Pennypack Ecological Trust Infiltration Trenches

Location

The T-VSSI project team constructed three stormwater Infiltration Trenches at the Pennypack Ecological Restoration Trust (PERT) within the Pennypack Creek Watershed, which spans a 12-municipality area, encompassing parts of Philadelphia, Montgomery, and Bucks counties. The infiltration trenches are situated along the sloping field between the lower parking areas and an upland pond located below PERT’s main buildings. The researchers chose this location because it is at a low point near PERT’s visitor’s center, below most of the impervious surfaces (parking lot, driveway, and buildings). At this location, the trenches will receive most of the stormwater runoff that is not intercepted by the Infiltration Gallery. Approximately 3 acres drain to the infiltration trenches. Of those 3 acres, approximately 50% are impervious and the remainder is either forest or mowed grass lawn.

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Trenches

Executive Summary

In July 2006, the T-VSSI project team constructed three stormwater Infiltration Trenches along the sloping field between the lower parking areas and an upland pond, below the main buildings of the Pennypack Ecological Restoration Trust (PERT). The trenches infiltrate stormwater runoff from the lower parking areas, access roads, walkways, and roof tops of PERT’s offices, visitor center, and greenhouse. The trenches are designed to capture non-point source stormwater runoff from a 3-acre portion of the upgradient drainage area and thereby reduce non-point source runoff within the upper Pennypack Creek watershed. The 3 acres generally comprise the area that remains, which is not captured by the Infiltration Gallery that is upgradient from the infiltration trenches.

The project site is designed to test the failure rate of a typical residential gravel-filled infiltration bed facility by replicating conditions when a homeowner does not properly maintain the infiltration bed, and to compare the performance and maintenance requirements of side-by-side sand and gravel infiltration beds. The installation included replacing the existing +100 feet of 6-inch diameter tar paper pipe that conveyed stormwater to the pond, with +20 feet of 8-inch PVC pipe that conveys the stormwater to a swale, which diverts the stormwater into three flows. One third of the total flow is diverted directly into a gravel infiltration bed. The stormwater flowing to that bed is not filtered to remove organic and inorganic debris, replicating conditions when a homeowner does not properly maintain the infiltration bed. The team will study how long it takes to clog the unfiltered trench with organic matter and measure the performance of this trench from new, through reduced performance, to failure. The remaining runoff, two thirds of the total flow, is diverted into a sand-filled bed and a gravel-filled bed. This gravel-filled bed has a filter/screen to remove organic and inorganic debris from the water. The sand-filled bed does not have a filter/screen to remove organic and inorganic debris. Any debris that accumulates can simply be raked off the surface.

Each of the three trenches is 5 by 10 feet with a depth of 4 feet. The earthen walls and floors of the trenches were lined with filter material before filling with stone or sand. The trenches are all open topped. One monitoring well was installed in each of the infiltration trenches to monitor stormwater levels and infiltration within the trenches after rainstorms. All three infiltration trenches will be monitored and analyzed to compare performance and maintenance requirements.

Design Goals

The goals of the infiltration trench designs are to:

  • Capture non-point source stormwater runoff
  • Retain stormwater
  • Infiltrate stormwater
  • Remove sediment, grit and debris

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