Performance assessment of sponge city infrastructure on stormwater outflows using isochrone and SWMM models

•Isochrone and SWMM model are combined to evaluate effect of sponge city construction.•Sponge city infrastructure reduce outflow volume and peak flow.•Sponge city infrastructure perform best against storms with smaller return periods. Sponge city construction in China is integrated with stormwater management infrastructure, which is based on the concept of low impact development, such that the urban flooding and nonpoint storm pollution are mitigated. We hypothesize that an analytic framework based on isochrone and SWMM models can be used to simulate and compare the rainfall-runoff process before and after sponge city construction and thus assess the performance of the sponge city infrastructure on controlling stormwater outflows. In this paper, the analytical framework driven by design storms (with return periods of 2, 5, 20, and 30-year) was developed and tested in the catchment in Fengxi New City, China. The modeling results were then analyzed with indicators like peak flow rate, outflow volume, rainfall-runoff ratio, and stormwater stored due to sponge city construction. Outflow hydrographs and volume curve analyses showed that the sponge city infrastructure significantly mitigates the risk of stormwater overflows in the catchment and dramatically relieves the pressure on the flood control system of its downstream areas by a firm limit of the peak flow rate. Also, the performance generally decreases as rainstorms become less frequent and more intense. We anticipate that the proposed framework provides a practical approach to quantifying the sponge city's response to rainfalls under complex models.