Page 132 - Enfield SuDS DESIGN & EVALUATION

Detailed Design Bioretention Raingardens
A bioretention structure differs from a
raingarden in that it employs an engineered
top soil and is used to manage polluted
urban runoff in street locations and carparks.
These features can contribute significantly to
the urban scene so should be designed to
meet urban design standards.
The runoff entering bioretention features will
normally carry silt and pollution from vehicles
and urban street use. Therefore, some
maintenance should be expected to remove
the build-up of inorganic silt.
The free-draining nature of engineered soils
leads to the washing away of nutrients from
the soil. The proportion of organic matter
should be relatively high and replenished
yearly by the application of a mulch layer of
well composted greenwaste or shredded
plant matter arising from maintenance.
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Enfield Council SuDS D & E Guide
Key design aspects for bioretention
raingardens include;
1. silt collection in forebays
2. space above the soil profile for water
collection and stilling before infiltration
through the engineered soil
gravel protects the infiltration capacity of
3. a surface mulch of organic matter, grit or
the soil
4. a free draining soil, 450 -600mm deep,
with 20-30% organic matter cleans, stores
and conveys runoff to a drainage layer
5. a transition layer of grit and/or sand
protects the under-drained drainage layer
that discharges to an outfall
6. a surface overflow for heavy rain or in the
event of blockage.
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