Page 133 - Updated SuDS Design & Evaluation Guide-Newham V3 Spreads

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Detailed Design Bioretention Raingardens Key design aspects for bioretention Permeable surfaces ■ ■ edge Detailed Design

Permeable surfaces enable SuDS designers
A bioretention structure differs from a
to direct rainfall straight into a SuDS
raingardens include;
raingarden in that it employs an engineered
adjacent planting should include dense
structure for cleaning and storage or
top soil and is used to manage polluted
ground cover to bind the soil in place
1. silt collection in forebays
urban runoff in street locations and carparks.
infiltration into the ground.
slopes running toward permeable
These features can contribute significantly to
2. space above the soil profile for water
surfaces should have a depression and
There are a number of permeable surfaces
the urban scene so should be designed to
collection and stilling before infiltration
available. All should have in common:
ideally an underdrain before reaching
meet urban design standards.
through the engineered soil
the pervious surface.
The runoff entering bioretention features will
the pavement surface
pavements are covered by guidance in the
gravel protects the infiltration capacity of
normally carry silt and pollution from vehicles 3. a surface mulch of organic matter, grit or 1. a pervious surface to allow water through The design and construction of pervious
2. an open-graded sub-base layer that
and urban street use. Therefore, some the soil provides structural strength to the SuDS Manual (Section 20) and the Interpave
maintenance should be expected to remove pavement with about 30% by volume website www.paving.org.uk
the build-up of inorganic silt. 4. a free draining soil, 450 -600mm deep,
with 20-30% organic matter cleans, stores available for water storage. There are no reported issues with surface
The free-draining nature of engineered soils and conveys runoff to a drainage layer 3. Silt washed off adjacent landscape areas clogging under normal use. A dedicated
leads to the washing away of nutrients from can lead to localised surface clogging. maintenance may be required after between
the soil. The proportion of organic matter 5. a transition layer of grit and/or sand This risk can be managed through design 10 and 20 years of use comprising a brush
protects the under-drained drainage layer
should be relatively high and replenished detailing as follows: and suction removal of grit joints and joint
yearly by the application of a mulch layer of that discharges to an outfall ■ slope adjacent landscape areas away replacement.
well composted greenwaste or shredded 6. a surface overflow for heavy rain or in the ■ use paved or turfed surfaces to
plant matter arising from maintenance. event of blockage. adjacent areas
127 ■ soil in adjacent planting beds should Soft landscape areas are set below kerb level at 128

be min. 50mm below the pavement this permeable paving installation.
Almac Car Park, Limerick, Ireland.

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