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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.
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