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During high river discharge extremes, the growth of dunes can reach a maximum beyond which a transition to upper stage plane bed may occur, enhancing the river's conveyance capacity and reducing flood risk. Our predictive ability of this bedform regime shift in rivers is exclusively built upon high Froude number flows dominated by asymmetric dunes with steep downstream-facing slipfaces that are rare in natural rivers. By using light-weight polystyrene particles as a substrate in an experimental flume setting, we present striking dune morphodynamic similarity between shallow laboratory flow conditions and deep rivers, preconditioned that both flow and sediment transport parameters are accurately scaled. Our experimental results reveal the first observation of upper stage plane bed in a shallow laboratory flume that is reached for a Froude number well below unity. This work highlights the need to rethink widely used dune scaling relationships, bedform stability diagrams, predictions of flow resistance, and flood risk.

Plain Language Summary Dunes are rhythmic shapes at the river bed that significantly increase water levels and the associated flood risk. They can reach heights up to one third of the water depth and often dominate the flow field. Under extremely high river discharges, dunes are observed to undergo a transition phase, after which they are washed out and disappear from the river bed. This morphological transition is expected to substantially reduce water levels and thus flood risk, by enhancing the conveyance capacity of the river during a peak discharge event. Once such circumstances can be predicted with certainty, it will become possible to meet safety standards while reducing the height of the river embankment.