Spatiotemporal response of an Alpine braided river reach to snow melt and flood events

Alpine environments are responding to accelerated climate warming through the release and mobilization of large amounts of unconsolidated sediment. Sediment fluxes delivered to Alpine streams may be buffered, filtered and/or modulated as they pass through braided river reaches, which play a key role in the downstream transfer and dynamics of bed material. The functioning of these braided reaches is however still poorly understood, particularly during high magnitude events whose effects are very difficult to monitor but play an ever more prominent role in river system evolution.

In this study, we investigate the transfer of bedload material and river bed morphological change in a braided reach of the Séveraisse River (France), over the course of the melt season and two large flood events with an estimated return period of 5 and 50 years. To quantify braided reach dynamics, a multi-physical approach is employed that combines both temporally and spatially resolved techniques. We use bank-side geophones and locally derived parameters that describe seismic wave propagation in the subsurface to accurately quantify bedload transport and gain a unique insight in its temporal dynamics, particularly during the flood events. River bed elevation changes are determined from intermittent UAV-based LiDAR and photogrammetric acquisition. These are complemented with hourly (daytime) time-lapse images that register planform changes during the flood events.

Our results show strongly contrasting morphodynamic behavior with different flow conditions. During ‘normal’ bedload transport conditions driven by annual snow-melt, channel aggradation occurs leading to progressively lower bedload export from the reach for a given discharge. During the flood with a 5 year return period, which occurred at the end of the melt season, the braided riverbed morphology is rearranged and net sediment export took place. Most interestingly, in the autumn an extreme flood event led to the development of a single channel, meandering planform with significant outer bend erosion on alternating banks. Although this morphological change may be only temporary, i.e. a braided configuration may be expected to be gradually re-instated, it has important implications on the general functioning and morphological evolution of the reach and the downstream transfer of sediment.