Overland Soil Erosion Estimates for Water Quality Modeling in SWMM

Soil erosion due to overland flow is an important aspect in stormwater/water quality management, especially in agricultural areas. Suspended solids (TSS) and other particulate-bound pollutants can be associated with soil erosion and many pollution control measures and agricultural best management practices are aimed at reducing soil erosion.

The Us EPA’s older SWMM4 software has a soil erosion routine (USLE), however, when the US EPA SWMM5 program was formulated, this routine was omitted. The authors introduced overland soil erosion modeling capability in PCSWMM with a form of the modified soil loss equation (MUSLE). MUSLE uses runoff flow values to calculate erosion as opposed to rainfall values used in the SWMM4 USLE implementation. MUSLE allows the sediment yield to be computed without defining a delivery ratio and also event simulations.

The MUSLE-derived erosion computations were achieved by running an additional SWMM5 hydrology simulation to estimate total daily flow volumes and peak flow rates from pervious areas of all subcatchments subject to erosion. A daily erosion time series was generated based on these runoff results to comply with MUSLE methodology of daily time steps. This paper presents the methodology used by PCSWMM to convert daily erosion time series to SWMM runoff time steps based on runoff flow to accommodate SWMM routing. The paper also discusses ability to use erosion estimates for water quality simulations of various TSS particle sizes as well as particulate-bound pollutants such as phosphorus.