SWMM Sensitivity to LID Siting and Routing Parameters: Implications for Stormwater Regulatory Compliance

Abstract

Misrepresentation of sensitive siting and routing parameters for low impact development (LID) affects hydrologic model outputs, stormwater infrastructure design, and regulatory compliance. ABSTRACT: Cities are experiencing new growth through infill development, or "redevelopment," where lower density land uses are redeveloped to high density with increased impervious surfaces. Cities need revised stormwater criteria to manage increases in stormwater runoff and flooding from redevelopment via low impact development (LID). Watershed-scale hydrologic modeling in the Storm Water Management Model (SWMM) can help policy makers revise regulations to adapt to redevelopment. However, LID modeling is still relatively new and the understanding of model sensitivity to certain parameters is lacking. In particular, LID siting and routing parameters such as outflow routing from, area treated by, and placement of LID are not well studied. Using a case study of a redeveloping neighborhood in Denver, Colorado, we tested 32 configurations of these parameters in a calibrated, SWMM for PC (PCSWMM) model. We found some configurations lead to counteracting model processes that result in similar runoff reduction across a variety of configurations, suggesting to policy makers that reduction targets may be met in a myriad of ways. The relative sensitivity of runoff volume to area treated and LID placement was found to be on average 3.0 and 11.2 times higher than other model parameters. Given this sensitivity, practitioners and modelers should consider LID siting and routing parameters in hydrologic modeling efforts that inform regulations.

Investigating Tradeoffs of Green to Grey Stormwater Infrastructure Using a Planning-Level Decision Support Tool

“…Monthly evapotranspiration values were derived from the Denver Water Administration Building gauging station [53] and used to simulate evapotranspiration processes in each SCM in the model. Validated and calibrated land use time series of surface runoff volumes were acquired from the Panos et al (2018, 2020) modeling efforts to simulate water quantity in the model for both non-redeveloped and redeveloped land use conditions [53,64]. Two sets of time series were extracted from the outlet in the Berkeley neighborhood PCSWMM hydrologic model.…”

“…Therefore, water quality was included in this study. However, water quality was not simulated in the Panos et al (2018, 2020) modeling efforts [53,64], thus event mean concentration (EMC) data was obtained from a study that developed regional values from the National Stormwater Quality Database [65]. Runoff concentrations from the "southwest" NCDC climatic region for total suspended solids (TSS), total Phosphorous (TP), and total Zinc (Zn) have been identified as pollutants of concern for wet weather flows in the study area and were chosen to be simulated in the model.…”

“…All SCMs were designed to capture stormwater runoff from a uniform design drainage area which was chosen as the average area of predicted infill developed parcels in the Future Baseline scenario, or 0.053 ha as determined by Panos et al (2018Panos et al ( , 2020 [53,64]. SCMs in Denver are commonly sized to be 5% of the impervious drainage area [66].…”

“…This study uses previously calibrated surface water runoff timeseries from the PCSWMM modeling of Panos et al (2020) to drive the model, thus calibration of flow in the i-DST SUSTAIN model was not required [64]. The i-DST SUSTAIN model output of hourly water quantity flow for the Current Baseline, Future Baseline, and Future SCM scenarios were compared to the PCSWMM calibrated modeling results [72] to provide model validation for i-DST SUSTAIN.…”

“…The i-DST SUSTAIN model output of hourly water quantity flow for the Current Baseline, Future Baseline, and Future SCM scenarios were compared to the PCSWMM calibrated modeling results [72] to provide model validation for i-DST SUSTAIN. Bioretention units were used in the Future SCM scenario for this analysis and all BR design parameters used in Panos et al (2020) were used in i-DST SUSTAIN [64]. BR units were sized to 1% and 5% of the parcel area draining to the SCM.…”