Storm overflows 

Choppy waters

What is a storm overflow?

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.

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 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 which is 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. If an overflow requires improvement the Environment Agency will let us know that the permit will need to change and we then make an investment proposal to our regulator, Ofwat, that they will review along with the cost proposals before including within our five-year business plan. 

What are we doing to reduce discharges from our storm overflows?  

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. That’s why we’ve announced that we’re investing £180m before 2025 to improve the performance of our storm overflows.  

Find out more about our investment in storm overflows

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  

  1. Our interactive map that shows in near real-time whether our storm overflows are operating  
  2. 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.