CIRIA report questions focus on sewer capacity

“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: richard.lillywhite@ciria.org.uk

*European CEN Technical Committee 165/WG4/AHG5 is preparing drafts for

technology without standards to be implemented late in 1999.