SuDS in London - a guide

3.6 Wet swales
and dry swales
Swales are linear components that provide
slow water conveyance. They provide
filtration, attenuation and storage of
surface water runoff from relatively small
catchment areas. They can be designed to
accommodate a range of rainfall events.
Generally, swales are sloping sided, flatbottomed,
vegetated open channels,
constructed at a gentle gradient. Steeper
gradients can be accommodated through
the use of check dams. Swale design is
limited by available space and is only
effective when close to catchment areas.
Swales can be dry or wet.
Dry swales allow surface water to infiltrate
and include a filter bed with an underdrain
to prevent waterlogging. They can be lined
or unlined depending on groundwater levels.
Wet swales retain water, behaving like
a linear wetland. They are best located
where sites are level and soils are poorly
drained, where they can deliver amenity
and biodiversity through specific wetland
planting. During intense storm events,
water is retained in the swale before being
conveyed to a downstream outlet.
Benefits
Conveyance: swales are a simple and
effective means of collecting and
distributing runoff, or as a means of
conveying runoff on the surface,
while enhancing open space or the
roadside environment.
Filtration: engineered soils can
help neutralise contaminants and
sedimentation caused by runoff. Designs
can include submerged anaerobic zones to
promote nutrient renewal.
Attenuation: swales are typically designed
to capture a one in 10 year storm event
by storing water within and on top of
the filtration media where the water can
disperse over time.
Amenity: swales provide shallow linear
planted features in the landscape that are
space-efficient and adaptable to location.
They integrate well alongside highways,
cycleways or pathways. They allow
bridging structures to enhance spatial
experience, creating places for play and
contact with nature.
Biodiversity: swales can be designed
with a variety of marginal planting and
wildlife meadow that contribute to habitat
creation and connectivity.
Erosion: swales convey and/or retain
flowing surface water where soft
landscape is likely to erode. Reducing
the velocity of water flow limits erosion
through the use of measures such
as weirs, check dams, erosion control
matting and planting.
Design considerations
Swales should be designed to suit the scale
and character of the specific location, taking
into consideration orientation, aspect and
proximity to other landscape or townscape
features. The design of soft or hard edges
depends on the urban design context.
Dry swale: Upton, Northants
43 3 SuDS components