SuDS in London - a guide

Bioretention rain garden in Vauxhall
Design considerations
Edge protection: typically, bioretention
components are sited below pavement
surface levels and can hold standing water.
It is therefore important that the interface
with pedestrian and vehicular movement
is carefully considered. Bioretention can
be profiled in various ways, with soft
edges and gentle side slopes, or hard
edges and vertical sides.
Inlets: inlets may be necessary, especially
when hard edge protection is required.
Erosion at inlet points can be prevented by
reducing the surface water flow velocity
via a sediment trap or a reinforced and
textured zone. Protection grilles should not
be used unless the inlet diameter is greater
than 350mm. An outfall provides overflow
when heavy rainfall means infiltration into
the soil is too slow.
Erosion: bioretention systems aim to catch
flowing surface water. Soft landscapes
may suffer erosion, so design the feature
to control the surface water runoff
movement through the use of weirs,
check dams, erosion control matting
and planting.
Pollution/contamination: pollution and
contamination sources affecting surface
and ground water may affect planting,
so the planting specification should be
designed to meet the site conditions.
Bioretention systems can remediate water
contaminants with the use of filtration
mediums, normally sand-based material
with a source of organic matter to provide
nutrients for planting.
Sedimentation: slowing surface water
flow allows fine particles to be removed.
Design should limit excessive sediment
accumulation that could reduce storage
volume, filtration and infiltration rates.
Exceedance: bioretention systems can deal
with only small catchment areas and are
likely to be overwhelmed during heavy
storms. The design should therefore allow
for contingency flow paths and/or
provide outfall.
Outfalls: if an outfall is required, consider
the location, particularly the relative
level of potential discharge locations, as
bioretention system outfalls can be deep
compared to conventional drainage.
Maintenance
Bioretention systems require routine site
maintenance operations to ensure efficient
operation. Inlets and outfalls require
periodic inspection.
Useful design guidance
CIRIA C753 The SuDS Manual, Chapter 18
48 3 SuDS components