Valley Creek Watershed

PennDOT District 6-0 | Chester County, PA

This project involved the compilation of digital terrain data of environmental features and environment studies in a GIS format along with preliminary engineering and final design for watershed analysis of Valley Creek and Little Valley Creek basins in Tredyffrin and East Whiteland Townships. The work was completed to mitigate hydrologic and hydraulic impacts of the proposed widening of U.S. Route 202, located within the valley of this High Quality, Exceptional Value (HQ-EV) watershed.

Due to the HQ-EV status, strong public interest within the watershed, and local municipal ordinances, stormwater management and floodplain protection are major concerns; and therefore, stormwater mitigation is required by local ordinances for the proposed project.

AEG surveyed and assessed all drainage and stormwater structures along the seven-mile highway corridor to determine if replacement or modification of existing stormwater facilities would be required due to the highway expansion. AEG then created a comprehensive watershed-wide, GIS-based assessment of the watershed, which identified public water supplies, groundwater, soils, geology, floodplains, and surface water hydrology impacts and mitigation. This information was used in conjunction with stormwater Best Management Practices, including wetlands, stormwater treatment ponds, and infiltration facilities to enhance base flow recharge and provide stormwater quality renovation in the watersheds.

AEG was responsible for identifying groundwater, soils, geology, floodplains, and hydrology within the karst terrain of the basin. This information was used to prepare a watershed-wide GIS database to perform floodplain and stormwater analysis and to identify areas suitable for stormwater mitigation and infiltration within the watershed. After field reconnaissance and spatial analysis, 23 proposed mitigation sites were identified.

The potential sites were field-verified for suitability via site testing and analysis, which was completed to determine optimum sites for base flow recharge and water quality enhancement by wetland treatment, stormwater detention, and stormwater retention.

The proposed stormwater structures and roadway infrastructure to be located within the FEMA 100-year floodplain and/or regulatory floodway were analyzed for possible floodplain encroachment. Floodplain and natural channel restoration by fluvial geomorphologic methods was also proposed to mitigate and control erosion-prone areas.

Mitigation included natural stream channel design measures in order to stabilize and restore eroding, sediment-producing streambank reaches, and the replacement of wetlands. A total of approximately 2,500 feet of stream restoration/stabilization was completed using natural stream channel and bioengineering techniques in concert with the wetland construction. Additional stream stabilization helped to improve a nearby municipal park.

Services provided included DEP Chapter 105/ACOE Section 404 joint Permit coordination and preparation, environmental impact assessment, NEPA documentation, floodplain analysis, surveys, E&S control, wetland delineation and mitigation design, stream stabilization and restoration, planting plans, post-construction stormwater management, and construction oversight.