Low-Head Weirs for Enhancing Infiltration: Central Pennsylvania Case Study

This case study describes the installation and performance of two experimental low-head weirs and infiltration areas (Weir Facility) located in the 183 ha Fox Hollow Watershed, an urbanizing watershed underlain by karst bedrock in central Pennsylvania. Soil/rock corings and infiltration tests conducted prior to installation indicated that the area was suitable for development as an infiltration field. Low-head weirs were constructed to cause temporary, shallow water retention to promote infiltration and groundwater recharge. A monitoring program assessed the performance of the weirs and allowed mass balance calculations. During 2003 through 2008, which included two of the wettest years on record, few events (less than 30%) resulted in significant outflow from the weirs and associated infiltration areas. For smaller rainfall events ( $<1.9\mathrm{cm}$ of rain) almost all (89–100%) of the runoff delivered to the Weir Facility infiltrated; whereas for larger events ( $⩾1.9\mathrm{cm}$ rain) generally over 50% of the runoff volume infiltrated, and peak rates and runoff volumes were substantially reduced. These data support the hypothesis that, in karst-underlain watersheds undergoing substantial development, it is imperative to identify and preserve critical recharge areas, and that simple low-head weirs with corresponding infiltration areas enhance and sustain recharge, thereby reducing potential storm-water runoff.