The Mayor's draft water strategy - london.gov.uk - Greater London ...

stations and hydraulic solutions to alleviate
sewer flooding. In both 2006/07 and
2007/08 Thames Water removed over 500
properties from the risk of flooding once
or twice in 10 years, but 12,477 properties
remain at risk of flooding once in 20 years.
However, these were not all in London.
5.15 In the longer term, there is a need for a
better understanding of the sewer capacity,
and more effective controls on increased foul
water inputs to the sewer system. Defra’s
proposals in the Draft Flood and Water
Management Bill, discussed in paragraph
4.25, should help. Bad practices from both
domestic and commercial users, such as
sewer misconnections discussed below, can
also contribute to the overloading of sewers
leading to flooding.
5.16 There are circumstances where solving
the problem of sewer flooding can be
extremely expensive. Some modern practices
(for example, converting basements into
dwellings) can increase the incidence of
sewer flooding.
Misconnection of the foul sewer and
surface drains
5.17 In many cases the pollution in London’s rivers
comes from a much less obvious source than
factories, farms or industries. If a householder,
or professional plumber, inadvertently but
illegally connects household appliances
or waste pipes to the surface water drain
instead of the foul sewer, then foul water
can find its way into London’s streams, rivers
and canals without any prior treatment. The
misconnection of several houses or businesses
in the same area can cause damage to the
local watercourse. This is important for the
Mayor’s work to promote river restoration – it
is unsatisfactory to seek major funding to
restore the river’s structure and character if
the water quality continues to be severely
compromised.
5.18 Thames Water estimates that one in every
20 houses in London has a misconnection. In
some areas, this figure is considerably higher.
For instance in the Pymmes Brook catchment
in Barnet, it is more likely that one in every
ten houses has a misconnection.
5.19 During periods of wet weather, flow rates
are generally high enough to wash away
any signs of pollution. Yet when river flows
are low, the sewage matter is more visible.
Sewage in the river causes oxygen levels to
drop. In the more severe cases, the river can
no longer support the aquatic wildlife during
the pollution incident.
5.20 If a misconnection is the likely cause of
the pollution, then Thames Water and the
Environment Agency will try to find the
offending house(s). If successful, they will
notify the householder(s). At the same
time they will pass on the details to the
environmental health department of the
respective borough. An environmental health
officer will then check that the householder
has rectified the problem. Similarly, a
misconnection of the surface water drain to
the foul sewer can cause the foul sewer to
flood (see paragraph 5.12).
75