SuDS in London - a guide

5.18 A23 Coulsdon Bypass, Farthing Way
Location
Coulsdon
London Borough of Croydon
Extent
34ha
Cost
£33m (Total scheme)
Date
2006
Credits
TfL
Atkins
SuDS components
Kerb drainage
Soakaways
Filter strip
Filter drains
Filter strip gravel
Summary
Highway runoff attenuation.
Project description
A groundwater extraction borehole at
Smitham Pumping Station is located in
Coulsdon Town Centre. The new section
of A23 between Marlpit Lane and Smitham
Station passes across the inner Source
Protection Zone (SPZ) for the extraction
borehole and has been designed to direct
runoff appropriately.
Objectives
• The drainage design redirects
runoff flowing from the new
A23 away from the inner SPZ
• Attenuation was needed to ensure the
area receiving the runoff can cope with
the volume of water it now receives
Actions and results
• The new A23 is drained, via a piped
system with kerbs and gullies, into
spillage containment devices and
a full retention fuel/oil separator,
before discharging into soakaways
• To the west of the new A23, the
existing ground rises steeply; being
chalk downland, there is likely to be
significant runoff when it rains heavily.
A separate system, not linked to the
highway drainage, collects this runoff
and discharges it into soakaways
• Non-piped drainage components
within the site principally relate to
linear soakaways at the bottom of the
embankment adjacent to footways
where water is caught at a low point
Benefits
• The design maintains the
flow of previously-existing
drains and watercourses
• The design of the drainage components
allows them to be maintained in
a safe and efficient manner
• Surface water is able to drain into
soakaways on adjacent land
Lessons learned
• Localised design changes were
necessary, due to the unexpected
presence of services
111 5 Case studies