Forthcoming Research Paper

Flaws in a Common Stormwater Modeling Method Have Negative Watershed Management Implications

by Briar Ownby-Connolly, Matthew Huth Connolly, Randy Chambers, Landon Knapp, and William Strosnider

Abstract

Urbanization increases stormwater runoff, often leading to eroded channels and impaired downstream water quality and habitats. To simplify pre- and post-development calculations of stormwater runoff, Technical Release 55 (TR-55) was established by the United States Department of Agriculture in 1986 and adopted by engineering firms nationwide. The commonly applied HydroCAD stormwater modeling software utilizes TR-55, which is meant to help engineers ameliorate pressures from increased runoff due to urbanization, guiding Best Management Practice (BMP) design. Herein we evaluate this conventional modeling method using two field sites to test limitations of TR-55 within HydroCAD. In-situ stream flow velocities, rainfall data, soil samples, and topographic surveys were used to compare model results with site data and observations. We identified Hydrologic Soil Groups as the most influential parameter, where misclassified soil groups, specifically for undeveloped wooded areas, led to current standard practices that underestimate infiltration, leading the model to generate pre-urbanization runoff values far greater than reality. As a result, the HydroCAD model overestimated pre-development peak discharge rates by approximately 710% and 240% at our two field sites. Due diligence in corroborating soil mapping is crucial, as uncorrected model outcomes can cause deleterious downstream effects via undersized BMP designs. The large error rate observed indicates that the standard assumptions of TR-55 within HydroCAD should be revisited not only in undeveloped wooded areas, but in other prevalent land cover types.