Optimizing the Spatial Allocation of Green Infrastructures to Restore Pre-Development Surface-Subsurface Hydrologic Regimes

Green infrastructures (GIs) include stormwater management practices that can restore pre-development surface-subsurface hydrologic conditions of urban areas. Their benefits include the reduction of surface runoff and enhancement of infiltration and groundwater recharge, etc. These effects could vary in different climatic conditions and for GIs of different spatial allocations. However, the impacts of these variables, particularly the spatial allocation of GIs, on the surface-subsurface hydrological regimes have not been well examined. In this study, a surface-subsurface coupled hydrological model, SWMM-MODFLOW, was utilized to simulate surface runoff and groundwater flow in response to GI implementations in an urban catchment in the Kitsap County, Washington State of the United States. The model was first calibrated and validated using the surface runoff, underdrain flow of porous pavement, catchment outflow and groundwater elevation monitoring data at the site. Then, an evolutionary optimization algorithm (NSGA-II) was applied to identify the near-optimal spatial allocation of one particularly type of GIs, namely bioretention cells. Represented by three indicators (i.e., number of practices, midpoint of practices, and gap between practices), their locations were optimized using objective functions of surface runoff reduction (i.e., peak and volume reductions) and uniform groundwater table elevation (i.e., less local groundwater mounding). In addition, the surface runoff control, groundwater table elevation and groundwater recharge of the optimal spatial allocation was assessed and also compared with that of several representative spatial allocations (e.g., upstream versus downstream, aggregated versus distributed, a range of implementation ratios) in both wet and dry conditions. The results can quantitatively support the regional planning of GIs, particularly in areas with both surface runoff and groundwater management considerations.