CIRIA report questions focus on sewer capacity

Sewerage undertakers should be more willing to consider a variety of options for the prevention of sewer flooding, according to a report by CIRIA.

"The most common solution for dealing with sewer flooding is construction works to increase capacity. However, this may not be appropriate if the cost of the works is too high," says Richard Lillywhite, research manager at the Construction Industry Research Information Association (CIRIA).

Despite massive spending by sewerage undertakers to increase sewer capacity, Mr Lillywhite claims: "The decline in incidents due to overloading is not really significant, and sewer flooding is still a major problem." And although the number of properties on Ofwat's Œat risk' DG5 register of frequent flooding has decreased from 19,500 in 1992 to 13,018 in 1997, the report points out that: ŒThe figures refer only to those considered to be in danger of flooding because of lack of flow capacity in the sewers.

Therefore, a property experiencing repeated flooding due to blockages may not be included, even though the floods may be the result of a problem in the sewerage system.

Over 50 per cent of sewer flooding incidents in England and Wales are still caused by non-capacity related factors, such as random blockages, burst rising mains, pumping station failures, high tides and sewer collapses.

In rural areas, less than 10 per cent of sewer flooding is caused by sewer overloading.

Taking this into account, CIRIA lists several alternatives to increased capacity, including better use of existing storage, anti-flooding devices (AFDs), vacuum and pumping systems, improved maintenance, local structural improvements and flow redirection.

The report acknowledges that many options require a great deal of site specific input, unlike an overall increase in system capacity. But capacity improvements alone will not help a property that is in a Πlow spot, or with a sub-standard private sewer connection.

Many sub-standard, privately built sewer connections have become the responsibility of the sewerage undertakers (Section 24 sewers). According to CIRIA: "Many flooding problems are still associated with section 24 sewers." In London, these were often built with intercepting traps under local legislation, increasing the likelihood of blockage. Basement conversions are also particularly prone to flooding, especially where the public sewer is higher than the actual property.

Using a range of cost-benefit analyses, from single properties up to sub-catchment level, the report shows that pro-active maintenance, AFDs and pumping systems can be the best options for single houses and basement conversions, with public sewer capacity increases more appropriate for whole street projects.

In other words, where there are recurrent problems and where sewer capacity is no longer the crucial factor, sewer network managers should now be considering a wider range of solutions. Report no. C506 is available from CIRIA at £35 for members and £80 for non-members.

Contact Richard Lillywhite on Tel: 0171 222 8891, Fax: 0171 222 1708 or email:

*European CEN Technical Committee 165/WG4/AHG5 is preparing drafts for technology without standards to be implemented late in 1999.



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