Assessment of Climate, Sizing, and Location Controls on Green Infrastructure Efficacy: A Timescale Framework

We investigate how the effectiveness of green infrastructure (GI) to mitigate the frequency and magnitude of significant discharge events and combined sewer overflows (CSOs) depend on both climate and sewershed characteristics and propose a theoretical framework for a holistic assessment of GI's efficacy. The framework is based on the comparison of three characteristic timescales that control the production of peak discharge: rainfall duration (t_r), travel time in the sewer network (t_n), and the duration of rain that would be required to fill the GI's storage (t_GI). Storm events can then be characterized by two ratios of these timescales: T_n = t_n/t_GI and T_r = t_r/t_GI. A third dimensionless number characterizes critical storms during which adverse events (such as CSOs) occur and allows us to identify the combinations of T_n and T_r for which GI may substantially mitigate those events. The results of numerical experiments with the model demonstrate that the storms for which GI can substantially reduce peak discharge and CSO volume typically occur in a narrow band of T_n and T_r. Within that band, the efficacy of GI may depend on the location of GI within the sewershed if network routing substantially affects the timing and magnitude of flood peaks. The proposed framework is applied to examine the efficacy of GI using historical precipitation data from two major U.S. cities: Philadelphia, PA, and Seattle, WA, and the results of this comparative analysis suggest that GI location is an important control on catchment-scale GI efficacy in Philadelphia, but less so in Seattle.