Effectiveness of Green Infrastructure on Watershed Scale Flow Regimes of Urban Streams in Cleveland and Denver

Increasing urbanization can trigger severe hydrological problems due to impervious surfaces that create larger peak flows and runoff volumes in urban streams. Green Stormwater Infrastructure (GSI) has been implemented at the watershed scale to mitigate those effects because it mimics natural hydrological processes. However, studies of GSI's hydrologic effects are usually place specific, and cross-site comparison can be difficult because of differences in modeling approaches. The goal of this study is to evaluate the impact of several common forms of GSI on the flow regime of urban streams in two contrasting US cities, while using an identical modeling approach. We chose representative urban watersheds in Cleveland (Ohio) and Denver (Colorado) with similar drainage areas (8.3-20.6 km2) and impervious surface (30-36%). The OSTRICH-SWMM algorithm was used to optimize the placement of GSI in well-calibrated PCSWMM models, with the objective of reducing the largest event's peak flow volume. Four levels of impervious treatment (baseline, 14%, 42% and 70%) were selected based on the ranges of GSI implemented across 20 US cities. Preliminary results show that the response of peak flows to mitigating impervious areas by GSI is dramatically different between watersheds. For example, treating 14-70% of impervious surfaces with bioretention cells reduces peak flow by 9-62% in the Cleveland watershed, while the same treatment in Denver reduces peak flow by 3-18% for a single largest storm during (2014-2018). With 70% of impervious treated, the slope of the relationship between rainfall and peak flow is 30% and 79% for West Creek and Havard Gulch repectively of the slope for the baseline condition. Simulation results will be used to inform a Bayesian network that links urban infrastructure and stormwater management to environmental outcomes. Identification of optimal GSI locations and levels of treatment will support management decisions that decrease urban flooding, reduce erosion and help improve the health of stream ecosystems.