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During the planning
process for a new Facilities Services Building in 2000, the College and
civil engineer convened to discuss cost and space issues in conjunction
with local regulatory stormwater management requirements. According to
Merion Township requirements, stormwater management facilities must be
included in all new construction or rehabilitation projects. Building
individual basins on the Bryn Mawr campus for each capital improvement
project was costly at $200,000 or $300,000 per project. The College had
previously installed expensive underground stone or piped structures to
store, infiltrate, and perform rate control for the discharge of
stormwater runoff.
Instead of building a
new management facility for each new addition or renovation as required
by municipal code, Bryn Mawr College changed the status quo with the
addition of a new wet pond. The campus is no longer obligated to adhere
to municipal stormwater regulations for future projects because of the
unprecedented scale of this undertaking in Lower Merion. The regional
detention basin on the Bryn Mawr College was designed in accordance with
Montgomery County’s Stormwater Management Plan.
The project is
innovative because the plan design incorporates a regional vision into a
campus endeavor. For a site design of the basin, please click
here. By out-of-the-box thinking about stormwater runoff as
a resource instead of simple wastewater, facilities services began to
conceptualize an association between area flooding and the campus’ water
requirements. The College’s plan to install a large retention pond was
at the same time beneficial to the Bryn Mawr community and downstream
neighbors, inciting the participation of relevant stakeholders from
surrounding communities.
A murky detention basin
full of decaying trees was converted into the new 2-acre wet pond by the
fall of 2001. A gabion retaining wall that was insufficient at slowing
discharge during storm events was replaced by an earthen embankment dam,
creating a pool of water with enough depth to store stormwater runoff. The wet pond can be emptied in anticipation of a storm event so that the
wet pond does not exceed capacity and discharge levels can also be
altered during times of draught or maintenance. With additional
funding, the college hopes to incorporate a pumping station using the
conserved water to irrigate two nearby athletic fields. Once the pump house is
installed, the wet pond will
provide 700,000 gallons
of the one million gallons needed to irrigate the fields.
A concrete apron
directs water into a forebay, except in the event of high volume storms,
during which the water discharges directly into the pond. The off-line forebay captures solids during low-volume, high frequency storms without
stirring up and dispensing sediment and debris. This forebay can be
cleaned without having to dredge the entire wet pond. The estimated
amounts of pollutants collected in a given year amount to 9 tons,
including 14,700 lbs. of suspended solids like sediment, 25 lbs. of
phosphates, 105 lbs. of nitrates, 2,955 lbs. of oxygen demand, and 23
lbs. of trace heavy metals. An outlet hood located
at the wet pond’s outfall discharges cool water from more than 18 inches
below the surface of the pond into a small waterway to ensure normal
stream temperatures. Also under the outlet hood is a connection for the
future irrigation system. The valve for regulating water levels is
located 2 feet above the floor of the wet pond.
A shallow aquatic bench
measuring no more than 18 inches in depth and about 15 feet wide was
constructed at the perimeter of the wet pond. Besides acting as a
safety measure for visitors, the bench provides shallow planting ground
for wetland species. Over 1,800 native plants were planted over a
15,000 square foot area by volunteers and the landscape contractor. For a plant list, click
here. The
plantings were installed through the summer of 2002 and help dissuade
Canada geese and other migratory water fowl from residing at the pond.
An algae bloom erupted
in May of 2002, so the College had to skim the surface. After the
native plantings were installed, the pond has not had any other major
problems with algae. Keeping an eye on algae blooms is an on-going
maintenance task.
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County:
Montgomery
Watershed:
Schuylkill River
Bryn
Mawr College is located in Lower Merion Township in Montgomery County
and sits on a slope downstream from the Township of Bryn Mawr. The
50-acre campus lies within the Mill Creek Watershed in the Schuylkill
River Basin. The catchment area in question, the Lower Merion Drainage
Area, is 56 acres. Apart from the campus itself, the drainage area
includes railroad tracks, high-density residential development, offices,
the Shipley School, Montgomery Avenue, which is a four lane road, and
other local roads. The stormwater control measures at these sites
provide primarily traditional conveyance measures without providing a
solution to pollution or to heightened water velocity and water volume
during storm events.
View
all
TVSSI BMPs in a larger map
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Glenn R. Smith
Bryn Mawr College Director Facilities Services
101 N. Merion Ave.
Bryn Mawr, PA 19010
610-526-7935
gsmith@brynmawr.edu
Rob Lambert
Project Engineer
Site Engineering Concepts, LLC
P.O. Box 1992
Southeastern, PA 19399
610-240-0451
rlambert@site-engineers.com
info@site-engineers.com
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