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

4 Managing rainwater
4.1 This chapter is concerned with the
drainage of rainwater away from homes
and businesses in London. Following
this, Chapter 5 examines the removal of
wastewater. In large parts of inner London,
a combined drainage network (called the
combined sewer) takes both rainwater
and wastewater away from buildings. The
problems associated with the combined
system are considered in Chapter 5.
4.2 Rainwater is either lost through evaporation,
seeps into the ground to replenish
groundwater levels, flows over the ground
and returns to streams and rivers, or enters
the drainage systems and then flows on to
a sewage treatment works. In the parts of
London not covered by the combined sewer,
the surface water drains carry rainwater from
pavements, road surfaces and rooftops into
local rivers and streams (see Figure 2.9).
4.3 Over the past 40 years, the intensity of
rainfall has increased during the winter
months across the country. The occurrence
of more than 15 millimetres of rain on a
day (referred to as heavy rainfall days) has
become more frequent in winter. Since
1989, heavy rainfall days have consistently
contributed more than ten per cent of the
seasonal total rainfall; this was not always the
case prior to 1989. Heavy rainfall can quickly
overload the drainage system, as well as carry
debris and pollutants from paved areas, such
as roads and car parks, and from gardens into
otherwise clean rivers, stream and ponds.
4.4 In 2007 a submission to the Shadow Cabinet
entitled A Blueprint for a Green Economy 60
notes that we need ‘Slow Water’. Slowing
water down through the use of green roofs
and sustainable urban drainage systems
filters and removes pollutants. The Draft
Flood and Water Management Bill now
proposes significant changes to the way in
which we manage surface water.
Policy 2 – Drainage in London
The Mayor proposes the following
hierarchy for the drainage of rainwater:
1 Store rainwater for use later
2 Use porous surfaces to let rainwater
to soak into the ground where soil
conditions allow
3 Slow the runoff by directing rainwater
into ponds or open water features for
gradual release to a watercourse
4 Slow the runoff by directing rainwater
into tanks or sealed water features for
gradual release to a watercourse
5 Discharge rainwater direct to a
watercourse
6 Discharge rainwater to a surface
water drain
7 Discharge rainwater to the combined
sewer, as a last resort.
4.5 The flood risk principles (from LIFE
Handbook 61 ) provide a useful guide to
understanding where application of Policy 2
is important, so as not to increase the risk of
fluvial flooding from surface water runoff:
• upper catchment – ‘let rain slow’
• middle catchment – ‘let rivers flow’
• lower catchment – ‘let tides go’