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SWMM Modeling Methods for Simulating Green Infrastructure at a Suburban Headwatershed: User’s Guide
Citation:
Lee, J., C. Nietch, AND S. Panguluri. SWMM Modeling Methods for Simulating Green Infrastructure at a Suburban Headwatershed: User’s Guide. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-17/414, 2018.
Impact/Purpose:
The main purpose of the report is to provide SWMM model users interested in GI considerations at a watershed scale a framework for using common computer analytical software tools to configure a SWMM model for GI scenario analysis.
Description:
Urban stormwater runoff quantity and quality are strongly dependent upon catchment properties. Models are used to simulate the runoff characteristics, but the output from a stormwater management model is dependent on how the catchment area is subdivided and represented as spatial elements. For green infrastructure (GI) modeling, we suggest a discretization method that distinguishes directly connected impervious area from the total impervious area. We recommend identifying pervious buffers, which receive runoff from upgradient impervious areas, as a separate subset of the entire pervious area. This separation improves model representation of the runoff process. The rational and demonstration of the performance of this approach is presented and discussed in detail in Lee et. al. 2017. Using these criteria for categorizing important land cover components governing runoff hydrology, an approach to spatial discretization for projects using the U.S. Environmental Protection Agency’s Storm Water Management Model (SWMM) is demonstrated for the Shayler Crossing (SHC) headwatershed, a well-monitored, residential suburban area occupying 100 ha, east of Cincinnati, Ohio. The model relies on a highly resolved spatial database of urban land cover, stormwater drainage features, and topography. The approach accommodates the distribution of runoff contributions from different spatial components and flow pathways that would impact GI performance. In headwatersheds with relatively homogeneous landscape properties throughout the system like SHC, all subcatchments are discretized with the same land cover types, and instead of using a j × k array of calibration parameters, based on j subcatchments and k parameters per subcatchment, the values used for the parameter set for one subcatchment can be applied in all cases (i.e., just k parameters), reducing the number of modeled parameters to consider during calibration. Depending on the size of the watershed being modeled and the heterogeneity of the landscape, grouping subcatchments into categories, such as steep slope vs gentle slope, for example, may be necessary. This would result in an additional parameter set for consideration during calibration, but still limits the domain of parameter values compared to when each subcatchment is parameterized independently. This report was written to outline the spatial database and SWMM model set-up steps required to simulate GI scenarios at a small watershed scale. We use the SHC headwatershed as the case study for describing the processes for model set-up and conducting simulations. While some modeling results are given, they are provided for context and guidance only, and were not meant for detailed discussion. The main purpose of the report is to provide SWMM model users interested in GI considerations at a watershed scale a framework for using common computer analytical software tools to configure a SWMM model for GI scenario analysis. The report is staged in a step by step, users guide, format for setting-up a SWMM model to simulate the effects of GI on small watershed rainfall-runoff hydrology.