The headwaters of Goose Creek, a tributary of Chester Creek, run through a narrowly wooded portion of the West Goshen Shopping Center property. Goose Creek has been visibly impacted (serious bank erosion and water quality problems) by the shopping center and major highway construction, which have significantly altered the stream’s natural hydrology. Existing stormwater management on site is minimal and includes surface drainage inlets and a sub-surface storm system designed to convey runoff as quickly as possible to the wooded area and Goose Creek stream. The site of Goshen Shopping Center also suffers from flooding problems, which can interfere with shopping center activity and result in costly interior store damages.
To remedy the current conditions, the conceptual retrofit outlines techniques to manage on-site and off-site stormwater volumes: Subsurface Infiltration Bed/Linear Trenches Under Parking, Bioretention Basins/Infiltration Trenches, and Site Naturalization.
Subsurface Infiltration Bed/Linear Trenches Under Parking
It is recommended that a large sub-surface infiltration bed or series of linear trenches be installed under the existing parking areas in front of the retail spaces in order to manage on-site and off-site stormwater runoff. The main parking lot is approximately 5 acres in size, providing ample area to reduce stormwater runoff volumes. It is recommended that this system be sized to manage runoff from the parking area as well as runoff being conveyed to the site from the inlets located on Paoli Pike.
This type of infiltration system consists of a uniformly graded aggregate storage bed or trench wrapped with geotextile fabric and finished with a porous or standard asphalt surface. The system should be designed with a positive overflow that allows it to fill up before overflowing to ensure maximum storage capacity within the bed. Large infiltration systems provide an excellent opportunity to manage the runoff (volume and peak rate) and to provide for water quality control of the most pollutant-laden “first flush” stormwater runoff discharges. The type of pavement used will affect the design but the system is not dependent on the use of porous asphalt. Porous pavement allows stormwater to drain directly through the pavement; however, if standard impervious asphalt is used, inlets can convey runoff into the sub-surface bed.
Bioretention Basins/Infiltration Trenches
There are several stormwater discharge points located on the north side of the property (behind the stores), which convey runoff directly to the stream. It is recommended that these outlet areas be retrofitted with bioretention areas, where feasible, to slow and filter runoff before discharge to Goose Creek. It is also recommended that roof leaders located in the rear of the buildings be disconnected and diverted to subsurface infiltration trenches/beds or to bioretention areas. A bioretention area should be designed as a shallow depressed landscaped area with amended soils, and should be allowed to fill up to a maximum depth of 6 to 10 inches before overflowing. The area should be planted with native species tolerant of both wet and dry conditions. This system would reduce runoff volume through infiltration and evapotranspiration. For a bioretention diagram by Cahill Associates, Inc. click here.
Installing infiltration trenches is another option. An infiltration trench is a linear element consisting of a stone-filled trench wrapped in geotextile. The surface of the trench can be vegetated. Infiltration trenches effectively reduce runoff volume and rate by providing stormwater storage and allowing for the infiltration of runoff. For an infiltration trench diagram by Cahill Associates, Inc. click here.
The West Goshen Shopping Center property is virtually 100 percent impervious. Much of the site’s parking is under-utilized, especially areas located behind the retail stores. Some of this impervious area is needed for delivery trucks and fire access but the majority of the parking spaces are unnecessary and are not used by consumers or employees. Therefore, it is recommended that the site owner work with the Township to revise and reduce the parking requirements, subsequently removing unused pavement and restoring the land to native woodland areas. This will reduce runoff and nonpoint source pollution, increase groundwater recharge and evapotranspiration, and extend the existing riparian corridor located on the north side of the shopping center.
Watershed: Chester Creek
The West Goshen Shopping Center is located on Paoli Pike between the Route 202 Bypass and Turner Lane in West Goshen Township, Chester County. The site encompasses approximately 32 acres and is surrounded by commercial areas, mixed density residential housing, and major highways and ramps. The site is virtually 100 percent impervious, occupied primarily by a strip mall with parking in both the front and rear of the retail spaces.
$916,169 (Estimated cost including design)
The concept design was prepared by Cahill Associates, Inc. as part of the Pennsylvania Environmental Council’s (PEC’s) Stormwater Retrofit Technical Assistance Program, which is funded through a Pennsylvania DEP Coastal Zone Management Program Grant and through the William Penn Foundation.
Pennsylvania Environmental Council SE Office
123 Chestnut Street, Suite 401
Philadelphia, PA 19106