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CP454  Alternative Approaches to Surface Water Separation Sewerage
Project Appreciation and Objectives

Many sewer systems suffer from lack of capacity during wet weather causing sewer flooding and premature operation of combined sewer overflows (CSOs), leading to serious damage/inconvenience to customers and/or prosecution by the environmental regulator. The traditional capital solution has been to provide additional capacity. However, separation of surface water is now seen as more sustainable. In combined and foul systems this has centred on removal at source.

A recent CIRIA project focussed on separation at property level with an emphasis on diversion by retrofitting new SUDS schemes. However, this can be expensive and disruptive to customers. This project will focus on options for separate within the sewerage network.

In combined sewer systems there are often zones with separate surface water systems that eventually discharge into the combined sewer that could be diverted with less disruption to customers. Some of these date from the catchement's original development whilst others are the result of more recent infitll/regeneration schemes.

In foul systems, surface drainage misconnections and leakage of surface water through manhole covers also contribute to the problem and offer an ill-dfefined, but potential opportunity for significant separation of surface water. The objectives of this project are to develop techniques that could be applied and to establish whether their use could cost-effectively reduce flooding and CSO spills, while minimising disruption to customers.

Benefits to Clients

  • Reduced hydraulic overloading of many sewer systems with consequent reduction in numbers of flooding incidents and CSO spills.
  • Released sewer capacity, thereby removing the need for upgrading of sewer systems.
  • Better evidence on surface water separation, for use in developing approaches in the next periodic review.

Work Programme

  1. Define the nature and extent/distribution/contribution of surface water systems that could be diverted and the options available to achieve this.
  2. Field visits to assess the additional sources of surface water in foul systems including monitoring of the inflow through manhole covers/frames.
  3. Quantify the contribution made by misconnections and manholes. Determine the options for reducing this.
  4. Develop a methodology for companies to use to assess individual drainage areas and to identify the most cost-effective mitigation plan.

Project Output

  • A methodoogy for assessment and planning of surface water separation on an area-by-area basis.
  • Back-up documentation with the results of the investigations.
  • Guidance on the use of the methodology with example applications.

Related Work

  • Cost Benefit of Understanding SUDs Retrofit, 2007.
  • Predisposition of Properties to Flooding, 2008.
  • UKWIR Sewer Misconnections, 2011.
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