Effect of Managing Tributary Flows on Flood Risk in Transitional Low‐Gradient River Systems

ABSTRACT Flooding in low‐gradient river systems, particularly, within inland‐to‐coastal transitional settings, poses significant risks to both human and natural systems due to complex flow dynamics and the convergence of riverine, tidal, and storm‐driven flooding regimes. Effective flood management strategies in these settings require a careful consideration of the interactions between tributary flows and the main river, as the synchronization of peak flows can intensify flood severity. Using a counterfactual hydrodynamic modeling approach, this study investigates how modifications to magnitudes and timings of tributary inflows, as part of practical flood mitigation interventions, influence flood dynamics and overall flood risk along the Vermilion River in south Louisiana, USA—a representative a case study. Simulations of various tributary desynchronization scenarios showed that, while managing individual tributary flows can lower water levels in the main river, simultaneous manipulation of multiple tributaries can introduce added complexities, since poorly timed tributary interactions could diminish the intended flood mitigation benefits. The analysis also revealed that alterations to tributary hydrographs can modify existing flow exchanges between the river and interconnected large natural storage systems, such as swamps, highlighting the importance of comprehensive flood management strategies that consider different flood dynamics in low‐gradient river systems. Overall, this study offers actionable insights for optimizing flood management strategies in similar systems worldwide, where intricate interdependencies among tributaries, natural storage features, and main rivers can influence flood mitigation outcomes.