Morphological Response of an Alpine Braided Reach to Sediment-Laden Flow Events

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGEInternational audienceBraided gravel-bed rivers show characteristic temporal and spatial variability in morphological change and bedload transport under steady flow and sediment supply rates. Their morphodynamic behavior and long-term evolution in response to nonstationary external forcing is less well known. We studied daily morphological changes in a well-constrained reach of an Alpine braided river that is subject to regulated sediment-laden flows, associated with hydroelectric power exploitation, as well as occasional floods. We found that net reach erosion and deposition were forced by upstream sediment supply, albeit in a nonlinear fashion. The spatial distribution of morphological change and inferred spatially-distributed sediment transport rates varied strongly along the braided reach and between successive sequences of flushing. Local morphological change was driven by two factors: (1) local relief, leading to the preferential filling of topographic lows and erosion of highs, particularly during longer duration floods, which allow braided dynamics to be maintained; and (2) system memory, leading to a negative autocorrelation in bed level changes where erosion was followed by deposition of similar magnitude and vice versa. This effect was associated with the temporary storage of high sediment loads from flushing due to the abrupt on-off nature of these flows and reveals the relatively efficient transport of sediment in a river that is heavily impacted upon by flow abstraction. In general, the internal morphodynamics of the braided river condition their own response to external forcing events and thus sediment transfer