A city-scale fully controlled system for stormwater management: consideration of flooding, non-point source pollution and sewer overflow pollution

Abstract An increase in severe urban pluvial flooding, non-point source (NPS) pollution and combined sewer overflow (CSO) pollution increasingly threaten human life and property. City-scale sustainable stormwater management (SSM) has become inevitable solutions. This research designed and evaluated a stormwater management system (sponge city) on city-scale. The differences between old and new districts, the coexistence of combined and separate drainage systems, pipeline defects and the corresponding groundwater/river infiltration were fully considered in the system design. Low impact development (LID) practices, drainage upgrading and terminal control approaches were integrated together. Sponge city implementation is a no-regrets adaption strategy that can offset the stormwater related joint pressures. Compared with baseline values, the maximum fraction of the conduit full depth decreased for the entire pipe network. Annual values of total inflow volume (TIV) and total flooded volume (TFV) decreased by 32.26% and 53.68%, respectively. Significant decreases in pollution release were also recorded, with reduction rates of 40.96% (chemical oxygen demand; COD), 71.18% (ammonia nitrogen; NH3-N), 41.57% (suspended solids; SS) and 50.40% (total phosphorus; TP). LID on its own allows the city as a whole to control runoff and to largely reduce NPS pollution; however, it has a limited ability to improve drainage capacity. Adopting grey approaches, such as drainage system rehabilitation and renovation, as well as storage tanks, was proved to be essential for drainage capacity improvement and pollution control on city-scale. However, final pollutant discharge depends on the relative amount of increased NPS loads and reduced CSO loads after implementing combined sewer separation projects. It is usually recommended to increase the NPS pollution control target when designing stormwater control system. Terminal facilities (such as riparian zones and storage tanks) are encouraged to better control NPS pollution. Methods and conclusions on the comprehensive consideration of urban complex conditions, systematic stormwater control and the effectiveness of different measures highlighted in this study on a city scale provide important reference for future urban sustainable rainwater management.