ANALYZING FLAWS IN TR-55: STORMWATER MANAGEMENT IMPLICATIONS FOR THE CHESAPEAKE BAY WATERSHED

Urbanization increases the quantity and reduces the time of stormwater runoff, eroding channels and impacting water quality. Wet retention ponds are widely used in Best Management Practices (BMPs) to mitigate these impacts. To simplify pre- and post-development runoff calculations, Technical Release 55 (TR-55) was established by the USDA and adopted by engineering firms nationwide. HydroCAD stormwater modeling software utilizes TR-55, meant to help engineers alleviate these pressures. Limitations within these procedures may lead to overestimations of pre-development runoff and under-sizing of BMPs resulting in environmental issues, where development can promote erosion of receiving channels.

This project evaluates these methods using three field sites to test limitations of TR-55 within HydroCAD. To address these limitations, stream flow monitors were installed at two sites around James City County and New Kent County, Virginia, to determine in-situ runoff quantity with a third observational site in the City of Williamsburg. In-situ stream flow velocities, rainfall data, soil samples, and topographic surveys were used to compare model results with site data, hand calculations, and observations identifying accuracy of model outputs and sensitivity of model parameters. This study identifies Hydrologic Soil Groups as the most influential parameter in estimations of pre-development runoff and offers suggestions for adjustments to parameters to increase accuracy. Overall, mapped soils groups are not reliable and can underestimate infiltration in pre-development wooded conditions, leading the model to suggest higher runoff values post-urbanization should be allowed. This study shows that uncorrected model outcomes can lead to harmful effects to receiving channels and watersheds.