Storm overflows explainer


What are storm overflows and why are they used?

Storm overflows are designed to act as relief valves when the sewerage system is at risk of being overwhelmed, for example during heavy downpours when a lot of rainwater runs into drains and the sewerage system in a short space of time.

If the system does get overwhelmed it can have dreadful impacts for customers, causing flooding or even backing up into people’s homes in the worst case scenario.

To prevent that happening water companies sometimes use storm overflows to release extra rainwater and wastewater into rivers or seas. The Environment Agency and Natural Resources Wales regulate the use of storm overflows and can grant permits for them in certain circumstances.

Spills can also come from storm overflows in emergency situations, for example, if there are sewer blockages or equipment failures at wastewater treatment works.


Why are they used so often?

Storm overflow releases happen when the capacity of the sewerage system is at risk of being overwhelmed. There are different reasons this can happen and they can vary across the country, too.

Ensuring sewerage systems are well maintained and have enough capacity to deal with the volumes of water and sewage passing through the pipes and treatment centres, helps reduce the need for Storm overflows, although in extreme situations they may still be needed, even if everything possible is done to maintain full capacity in the network.

Customer behaviour can have an impact too. You have probably seen the reports of fatbergs – huge blockages caused when wet wipes, fats and grease clog up the pipes and reduce their capacity. If there is less space in the pipes, there is less resilience in the sewerage system. That is why it is important we don’t flush the wrong things down the loo or pour them into sinks.

Other things can also affect the capacity in the sewerage network such as population growth and development with more wastewater and rainwater run-off from new housing estates. Alongside that, is the collective impact of what is called urban creep, with changes to properties such as extensions, patios, paved-over gardens and driveways or astroturf, all of which reduce the amount of grass and soil available to soak up the rain, so water runs into the drains more quickly.

This point on capacity can be exacerbated by the change in our climate, as we are seeing drier spells followed by heavier and more intense rainfall. This kind of heavier rain can fill the pipes and system rapidly and lead to use of storm overflows.


What can be done to reduce the use and harm from storm overflows?

The current levels of harm caused by storm overflow discharges into rivers cannot continue, and the water sector must tackle this.

Ofwat is committed to playing its part, and has a key role in Defra’s Storm Overflows Taskforce, which has the long-term goal of eliminating harm from storm overflows. That work has resulted in commitments that water companies will:

  • make real-time data on sewage discharges available at bathing sites all year round.
  • accelerate work to install monitoring devices to create a complete picture of their activity by 2023
  • publish annual monitoring data on their websites about their use of storm overflows, so progress in reducing their use can be tracked. The Environment Agency will collate and publish a report on this.

In addition, work is underway to explore different policy options and costs to tackle the harm caused by storm overflow spills.

At the end of 2021, a new piece of legislation was introduced, called the Environment Act, which puts more duties and responsibilities on companies, regulators and the government to go further on reducing the use of storm overflows.

For companies to gain trust and meet the expectations of their customers around storm overflows, there will need to be investment, a commitment to innovate, and a transparent approach.

There has been significant investment going into the system and more to come with £5bn to care for the environment between 2020-25, including £1.2bn to tackle storm overflows.

Alongside that investment, innovation and other actions are needed. We are trying to ensure that the full range of causes are recognised, and that in turn the right mix of solutions are identified and implemented. These will need to encompass short, medium and longer-term measures including on reducing the amount of water going into the sewerage system, and improving how the sewerage system operates and is maintained.