Stormwater Retrofits in Urbanized Watersheds
Retrofit for Water Quantity and Quality Benefits
Southeastern Pennsylvania is one of the fastest growing areas in the Commonwealth. As a result of this tremendous growth over the past several decades, watersheds in Pennsylvania have been impacted by increased stormwater runoff and reduced water quality from developed areas. Detention basins have dominated as the primary method of stormwater management.

While detention basin provide peak rate attenuation of runoff produced from development, they fail to reduce stormwater volume, and as a result many streams in urbanized watersheds have suffered from increased bankfull conditions, erosion, increased sedimentation and flooding, and overall reduced water quality. Cahill Associates has been working to retrofit detention basins to control stormwater volume and quality in highly urbanized watersheds.

In the Valley Creek Watershed, CA worked with Green Valleys Association to retrofit a detention basin adjacent to a high-density townhouse development. A 364-foot infiltration trench was installed within the basin and the flow from existing stormwater conveyance pipes was intercepted. The trench functions to reduce the runoff volume and to reduce nonpoint source pollutants previously discharged from the basin.

CA also worked with Plumstead Township to retrofit basins in the Pine Run Watershed in Bucks County. At the Township's request, Cahill Associates developed a retrofit design for 3 basins in a single-family development. The design included a series of infiltration trenches to intercept stormwater runoff, the removal of concrete low-flow channels, and basin naturalization through re-vegetation with native species.
Examples

Plan view of the Valley Creek retrofit, funded by PADEP's Growing Greener program.

Construction of the infiltration trench

Conceptual diagram showing proposed retrofit strategy

Soil percolation testing is critical before any retrofit solution can be recommended.