger infrastructure (building larger pipes, pumping stations, etc) – this includes wetland treatment (for groundwater), sewer lining/sealing (groundwater), as well as building larger sewers, storm tanks and treatment works.

Do bathing waters in the UK and across Southern Water’s region, meet regulatory standards?

Before privatisation, only 28% of bathing waters in the UK met the minimum public health standards. Today, the situation has significantly improved, with 79 of the 84 bathing waters in our area recognised by the Environment Agency as either good or excellent. We’re keen to continue playing our part in supporting water quality across our region.

Do storm releases impact water quality?

Although storm releases are heavily diluted, they can impact water quality. The impact of a storm release can vary based on the location of the
release, the amount released, how long it was released for, and the tides when discharged. Each outfall/permit is designed to consider the dilution factor, sensitivity, and amenity of the watercourse. We alert local authorities when there is a release.

Can you close a beach if there has been a release?

This decision is for the local authority. They manage the beach and are responsible for public health. We can advise when there has been a release as we have installed alarms and sensors to alert us; these have been installed on 98% of our storm overflow sites but will be on 100% by 2025. A release rarely results in a beach closure due to the locations of our outfalls, the length of time they’re used, and the amount discharged.

Is Southern Water responsible for bathing water quality?

We are a key custodian of water quality, but there are several factors that all impact water quality, these include storm releases, agricultural run-off, animal waste and marine activity. We recognise that we must play our part in protecting rivers and seas and be catalysts for change.

Why don’t you stop new developments connecting to your network?

We have no statutory rights to prevent new connections on our network. We can only make recommendations to local authority planning teams.

Are combined sewers still being built?

Modern systems have one pipe for foul and one for surface water. The surface water pipe releases rainwater back to the environment. Separate sewer systems have been built in the UK since the 1960s – before this, the sewers were combined. We have no legal powers to prevent new connections being made to existing combined sewers.

Glossary of terms
  • Combined sewage system – the system that contains both wastewater from homes and businesses, and rainwater from roofs, gardens and roads
  • Storm overflow – a pressure release valve that sends excess wastewater to outfalls during a heavy rain event
  • Outfalls – pipes that spill excess wastewater into rivers and seas
  • Beachbuoy – our storm overflow reporting system
  • Pathfinder – area we are trialling solutions to slow the flow of rainwater
  • Storm releases – the excess water that is diverted into rivers or the sea
  • Environment Agency – regulator that monitors our storm overflow activity and provides the permits to allow storm releases under strict conditions
  • Nature-based solutions – solutions such as ponds, tree pits, wetlands, rain gardens These solutions focus on greening spaces that also benefit the local community and wildlife