A commercial shopping center, Horsham Gate’s site is covered by over 80% impervious surfaces. Careful planning was needed to capture, cleanse and control stormwater runoff utilizing Best Management Practices. The BMPs selected for this site include three subsurface detention/infiltration systems and four rain gardens. Approximately 25% of the site drains to the Wissahickon Creek watershed, with the bulk draining to the Pennypack Creek watershed.
The site’s infiltration systems feature multi-stage outlet structures to assist with water quality by increasing the dewatering time and reduce the peak outflow, with an open graded stone base to allow infiltration to occur. All proposed inlets in parking areas upstream of the basins have snout attachments to assist with water quality. Inlets discharge captured runoff to the underground infiltration system, composed of two conventional basins and one manufactured system. The basins use 60-inch perforated pipe in a gravel bed; the Stormtrap® is a series of concrete vaults set on a gravel base, and provides a great amount of storage capacity. The roof and service areas drain to the Stormtrap® basin. Together, all three basins provide a 100-year infiltration and recharge capacity of over 106,000 cubic feet (2.4 ac-ft) of water.
This site is located within the 100% Release Rate District, which requires that the amount of post-development runoff be equal to or less than the pre-development runoff in a meadow condition, despite having been a developed existing condition. Criteria for improved water quality includes limiting the amount of closed storm sewers, increasing the length of grass or naturalized surface drainage, detaining stormwater runoff over an extended period of time, and increasing infiltration to replenish the ground water and reduce volume of runoff entering the stream and rivers.
To better control and manage stormwater runoff, Horsham Township requires non-structural BMPs to be incorporated into all new construction projects. The size of the BMP is based on a percentage of impervious surfaces and soil type. In this case, 15.21% of the total runoff must drain to a non-structural BMP. The township code also requires parking lot landscaping, making rain gardens a good choice as the non-structural BMP.
The four rain gardens drain 2.2 acres of parking lot area, or 23.4% of the site. The rain gardens are located in the linear islands that divide the parking lot, and serve as the first point of sediment capture, pollutant sequestration, velocity reduction, and infiltration of sheet flow runoff from the parking lot. Runoff velocity is slowed as it flows over the depressed curb into a section of 3 to 5-inch diameter river rock. The rain gardens are designed with a 6-inch depression within the ten-foot wide islands that capture surface water before it percolates into the 3-foot deep enriched subsoil and gravel layers. Fine particulates, trace metals, and trash are captured in the rock/mulch layers. Microbes in the soil remove pollutants, such as organic carbon matter, bacteria, and hydrocarbons. Fibrous roots absorb water and nutrients, and hold the soil. Some water will move back into the atmosphere through evapotranspiration. The gardens were designed with improved varieties of predominantly native woody and herbaceous plants able to withstand the harsh microclimate of a parking lot. Click here to view a plant list. Runoff amounts that exceed the rain garden capacity bypass the garden and enter the subsurface storage and infiltration system through an elevated yard drain and perforated pipes. To view a list of the plants in the rain garden, click here.
Routine maintenance includes visual inspection of the facilities, inspection of mechanical components removal of debris and litter, and vegetation management to ensure that plant life is flourishing. The inspections are conducted annually and after each major storm event.
Watershed: Pennypack Creek
Ann Marchino, RLA
Glackin Thomas Panzak
Paoli Executive Green
Building 1, Suite 300
Paoli, PA 19301