Hydrodynamic evaluation of paddy field dams for flood mitigation using the SWMM model

This study investigates the flood mitigation potential of paddy field dams (PFD) in a flood-prone agricultural catchment in central Taiwan using a coupled hydrological-hydrodynamic modeling approach with the Storm Water Management Model (SWMM). Simulations incorporating bi-directional weir structures in paddy field boundaries demonstrate substantial flood peak attenuation, with discharge reductions exceeding 60% in minor drainage channels, while reductions in primary conduits reached 11.5% during a historical storm event and ranged from 10.4% to 17.6% across 10- and 50-year design storm scenarios. Spatial analysis reveals that PFD performance is highly heterogeneous and governed by localized hydrodynamic-topographic coupling. Besides, the retention capacity of PFD is significantly enhanced in low-lying fields near drainage confluences where backwater effects prolong hydraulic interaction. Moreover, a dynamic interaction was identified between the temporal evolution of piezometric pressure within the PFDs and the hydraulic head fluctuations associated with the flood peak in adjacent drainage channels. This hydraulic coupling governs the directionality of transverse weir flow and modulates the effective floodwater storage capacity of the PFDs, thereby exerting a first-order control on their overall flood mitigation performance. These findings demonstrate that the effectiveness of PFDs can be substantially improved when their design and placement account for field elevation gradients and local channel hydraulics, rather than following uniform or spatially arbitrary installation strategies. This study establishes a hydrodynamically informed framework for targeted implementation of decentralized flood mitigation measures, offering a practical alternative to conventional uniform deployment approaches, and highlight the potential of multifunctional paddy field infrastructure to enhance flood resilience. • Evaluates the spatial performance of Paddy Field Dam (PFD) through a sophisticated hydrodynamic model - SWMM. • Assesses hydrodynamics governing bidirectional exchanges between PFD units and drainage channels. • Introduces a strategic framework for the spatial optimization of PFD placement to identify mitigation hotspots. • Demonstrates that PFD performance is sensitive to localized spatial configurations and weir crest elevations.