Dune Evanescence at Flood Stage in Lowland Sand‐Bed Rivers: A 2D Modeling Approach With Dynamic Resistance

Bedforms in lowland sand‐bed rivers, which contribute substantially to total flow resistance, are known to adapt to changing flow conditions. While the growth of subaqueous dunes with increasing bedload transport and their wash‐out at higher transport stages are well documented in laboratory experiments, their possible evanescence in lowland rivers characterized by very low Froude numbers remains poorly observed, as do the effects on flood propagation. In this study, a 2D depth‐averaged fixed‐bed hydrodynamic model is enhanced to account for bedform evolution and applied in a real river setting to analyze conveyance shifts caused by dune evanescence during floods. Bed resistance is dynamically updated based on local flow conditions using different roughness predictors for dune dimensions and complemented by additional dissipative components. The model is applied to the lower 200 km reach of the Po River (Italy), where traditional fixed‐resistance approaches misrepresent the stage‐discharge rating curve, overestimating peak water levels by more than 2 m (~25% of the water depth). The results confirm a marked resistance reduction at flood stage due to dune wash‐out into upper‐stage plane bed, occurring at very low Froude numbers (∼0.2) and initiating at approximately 60% of the maximum flow capacity. Enhanced with the Van Rijn roughness predictor, the model enables accurate water level predictions under all flow conditions. In real river settings, flow‐bedform interactions are complicated by irregular morphology and mutual feedback mechanisms. This study demonstrates that an appropriately tuned roughness predictor may substantially improve flood prediction and flow capacity assessment in lowland alluvial rivers.