Evaluating strategies, opportunities, and tensions for implementing green stormwater infrastructure at the site scale

Green stormwater infrastructure (GSI) is increasingly relied upon to improve the negative hydrologic and ecological impacts of urban development. Site scale strategies are often dependent on owners to adopt, manage, and fund. Although cities commonly offer rebate incentives, adoption remains correlated to high income residents and single-family homeowners. Renters unable to control property decisions, especially in multifamily apartments, are left behind by current programs that benefit owner occupied properties. Similarly, reductions to property fees offered for adoption provide little incentive to rental property owners who pass costs onto renters. Important to this conversation is that flood risk falls disproportionately on low-income residents and multifamily apartments, making the geography of GSI adoption misaligned with flood risk. At best, current strategies lead to inefficient use of funds with limited impact, while at worst, the strategies deepen hydrologic and financial inequalities. This study investigates the performance of current untargeted rain cistern implementation strategies’ performance relative to strategies that integrate cisterns for multifamily apartments, and a biofiltration pond. Survey data informs the renter and owner adoption rates used in this model. Multifamily scenarios are investigated under different impervious cover percentages, and increasing precipitation intensities, inquiring performance insight to future development and climate change projections. To answer these pressing questions, a Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model is used to simulate total runoff volume and peak discharge rate for a sub catchment in Austin, TX. Unsurprisingly, a general trend of decreasing performance under increasing precipitation intensity was seen for all GSI. Key results show reductions of total runoff for single-family scenarios range between 0-9%. The multifamily strategy, with a 16,000-gallon cistern, reduced total runoff volume 33-18% from the median to 25-year storm. Increasing impervious cover from 50 to 60% for the 16,000- gallon cistern multifamily scenario reduced total runoff volume to 35-24%. The results show integrating cisterns for multifamily apartments can lead to better outcomes under median storm intensities, increasing storm intensities, and denser urban development. This study contributes to the current body of knowledge and practice by recommending policy changes and suggesting new multifamily strategies absent from the literature that achieve improved results.