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STORMWATER WETLANDS REGIONAL
CONTROL
Stormwater Wetlands
DESCRIPTION
tormwater wetlands are similar to
retention ponds but with more
Semergent aquatic vegetation and
a smaller open water area (less than
25% of the water surface area).
Stormwater Wetlands are shallow pools
that create growing conditions suitable
for the growth of marsh plants. They
typically have less bio-diversity than
natural wetlands. Because wetlands
are heavily vegetated, they serve as a
natural filter for urban run-off.
Stormwater wetlands detain urban runoff,
remove pollutants through
biological treatment and settlement and
provide habitat and aesthetic benefits.
Wetlands can be integrated into
developments as a community water
feature.
A Stormwater Wetland in Scotland
Construction Cost
L and Take
PRIMARY CONSIDERATIONS
Maintenance Requirements
HIGH
MEDIUM
HIGH
BENEFITS
�Water
Quality Control YES
�Water
Quantity Control YES
�Amenity
Value YES
�Habitat
Creation Value YES
�Biological
Treatment YES
DESIGN
�The
average depth of the wetland
should be 0.5 to 0.75m. The depth of
General Design Criteria / Features open water should not exceed 2 metres.
�Design
to retain water for 14 days
Water deeper than 2m inhibits rooted
during the wettest months to allow for
plant growth, thus providing areas of
biological treatment and for settlement
open water. However, wetlands can be
of solids.
designed for flood control by providing
�Design
to have a minimum permanent
flood storage of up to 2m, above the
pool of 3 x Vt (Wallingford Procedure).
level of the permanent pool.
�The
simplest form of a constructed
�The
open water area should be less
wetland comprises of a basin with a
than 25% of the water surface area.
forebay and wetland vegetation area.
�Shallow
side slopes should be gradual
�An
adequate water flow is required to
(e.g. 1 in 4), as in natural wetlands and
ensure a permanent pool of water.
should not exceed 3:1 (Horner et al,
�Wetlands
should have a length-to-
1994) to reduce safety hazards and
width ratio of at least 5:1, which helps
enable maintenance.
prevent short-circuiting (US EPA
�Wetlands
should be constructed to
Factsheets).
have no lateral slope perpendicular to
�The
distance between inlet and outlet
the flow path to avoid concentrating the
should be maximised. The use of
flow in preferred channels.
islands and peninsulas will ensure this.
�The
drainage area should be greater
than 5 hectares.
Habitat Enhancement
�Sediment
forebays are recommended �A
wetlands ecologist should be
to decrease the velocity and sediment consulted about planting so that plants
loading. Forebays should be a separate are selected which are capable of
cell. It is suggested that they should be 2 pollutant removal, adapted to saturated
to 3m deep and contain at least 10% of soil conditions, tolerant of periodic
the wetland's treatment volume. inundation by run-off and which can
Coarse particles remain trapped in the withstand the dry periods that naturally
forebay and maintenance is performed occur in the local area. A diverse native
in this smaller pool, eliminating the selection should be planted shortly after
need to dredge the entire wetland. constructing the wetland. Mostly
Alternatively a detention basin may be perennial species should be selected
placed before the wetland, to remove with priority to those that establish
settleable solids and protect the wetland rapidly. Vegetation reduces the effect of
from extreme fluctuations in water wind which can cause short circuiting of
levels during large storms.
the wetland.
�Effective
wetland design displays �If
possible, stormwater wetlands
"complex microtopography"; wetlands should be located close to natural
should have zones of both very shallow waterbodies, to enhance colonisation.
and moderately shallow wetlands using However, existing important habitat
underwater earth berms to create the areas should be avoided.
zones. This design will provide a longer
flow path through the wetland to
encourage settling and plant diversity
and to discourage undesirable plant
monocultures.
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GREATER DUBLIN STRATEGIC DRAINAGE STUDY - ENVIRONMENTAL MANAGEMENT POLICY
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