A North Carolina Piedmont Application of Protocols for Studying Wet Weather Impacts and Urbanization Patterns

Eight watersheds spanning a gradient of urbanization were used to identify linkages between indicators of aquatic ecosystem health and hydrologic and geomorphic metrics derived from a 20-year continuous stream flow record. The geomorphic metric used to evaluate linkages is erosion potential calculated using excess shear stress, which describes the energy exerted on the stream channel that is capable of causing bed scour and bank erosion. The hydrologic metric used to evaluate linkages is the duration of time that in-stream flows are above the 0.5-year return interval peak discharge rage, which is a measure of stream flashiness. The 20-year continuous stream flow records for each watershed were generated using EPA Storm Water Management (SWMM5) Models calibrated to 18 months of measured flow data. A benthic index of biotic integrity was used as one indicator of aquatic ecosystem health, and was calculated from aquatic macroinvertebrate sampling data collected by the United States Geological Survey in the eight watersheds, ranging from three to eight square miles in size. Various land use and stormwater management practices were modeled in one of the eight gradient watersheds to determine the effect of these practices on the hydrologic and geomorphic metrics. The influence of development density (percent imperviousness) and of land use patterns, such as the proximity of dense urban development to stream corridors and headwater development versus downstream development, on hydrologic and geomorphic metrics was examined. Additionally, the effects of stormwater management practices such as traditional stormwater detention, water quality control and low impact development on hydrologic and geomorphic metrics were also examined. Hydrologic and geomorphic metric values generated by the various practices were compared to those generated along the urbanization gradient to identify the management practices that best generated conditions favorable to meet ecologic targets.