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Existing stormwater
management facilities at Wild Goose Farm, a residential development, include
three dry detention basins, a conveyance system comprised of a series of
concrete swales, and a storm sewer system. The current system was designed to
manage the stormwater rate control, but not to reduce water levels or
improve on-site conditions. In fact, poor design and maintenance of
current control structures has rendered these devices ineffective. All
stormwater runoff is diverted directly into Goose Creek, and therefore
water velocity and pollutants negatively impact the health of the creek.
If the
following retrofit
recommendations were carried out, stream bed and base-flow deterioration
would be reversed, downstream flooding and overall pollution would be
reduced, and the beauty of Wild Goose Farm would be enhanced.
Stormwater Best Management
Practices (BMPs) include the 1) Naturalization of the three basins
with outlet and channel replacement; 2) Addition of level
spreaders and sediment forebays; 3) Cooperation of homeowners
through a Front yard/Backyard Rain Barrel or Rain Garden Program.
The success of these
recommended stormwater management facilities depend on regular
inspection and maintenance.
1) Naturalization of the Three Basins with Outlet and Channel Replacement
It is recommended that
the Homeowner’s Association and proposed project partners retrofit the
three detention basins. The first step in this process is the
modification of outlet control structures. If the lowest orifice is
removed and a higher drain is put in place, stormwater could be
regularly detained. The advantages of detaining stormwater run-off
include improved conditions for fauna and flora, evapotranspiration by
vegetation, settlement and elimination of sediment and contaminants, and
improved water recharge to natural underground aquifers.
Planting native plants
that are tolerant of varying moisture conditions will morph the
presently bare sod basins into beautiful meadow-like wetlands. The roots
of planted grasses, shrubs and trees will cleanse water before it
filters into Goose Creek or into underground aquifers. Concrete low-flow
channels (like the one pictured) should be removed and re-vegetated with
moisture-tolerant plants similar to the basin plantings. Because
naturalized basins need only be mowed annually or bi-annually, the
maintenance practice of weekly mowing would be no longer necessary.
Inlets, outlets, and all structural components should be cleared of
vegetation on a regular basis though, to avoid improper functioning.
Two of Wild Goose Farm’s
three current basins could also benefit from the addition of meandering
vegetated channels with check dams where the flow path within the basin
would be extended. With such an improvement, better control rates would
be obtained during heavy rainfall, plants would have more moisture in
dry periods, and the overall vitality of the system would be improved.
For a diagram of a typical bioretention basin, created by Cahill
Associates, Inc. click
here.
2)
Addition of Level Spreaders and Sediment Forebays
Installing a level
spreader would calm and diffuse water entering the basin through the
in-flow point. This would leave plants undisturbed by water velocity,
but would also be beneficial by equally distributing moisture to
vegetation. Adding a sediment forebay,
where sediment and debris are centralized within a “berm” is recommended
so that they can be readily removed from the basins, which will reduce
time for clean-up and maintenance. This forebay would capture and
pre-treat stormwater run-off before its entry into the basin’s wetland
area. A low-rise “berm” can be constructed out of rock, concrete, or
excavated earth.
3)
Cooperation of Homeowners through a Front yard/ Backyard Rain Barrel or
Rain Garden Program
The site offers many
opportunities for individual residents to install rain barrels or rain
gardens on their properties to manage their own stormwater from roofs or
driveways. The active participation of homeowners will reduce some of
the pressure on community facilities to effectively manage all aggregate
stormwater. The storage and reuse of stormwater is a management tool
that can be very effective with the public’s participation.
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