Stormwater Runoff Reduction Achieved by Green Roofs: Comparing SWMM Method to TR-55 Method

Abstract

This research uses the EPA Storm Water Management Model (SWMM) to simulate runoff generated by impervious roofs and green roofs. Simulation results are compared with previous simulation results using Natural Resources Conservation Service Technical Release-55 (tr-55) and measurement by the British Columbia Institute of Technology (BCIT). Findings show that SWMM's Green-Ampt method can calculate more accurate runoff coefficients of impervious roofs than tr-55's Curve Number method. Annual runoff coefficients are 0.88 by SWMM, 0.6 by tr-55 and 0.93 by BCIT's measurement. However, using SWMM's Green-Ampt method alone does not accurately simulate green roof runoff. Green roof runoff is more accurately simulated by combining the Green-Ampt method with evapotranspiration of green roofs. According to BCIT's measurement, runoff reduction rates of monitored green roofs are 24% and 21%. As determined by combining the Green-Ampt method with evapotranspiration of green roofs, the potential runoff reduction achieved by green roofs is 20%.