Bryn Mawr College Regional Basin Retrofit

Description

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.

Location

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.

Partners

Montgomery County Conservation District
Lower Merion Township Environmental Advisory Council
Lower Merion Conservancy
Delaware RiverKeeper
Villanova University
Yerkes Associates (site design)

Cost

Pennsylvania Department of Environmental Protection Growing Greener Grant (2000): $150,000
Pond cost: approximately $750,000

Contact

Bryn Mawr College Facilities Services
101 N. Merion Ave.
Bryn Mawr, PA 19010
610-526-7938

Site Engineering Concepts, LLC
P.O. Box 1992
Southeastern, PA 19399
610-240-0450
info@site-engineers.com

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