A New ‘Philadelphia Story': Getting Ahead of the Curve in Managing Groundwater-Related Impacts of Stormwater Infiltration

There is an emerging trend in urban stormwater management, as more and more major U.S. cities are considering utilizing green stormwater infrastructure (GSI) to reduce stormwater impacts to their sewer systems. GSI refers to a number of strategies for handling stormwater close to its source rather than after it has entered a sewer system. GSI often relies heavily on systems designed to infiltrate the stormwater. For example, the Philadelphia Water Department’s recently submitted Long Term Control Plan Update for Combined Sewer Overflow Control calls for “greening” more than one-third of the city’s impervious cover in the coming 20 years and will include the installation of stormwater infiltration trenches along the city streets. Although GSI is being widely tested and implemented, urban application at the scale Philadelphia proposes to implement is unprecedented.

One of the key concerns associated with urban GSI is the potential for basement flooding and infrastructure damage, due to groundwater table mounding that will result from the enhanced recharge. PWD is addressing this concern using numerical groundwater models. A regional finite element model has been developed for southeastern Philadelphia to evaluate mounding on both a local and a city-wide scale, to address the potential long-term impacts of the GSI program. Transient site-specific model simulations are used to aid in the siting and design of the individual infiltration trench systems. Preliminary analyses suggest that although the highest water table mounding is generally localized immediately beneath the trench, the trench size, placement, loading ratio, and underlying stratigraphy are critical. On a regional scale, mounding is greatest in the Piedmont physiographic province, but is somewhat mitigated by infiltration into the city’s sewer system. An updated city-wide water table map, last developed in 1980, is being developed and will be incorporated into additional groundwater modeling efforts.