Coastal Urban Waterlogging Simulation and Drainage System Optimization: A Case Study of the Shajing River Drainage Area in Shenzhen

• • Under a 5-year return period storm, 47 manholes overflowed and 31.29% of stormwater pipes operated at full capacity, identifying 12 waterlogging-prone points, of which 8 were due to insufficient pipe capacity and 5 due to river backflow. • • Enlarging pipe diameters reduced overflowing manholes by 32 (from 47 to 15) and full-flow pipe length by 20%, significantly decreasing surface ponding area. • • Implementing LID measures reduced overflowing manholes by only 4 and full-flow pipe length by 1.5%, but decreased maximum flooding depth by nearly 1 m, indicating its role in reducing runoff and easing drainage burden. • • River backflow, a critical factor in coastal waterlogging, was observed at 5 of 12 waterlogging points, emphasizing the need to incorporate tidal and storm surge effects in drainage system design.