We know our storm overflows operate more often than we or our customers would like, and reducing discharges is a priority for us. However, replumbing the whole of Yorkshire is not a quick thing to do as well as being significantly disruptive and costly to customers. Alongside our plans to improve Yorkshire’s rivers, we want to move faster than the government targets.
What is a storm overflow?
We have three types of sewers in our network: foul sewers, surface water sewers and combined sewers.
- Foul sewers carry waste from things like toilets, sinks, showers and washing machines
- Surface water sewers contain rainwater which runs off impermeable areas such as paving, roofs and highways
- Combined sewers have a mixture of foul and surface water. In Yorkshire, over half our sewer network is combined.
Foul and combined sewers transport wastewater to the nearest wastewater treatment works where it is cleaned and safely returned to the environment. Surface water sewers usually drain into a local watercourse.
We all know rainfall in Yorkshire can be unpredictable, so when our network was designed, storm overflows were installed to act as a relief valve when the network is full. Overflows reduce the pressure on combined sewers and stop the system from backing up and flooding homes and gardens by allowing heavily diluted wastewater to discharge into watercourses after the capacity of our storage has been used up. Most of our storm overflows have preliminary treatment such as screens or storm settlement before they operate.
How can I identify different types of outfall?
Outfalls are not always easy to identify. CSO outfalls often have larger more robust structures. Internally, they may include gates, weirs and other control mechanisms. These often have ‘YWS’ on the flap valve if there is one present. However, many CSO outfalls are located far from the CSO itself. Some of these outfalls link with a surface water line.
Surface water outfalls are generally simpler structures, often just a pipe or culvert opening that allows stormwater to flow out.
How many overflows are there in Yorkshire?
We have 2,190 overflows on our network which makes us the water company in England with the second highest number of them. This is due to the industrial heritage of parts of our region, and the way the network was created at the time. We have the second highest percentage of combined sewer network compared to other water companies. However, having more overflows doesn’t necessarily mean that we’re discharging more wastewater into rivers as some overflows cover a much bigger network than others.
We inherited these systems when we adopted them from local councils in 1989 but that doesn’t mean we’ve not been investing in our network or optimising its performance.
We’ve made some big changes in our wastewater networks since then and over the past few years we’ve been investing almost £800m in reducing phosphorus levels in the treated effluent we release back into the environment.
How are these overflows regulated?
Overflows have a permit provided by the Environment Agency. This sets out the conditions for when an overflow can operate and how it should be configured. We keep a close eye on all our overflows and report activity that goes outside of the permit conditions to the Environment Agency, who then investigate.
How are we reducing discharges from our storm overflows?
We have committed to a 52% reduction in storm overflow operation by 2030.
We've already made significant strides in this area. Between 2020 and 2025, we allocated an additional £182 million to our existing £147 million investment fund to accelerate our commitment to improving river health and achieving a 20% improvement . We have also committed to achieving one discharge per bathing water season at inland bathing waters 5 years ahead of our regulatory date of 2035.
But we're not stopping there! We're aiming to push this improvement even further with our largest investment program over the next five years.
We've proposed an ambitious program of works, investing approximately £1.45 billion to enhance over 480 storm overflows, significantly reducing their impact on watercourses. This will include, but is not limited to, the following solutions:
- Disconnecting surface water from our sewer network and gradually discharging it to the environment
- Using active system control of the network and storm tanks to maximize sewer network storage
- Constructing storage on the combined sewer system at the overflow and returning it to the sewer network
- Increasing the treatment capacity of our wastewater treatment works
How do we monitor storm overflows?
Over the past few years, we’ve been installing monitors on our overflows to help us gain a better understanding of our network and any impact the overflows might be having on the environment. Here in Yorkshire, all of our storm overflows are monitored.
There are two ways you can view data on our storm overflows
- Our interactive map that shows in near real-time whether our storm overflows are operating
- Our annual storm overflow data for 2022. Every year, we collate all this data and publish it on our website after sharing it with the Environment agency. We have a dedicated data page where you can look through the data and read the summary results.
More from us
Our storm overflow map
See whether any overflows are currently operating as well as the date, time and duration of the last time it operated.
Our work at Pudsey Beck
We're creating wetlands to filter and store storm water, helping to reduce storm overflow use and provide a home for wildlife.
Bin it. Don't block it.
You might be surprised at the things that shouldn’t go down the drain…
Doing right by Yorkshire
From fixing leaks to making new wetlands. From helping customers struggling with bills, to giving back by helping communities thrive… We want to do right by you, by doing right by Yorkshire.
