SuDS in London - a guide

2.4 London’s
chalk aquifer
Beneath London is a large chalk aquifer.
This was substantially depleted during the
19th and 20th centuries due to extraction
by industrial activities. This resulted in
the aquifer being depleted to 88m below
sea level. However, in the last 60 years,
as industrial activities moved away from
central London, the chalk aquifer has
started to rebound by as much as 3m
per year.
Some geology in London is susceptible
to shrink-swell movement, caused by the
presence or absence of water. This can
have a substantial effect on underground
structures and foundations.
Since 1992, the General Aquifer Research
Development and Investigation Team
(GARDIT) has licensed the removal of
groundwater from London’s chalk aquifer.
The aim is to control and eventually
stabilise the rise in groundwater levels.
The SuDS designer should take account of
the chalk aquifer because:
• In areas with high groundwater levels,
water can enter the SuDS component
and reduce its storage capacity
• There is a risk of flotation and increased
loads imposed by groundwater
• High levels of groundwater can reduce
the infiltration rate of SuDS features
• Groundwater can change the stability of
underground structures and foundations
Further information:
CIRIA C753 The SuDS Manual, Chapter
26 Management of the London Basin Chalk
Aquifer: Status Report 2015,
Environment Agency.
Exposed chalk
2.5 London’s soils
London’s soils are derived from the
underlying geology. Across London there
are variously clays, sands and gravels
which are often found in a river
environment. Soil management is
fundamental to the successful
functioning of SuDS components.
SuDS should be designed according
to the geology and soils of the area.
Designs should consider the availability
and properties of existing soils, the
surrounding ground and the requirements
for imported soils. Soils should not be
imported unless this is unavoidable.
Soil properties typically influence:
• Water quantity: the physical properties
of soil affect the attenuation
capacity as they dictate its drainage
and water-holding properties
• Water quality: the filtration capacity
of soils influence water quality by,
for example, affecting the amount
of elements such as nutrients or
contaminants, taken up by the
soil or dissolved into the water
20 2 The London context