Sediment supply control on morphodynamic processes in gravel-bed river widenings

Abstract

Reach-scale widening of channelized rivers can locally reactivate morphodynamic processes and increase habitat heterogeneity. Following process-based restoration principles, construction efforts are often limited to bank protection removal and local bank erosion initiation. Despite significant advances in our understanding of fluvial processes, the influence of sediment supply on widening morphodynamics is still inadequately understood. Using mobile-bed laboratory experiments, we studied the morphological development of river widenings in channelized subalpine gravel-bed rivers in response to different sediment supply levels. The initial setup included a channel with a longitudinal slope of around 1% and a floodplain on one side, which was 25 channel widths long and three wide. After removing the fixed bank, the floodplain was available for erosion and channel shifting. Seven experimental series with sequences of steady bed-forming discharge and unsteady larger floods were conducted. Sediment was supplied at rates of 20%, 60%, 80%, and 100% of the initial channel's transport capacity. The laboratory experiments were combined with two-dimensional hydrodynamic numerical simulations to closely track both the morphological development and the resulting flow conditions. Sediment supply was found to be a critical driver of morphodynamic activity in river widenings, with sediment supply rates close to channel transport capacity promoting active channel widening, sediment relocation, and lateral channel-floodplain connectivity. In contrast, low sediment supply maintained a stable, single-thread channel disconnected from its floodplain. Further, our results indicate that an increase in sediment supply or structural measures triggering local flow deflection can reactivate morphodynamic processes in previously sediment-starved reaches after a transition period of several years to decades. Our results highlight the fundamental importance of sediment transport in reach-scale river restoration.