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Water and sewerage companies have taken action to remove the risk of sewer flooding from 3,300 properties in England and Wales since 2000. Ofwat has played an important role in achieving this reduction, according to a report published today by Sir John Bourn, head of the National Audit Office. However, Ofwat and companies face a number of challenges in reducing the risk of sewer flooding further, and in planning for the long term future of sewer networks.

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Every year since 1994, there have been between 5,000 and 7,000 incidents of sewer flooding in properties. These can occur when sewers become overloaded due to heavy rainfall, sometimes exacerbated by extra demands placed on some sewers by new housing developments, or by sewer blockages and to a lesser extent sewer collapses. Companies are only required by law to pay an average of 125 in compensation to affected customers for each incident, and these payments do not vary with the damage suffered, although obtaining insurance cover against all types of flooding incidents is the householder’s responsibility.

Work by companies to alleviate known sewer flooding problems has accelerated since 2000. The number of properties reported to Ofwat as being at risk of sewer flooding has reduced to just over 11,000 during this period, and Ofwat has made provision for such work when setting price limits. Such work is expensive – the companies forecast that it would cost a further 1.1 billion to deal with the most severe known internal flooding problems, and a further 0.5 billion to tackle severe external flooding. Ofwat should encourage companies to have a robust economic approach to assessing the balance between cost and benefits to help them to prioritise between known problems.

Although Ofwat has concluded that there is no evidence of deterioration in sewer networks since the early 1990s, there are risks that any problems could go undetected for some time before they affect performance to customers. Developments such as climate change, new housing development and European Union legislation will place new demands on sewer networks and could increase the risk of sewer flooding incidents in future. Water companies, the industry’s regulators and the government should work together to clarify responsibilities, and to produce more robust assessments of the long-term requirements of sewer networks and how these will be met.

"While incidents of sewer flooding in properties are relatively rare, few could deny that it is highly distressing. This must be especially so for those who suffer repeat incidents. And the harm to the environment should not be dismissed. That is why I have made these recommendations to encourage the work that is already being done to reduce the risks of sewer flooding."

Sir John Bourn, head of the National Audit Office

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