Monitoring the Effectiveness of Stormwater Infiltration Trenches at the Pennypack Preserve, Montgomery County, Pennsylvania

Abstract

As a result of urbanization, impermeable surfaces cover about 40% of land area around Pennypack Creek (Philadelphia, PA). The resulting increase in stormwater runoff leads to flooding, bank erosion, and stream habitat degradation. Stormwater Best Management Practices (BMPs) have been constructed in the Pennypack Preserve (Montgomery County, PA) to evaluate the effectiveness of various mitigation techniques. The BMPs include a row of 3 infiltration trenches constructed in July 2006 to compare different designs. Water level data were recorded in monitoring wells from December 2006 through June 30, 2009, in addition to controlled infiltration experiments. The monitoring wells in the infiltration trenches allowed monitoring of any gradual loss of mitigation effectiveness and for quantitative comparison of the different trench designs. The right and center trenches are filled with gravel, and the left trench is filled with sand. The center trench is distinguished from the right by a leaf filter. Both seasonal data and controlled experiments showed water in the trenches drained at different rates in the different sections. The right gravel-filled trench tended to have the highest peak water levels followed by the center gravel with filter trench. The center trench showed a slow drainage rate from the beginning of monitoring, thus any effect of the center trench's leaf filter was obscured. The sand-filled trench showed the fastest drainage rate of the 3 trenches. The drainage rate was uniform within each trench over the 2.5 years of monitoring. The seasonal data showed that the right gravel-filled trench showed the most water level peaks, and the center gravel-filled and left sand-filled trenches responded to fewer storms over time. However, the uniform drainage rate suggests this decline in response was not due to degradation but rather a change in water delivery. As of May 2009, water from most storms flowed into only the right gravel trench due to berm erosion. At the end of the study, water level data showed that the trenches continued to receive stormwater without overflowing and drain within 72 hours, the Pennsylvania Department of Environmental Protection recommended design limit. Monitoring water level data in the infiltration BMP showed drainage effectiveness over time but also pointed out how unplanned design differences (water delivery and construction variation) affected trench behavior. Monitoring is the key to developing successful designs, implementation, and maintenance of stormwater BMPs.