Urban waterlogging hazard and sponge city mitigation in the lower Bahe River Basin, Xi'an: A complementary MIKE–HEC-RAS assessment in a semi-arid to semi-humid transitional zone

Study region The Ba River Basin is a representative urban watershed in the semi-arid to semi-humid transitional zone of northwestern China, characterized by dense population, rapid urban expansion, and extensive impervious surfaces. Under intense rainfall, rapid runoff concentration in urbanized areas is primarily associated with impervious surfaces, local topographic depressions, and limited drainage capacity. Study focus This study combines MIKE FLOOD simulations with HEC-RAS hydraulic analyses to assess urban waterlogging and river-related flood hazards under multiple rainfall scenarios. MIKE was used to simulate rainfall-induced surface inundation and river–floodplain interactions, while HEC-RAS was applied to evaluate river conveyance and levee safety. The framework provides a joint assessment of pluvial inundation and river flood risk in an urbanized semi-arid basin. This study also evaluated the efficacy of sponge city initiatives in mitigating flood risk. New hydrological insights for the region Areas with gentle slopes, dense urban development, and extensive impervious surfaces are highly susceptible to severe waterlogging during intense rainstorms, with maximum ponding depths reaching 1.57 m. No river overflow was simulated under the 100-year rainfall scenario, indicating that flooding was primarily driven by inadequate drainage capacity and urbanization effects. Sponge city construction significantly reduced waterlogging hazards and the affected area. Under the 20-year return-period rainfall scenario, the high-risk area was reduced by 47.56% and 72.52% in Scenarios 2 and 3, respectively. These findings provide support for flood-resilience planning in similar regions.