Sustainable drainage

Current situation

In urban areas we have two main types of drainage systems. In some places, sewage and surface water are collected separately. In other places the sewage from buildings and the surface water from roofs and hard surfaces are collected and dealt with together as part of the same drainage system. These are known as combined sewers. The diagram below shows a typical combined sewer system where overflows discharge into rivers.


When there is heavy rain, combined sewers cannot always hold all of the storm water, and sewer flooding occurs. In these cases engineering solutions are often used to try to stop sewer flooding from happening again in future.

These types of solution are considered low risk and from an engineering perspective are relatively easy to deliver. But, they tackle the symptoms and not the causes of flooding.

In the future, particularly as climate change increases the intensity of rainstorms, the capacity of many parts of our current drainage systems will be insufficient to meet needs. Building bigger sewers to cope with higher flows is unlikely to be a sustainable solution. It would be very expensive and in extreme weather conditions the sewers and downstream rivers would still be overwhelmed.

Managing demand for drainage by using more natural drainage processes or encouraging more efficient use of drains and sewers are potential alternative approaches, but they also come with different risks.

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Further information

Our publications on sustainable drainage are listed below.

Ofwat publishes a framework with the Environment Agency on preparing a drainage strategy
Good practice guidance commissioned by the Environment Agency and Ofwat. Prepared by Halcrow
22/May/2013 (HTML – 3 KBytes)

Drainage strategy framework for water and sewerage companies to prepare drainage strategies
Good practice guidance commissioned by the Environment Agency and Ofwat
10/May/2013 (PDF – 2 MBytes)

Future Impacts on Sewer Systems in England and Wales
A report prepared for Ofwat by Mott MacDonald
29/Jun/2011 (PDF – 1 MBytes)

Sewer flooding workshop 23 March 2011
31/Mar/2011 (HTML – 2 KBytes)