Landcom Book 4 Maintenance - WSUD

Water Sensitive Urban Design
Examples of constructed wetlands
3.2 Constructed Wetlands
and Ponds
The deep open
water bodies
typical of ponds
provide larger
detention volumes
as compared
to constructed
wetlands.
Constructed wetlands use enhanced
sedimentation, fine filtration and biological
uptake processes to remove pollutants
from stormwater. The wetland processes
are engaged by slowly passing runoff
through heavily vegetated areas where
plants filter sediments and pollutants from
the water. Biofilms that grow on the plants
can absorb nutrients and other associated
contaminants.
Ponds also provide physical, biological
and chemical mechanisms for pollutant
uptake. The main physical difference
between a pond and a constructed
wetland is the ratio of surface area to
volume and the coverage by vegetation
(ARQ, 2006). Typically ponds have a depth
exceeding 1.5 meters as compared to
constructed wetlands, which have an
average depth in the macrophyte zone of
0.3 metres. Furthermore, a regular water
level fluctuation regime is promoted in a
constructed wetland through the design
of the outlet structures. Combined, the
high surface area to volume ratio and
the promotion of regular water level
fluctuation through the design of the
outlet structure in a constructed wetland,
supports a densely vegetated system
and hence a greater biological uptake of
pollutants as compared to a pond.
The deep open water bodies typical of
ponds provide larger detention volumes
as compared to constructed wetlands.
Higher detention times promote improved
sedimentation; however the risk of large
open water bodies is short circuiting,
redox potential conditions and elevated
levels of nutrients. These processes can
have a reverse effect on stormwater
treatment causing release of pollutants
from the sediment into the water. High
nutrient concentrations can also lead to
nuisance macrophyte growth or algal
blooms that can be toxic and aesthetically
unpleasant.
For these reasons, ponds are commonly
designed in combination with constructed
wetlands, providing polishing of
stormwater quality, attenuation of flows
and protection of downstream waterways,
and storage for reuse applications.
Book 4 | MAINTENANCE 9