SuDS in London - a guide

These components are designed to
promote infiltration where capacity and
permeability of soils and the depth of
groundwater allows. However, infiltration
systems may not be appropriate in many
parts of London due to groundwater
extraction issues (see Chapter 2).
Benefits
Infiltration components allow groundwater
to be replenished. They can incorporate
marginal and wetland habitat. Planting
slows the flow rate by improving the
drainage properties of the soil, creating
a more effective SuDS component.
Infiltration can be used to manage overflows
from rainwater collection systems, such as
water butts and runoff from small areas
(for example, drives and roofs).
Design considerations
Infiltration components can be retrofitted,
designed as a series of small linked
elements, or as a single larger one.
Runoff flow to be directed to a SuDS
infiltration component can be collected
laterally along the edge of an impermeable
surface. Kerb openings and roadside lateral
inlets help to direct, control and reduce
flow velocities.
A minimum of 1m from the base of the
infiltration component to maximum
groundwater level is required. Upstream
pre-treatment may be needed to remove
sediment and silt.
Performance of SuDS components may
be compromised if surface soils become
compacted, so should be designed to
withstand high intensity pedestrian use.
Performance depends on the capacity
of the soils surrounding the component.
When rainfall rate exceeds the design
capacity, a flow route or temporary storage
should be provided.
Soil infiltration can be enhanced by:
• Managing construction traffic to prevent
compaction during construction
• Mixing sand with soil to retain
its drainage properties
• Adhering to tight construction tolerances
• Soil decompaction
• Reusing existing topsoil to allow the
inherent seed bank in the soil to
regenerate quickly, reducing erosion and
enhancing the potential for infiltration
Maintenance
This can usually form part of the wider
routine landscape maintenance.
Control structures require periodic
inspection. Existing parkland, particularly
in critical drainage zones that are
subject to intense use, should be
periodically decompacted.
Useful design guidance
CIRIA C753 The SuDS Manual, Chapter 13
38 3 SuDS components