Tuesday, July 31, 2012

Stormwater and the New Westminster Parking Lot

The new parking lot taking shape at Westminster Choir College may look conventional, but it handles stormwater differently than has been the norm in Princeton.

Grassy strips between parking areas--shown here in Westminster's current lot--are typically higher than the pavement, forcing runoff immediately into stormdrains.

But in the new lot, some of the partitions are actually lower than the pavement. The dips in the row of Belgian curbstones on the left will allow water to flow from the asphalt into the partition strip, where the dirty "first flush" of rainwater runoff (carrying whatever dirt and oil has accumulated on the pavement since the last rain) will be filtered by vegetation. The water will then either infiltrate into the ground, recharging the groundwater, or overflow into this drain.

No longer visible (this photo was taken May 30) is the matrix of 3 foot diameter pipes underneath the pavement that will serve as a giant cistern, holding storm runoff in order to reduce the parking lot's contribution to flash floods downstream in Harry's Brook.

Water from elsewhere in the lot will flow down this drain into a small retention basin at the edge of the Westminster property. The goal of all of this holding capacity, both under and above ground, is to have runoff leave the property more slowly than before the parking lot was built. This approach of slowing the water down gained momentum in 2000, when Maryland published a design manual with the goal of reducing pollution in the Chesapeake Bay.

One way to beautify these swales and basins is with native wildflowers, as we've done at Princeton High School, just across Walnut Lane from Westminster. The high school's "ecolab" wetland--essentially a retention basin converted from grass to native floodplain wildflowers--takes runoff from the school's roof and also the parking lot next to the performing arts center, and is in full bloom right now.

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