Storm overflows

How we plan to tackle the issue with storm overflows

Storm overflows are an important part of the sewerage network and include combined sewer overflows (CSOs) and storm tank discharges. They act as a pressure relief valve when there is too much rainfall, allowing rain water, mixed with sewage, to rise inside the sewer and eventually enter a separate pipe which flows into a river or the sea. Sewers operate this way to help prevent the flooding of streets, homes and businesses. When we do need to use them, they can sometimes affect river and bathing water quality, albeit temporarily.

Many people have told us they do not like the idea of untreated sewage going into our rivers and seas, no matter how diluted. We agree. However, it will take many years to change how excess rainfall is managed, especially in our region.

Why do we need storm overflows in the North West?


  • Analysis of Met Office data shows that average annual water runoff in the North West is 28% higher than the average for England and Wales which means more water runs into our sewers.
  • We have a significantly higher proportion of combined sewers than any other water company. Over 54% of our public sewers combine foul and surface water compared to an average of 33%. Combined sewers respond quicker to a storm with the capacity filling up more quickly when compared to more separate systems.
  • We have 25% more sewer overflows than the industry average.

Our investment


  • To 2020, we invested £1.2bn to improve overflow discharges to reduce spill frequency, volume and impact upon the natural environment. This has improved the operation of over 1,200 intermittent overflows.
  • Between 2020 and 2025, we will invest a further £230m. This includes enhancements to 29 storm overflows that will improve 184km of river and result in 10,000 fewer hours of spilling.
  • We already planned to conduct 195 investigations by 2025 to better understand high spilling overflows to inform spill reduction schemes for future investment. Through the sector’s Green Recovery programme, we will investigate a further 300 overflows.
  • We have invested over £1m in a new data system to help us analyse over 500 million rows of data. We formed a new multi-disciplinary team including 12 new recruits to examine the data to better target our efforts to reduce spill frequency.

Improving river health - supporting others to improve and care for rivers

The Environment Agency estimates that storm overflows lead to around 5 per cent of river and sea pollution in the North West, with water quality in the natural environment affected by rain running off highways and farm land and private drainage being incorrectly connected. We are committed to improving the water environment and working with partners like the Rivers Trust and the RSPB to tackle these issues.

We want to work with river and beach users, regulators and politicians to plan how we can reduce the need for these overflows. We will continue to work with stakeholders across our water catchment areas to set clear environmental and social objectives for improving water quality.

Our approach is very much aligned with the action plan set out in Water UK's 21st Century Rivers Report. This document calls for a new deal for rivers and for everyone — from river users and customer groups, to environmental NGOs – to work together on a new approach to restore the health and resilience of rivers as sources of habitat, life and joy.

The public sewer system has too long been the catch-all last defence for managing surface water in our communities. Better managing the flow of water and reducing its interaction with the piped sewer network is key to helping improve river quality.

What you can do to help