CAN WE QUANTIFY REDUCTIONS IN SEDIMENT AND NUTRIENTS DUE TO STORMWATER CONTROL MEASURES? A PRE- AND POST- INSTALLATION STUDY ON AN URBAN HILLSLOPE

Overland runoff was monitored before and after the installation of two stormwater control measures (SCMs: a berm and bioswales) on a hillslope in suburban Philadelphia. A unique subsurface bottle was used to capture first flush runoff and compare overland runoff volumes, nutrient concentrations, and total suspended sediment in samples from uphill and downhill of the berm and the bioswales. After the SCMs were installed, captured runoff volumes were lower in downslope locations, particularly downhill of the bioswales. No significant trends were observed in dissolved nutrients or total suspended sediment. Even the upslope concentrations were variable - casting doubt on whether urban overland runoff geochemistry can be representative in the face of significant heterogeneity. A physically based model (GSSHA: Gridded Surface/Subsurface Hydrologic Analysis) was generated using high-resolution LiDAR data collected before and after the berm and bioswale installation. The model showed that overland runoff initially was captured in all three bioswales, but overflow from the second bioswale can be significant during higher intensity rainfall events. Stream discharge was also modeled and calibrated to measured water levels in the headwater stream. Although modeled and measured runoff volumes from the hillslope decreased, there was no measurable decrease in stream discharge. This study points out the challenges in evaluating nutrient and sediment fluxes in stormwater control measures and suggests they can be better assessed through small-scale overland flow measurements or high-resolution modeling.