What Are Storm Overflows?
Storm overflows are a feature of the UKβs combined sewer system β a network that carries both household wastewater and rainwater in the same pipes. During periods of heavy or prolonged rainfall, the volume of water entering the system can exceed its capacity. When this happens, storm overflows act as safety valves, releasing a mixture of heavily diluted rainwater and wastewater into rivers or the sea to prevent sewage from backing up into homes, streets, and businesses.
The combined sewer system was designed and built over a century ago, when rainfall patterns, population levels, and urban development were very different from today. While these overflows serve a vital function in protecting public health and property, we recognise that their impact on the environment is a growing concern for our customers and communities. At Northumbrian Water, we are committed to being transparent about how our overflows operate and to reducing their frequency and impact through sustained investment and innovation.
Why Do Storm Overflows Exist?
To understand why storm overflows are necessary, it helps to know how the sewer system works. In much of the North East, a single pipe system carries everything β water from sinks, baths, toilets, and washing machines, as well as rainwater from roofs, driveways, and roads. On a dry day, this system has ample capacity. But during a heavy downpour, the volume of rainwater entering the system can increase dramatically, sometimes by a factor of fifty or more.
Without storm overflows, this excess water would have nowhere to go. Sewage could back up into homes, flood streets, and overwhelm treatment works, leading to far worse environmental and public health outcomes. Storm overflows are designed to activate only during these exceptional conditions, releasing water that is primarily rainwater with a small proportion of diluted sewage.
Our Monitoring and Transparency Commitment
Northumbrian Water has installed event duration monitors on all of our storm overflows. These devices record when an overflow activates and how long it operates. The data is published on our website and shared with the Environment Agency, ensuring full transparency and accountability.
We were one of the first water companies in England to achieve 100 per cent monitoring coverage, and we are proud to lead the industry in this area. Our customers can access overflow data through our interactive map, which shows the location, status, and recent activity of every storm overflow in our region. We believe this openness is essential to building trust and driving improvement.
What We Are Doing to Reduce Overflows
While storm overflows will remain a necessary part of the sewer system for the foreseeable future, we are taking significant steps to reduce their frequency, duration, and environmental impact. Our approach includes:
- Increasing sewer capacity: We are investing in larger pipes, additional storage tanks, and improved pumping stations in areas where overflows operate most frequently. These upgrades enable our network to hold more water during storms, reducing the need for overflows to activate.
- Sustainable drainage systems (SuDS): We are working with local authorities, developers, and landowners to install sustainable drainage solutions that manage rainwater at source. Rain gardens, permeable pavements, green roofs, and swales help to slow the flow of rainwater into the sewer system, reducing the risk of overflows.
- Separating surface water: Where possible, we are separating rainwater from the foul sewer system, directing it to dedicated surface water drains or natural watercourses. This reduces the volume of water entering the combined system and significantly lowers the risk of overflow activation.
- Advanced treatment: At key overflow locations, we are installing screening and treatment technologies to reduce the impact of any discharge on the receiving watercourse. This includes fine screens that capture solids and UV disinfection that reduces bacteria levels.
- Catchment management: We work with farmers, landowners, and environmental organisations to reduce runoff from agricultural land and improve the health of river catchments. Healthy catchments are more resilient to rainfall and less susceptible to the impacts of storm overflows.
The Scale of the Challenge
Addressing storm overflows is one of the biggest environmental challenges facing the water industry. The cost of completely eliminating overflows across England has been estimated at over Β£350 billion β a figure that would have significant implications for customer bills. At Northumbrian Water, we are focused on delivering the greatest environmental benefit for every pound invested, prioritising those overflows that have the most impact on sensitive habitats, bathing waters, and shellfish waters.
We are also engaging with regulators, government, and other water companies to develop a long-term national strategy for reducing overflows. This includes advocating for changes to planning policy that would require all new developments to incorporate sustainable drainage, and for greater investment in separating surface water from the combined sewer network.
What You Can Do
Customers can play an important role in reducing the pressure on the sewer system. Simple actions such as not pouring fats, oils, and grease down the drain, avoiding flushing wet wipes and sanitary products, and disconnecting downpipes from the combined sewer where possible can all help to reduce the risk of blockages and overflows. Every small action contributes to a healthier sewer system and a cleaner environment.
We welcome feedback and questions from our customers about storm overflows. Transparency is at the heart of our approach, and we are always happy to explain what we are doing and why. Visit our storm overflow hub on the Northumbrian Water website for the latest data, news, and updates.