Combined Sewer Overflows (CSO) and Natural Flood Management (NFM)

The Rivers Trust Map

What are CSOs and why a problem?

Combined Sewer Overflows (CSO) occur when our ‘combined sewer’ system mixes surface water drainage and foul water drainage. When the drainage system is at capacity, the combined (including foul) sewage can be released into our watercourses. CSOs are an issue that affects our valley regularly, which is a matter of great concern – the Rivers Trust have produced a shocking map showing the regularity of such occurrences:

 When the valley floods, CSOs result in polluted flood water entering homes and businesses. 

How can NFM and SuDS help?

Slow The Flow has recently responded to the Defra “Consultation on the Government’s Storm Overflow Discharge Reduction Plan”, on how Natural Flood Management (NFM) and Sustainable Drainage Systems (SuDS) can help to combat the issue. 

Our response was as follows:

Re. Schedule 3: Wales’ experience in implementing Schedule 3 ( shows that it can be done effectively. There are only positives to be gained from its implementation, and this should be done immediately.  

Re. Giving water companies the right to alter drainage systems on private property: This is a good idea, water companies should be encouraged to help others implement more sustainable drainage practices. 

Additional comments:   

   1.          The automatic right to connect to the surface water sewer should be removed.  

  2.          There should be a dramatically increased / publicised financial benefit to disconnection from the surface sewer.  

  3.          Water companies should be incentivised to invest in both rural NFM and urban SuDS schemes, increasing the prevalence of both at a catchment scale.  

  4.          Re. Q’s 6-9, We have chosen ‘Agree’ rather than ‘Strongly agree’ because the timescales should be more ambitious, 2050 is too long to wait.

  5.          Attenuation/detention ponds, and ‘Green’ NFM interventions generally (, should be encouraged in preference to tanks. ‘Green Infrastructure’ interventions carry significant benefits over water storage tanks, from an economic and environmental point of view, including improvements to water quality.  Ponds are also far quicker to construct than buried tanks.

  6.          Grey water systems should be put to much better use, thus reducing dependence on clean water for flushing toilets, washing machines and watering gardens etc. Widespread use of grey water would reduce the load on surface water sewers, especially during storm events, reduce household water bills and allow for more water to go back to the environment as nature intended, instead of having to remove more and more water for purification.

Slow The Flow also discusses CSOs in some of our position statements, at