Critical transition of incising gravel channel to evacuate alluvial lateral supply

ABSTRACT Incising gravel channels respond with a robust critical transition to the lateral alluvial supply. Previous experiments have studied how supply affects bedload transport, but I studied a setup with converging banks, where the supply–transport relation implies feedback. While increasing flows, a short range of transitional flows showed sharp increase of bedload, accompanied by channel response via longitudinal homogenization (connectivity) and maximum sediment storage. To slightly vary initial bed mixing (channel history), I repeated the experiments three times, which validated the robustness of the transition. This transition synchronized hydraulics and transport along the channel, leading to a wetted width consistent with downstream hydraulic geometry that allowed a critical sediment evacuation with minimum energy and bed alteration. For flows higher than the transitional, longitudinal connectivity persisted, as most of the landslide material was redistributed by fluvial action. Finally, the largest flows dissipated energy excess via coarsening and channel migration, akin to non-incising rivers with floodplain. A conceptual model of the geomorphic cycle of gravel river reaches might contain the proposed critical transition, but only if this transition proves to be robust to bank cohesion and to spatial and temporal size of the experimental setup.