Climate-resilient vortex dropshaft design: Mitigating urban flood risks by controlling flow regime transitions under extreme rainfall

Climate change-induced extreme rainfall events pose unprecedented challenges to urban drainage systems, making the optimization of drop shaft hydraulic performance critically important for effective stormwater management. This study investigates the flow regime transition mechanisms between plunging and vortex flows in drop shafts with tangential intakes using an integrated approach combining three-dimensional (3D) numerical simulations, physical experiments, and analytical solutions. Five distinct flow regimes are identified: weak drop flow, moderate drop with weak rotation, strong drop with moderate rotation, partial blockage with strong rotation, and severe blockage. Three comprehensive criteria are developed for flow regime identification: pressure ratio criterion, velocity ratio criterion, and dimensionless drop height criterion. The moderate rotation regime achieves maximum drainage efficiency while maintaining controlled energy dissipation, which is essential for handling the high flow rates generated by extreme precipitation events. Extreme contraction ratios lead to performance degradation through either insufficient swirling effects or flow blockage, potentially causing system failure during critical storm events. These findings provide practical design guidelines for optimizing drop shaft performance in urban drainage systems capable of managing climate change-driven extreme rainfall, with the proposed criteria enabling real-time monitoring and operational control of flow regimes during intense storm events.