Monitoring the Effectiveness of Stormwater Management Practices In the Pennypack Creek Watershed, Philadelphia, Pennsylvania

The Pennypack Creek Watershed is located in suburban Philadelphia, PA. Paved roads, parking lots, rooftops, and lawns cover 40% of land area around the Pennypack and limit infiltration of stormwater. As a result of urbanization, there is increased flooding, bank erosion, and stream habitat degradation. Stormwater Best Management Practices (BMPs) have been developed to mitigate this growing problem.

In July 2006, three types of stormwater BMPs were constructed along the Pennypack to study how the impact of urbanization on a watershed can be reduced. The constructed BMPs include an infiltration gallery under a porous paved parking lot, three infiltration trenches with differing characteristics, and a riparian stormwater wetland. We instituted a monitoring program before and after the construction of BMPs that includes measuring water levels in the local stream reach, water levels inside the constructed BMPs, and bank erosion pins.

Preliminary analysis suggests that the wetland is having a small yet detectable effect on bank erosion at the adjacent stream. Water level response in the stream does not appear to have changed significantly, however. Wells in the infiltration trench allow for observation of gradual loss of effectiveness. Water is piped from a parking area to the row of trenches down slope. The right and center trenches are filled with gravel, and the left trench is filled with sand. The pipe leading to the center trench has a leaf filter. Over time, the rate of trench emptying is expected to decrease due to clogging, but the leaf filter and sand were designed to reduce clogging. As of May 2008, all three trenches continue to infiltrate stormwater and empty within 2 to 5 hours, with the fastest infiltration in the sand trench. Although not commonly part of BMP design, monitoring is the key to planning successful implementation and maintenance of stormwater BMPs.