Seasonal bedload pulses in a small alpine catchment

Abstract

Natural fluctuations of bedload transport under steady hydraulic conditions have been often attributed to the propagation of sediment pulses in river channels, leading to hysteretic patterns in bedload rating curves at both the hydrograph and the seasonal timescales. However, direct field observations of the sediment pulse effect on bedload transport rates have been rarely reported. Two bedload datasets recorded in a small Mediterranean badland catchment of the Southern French Prealps (Moulin Ravine at Draix) are used here to investigate the effect of sediment pulses originating from hillslopes on the bedload response to flow conditions. The first dataset comes from a long-term erosion monitoring program providing time-integrated bedload yields since the late 1980s by means of a large sediment trap. The second dataset is based on a slot sampler providing bedload fluxes at a 1-min time resolution since autumn 2011. Sediment pulses have been tracked with regular scour chain surveys along the alluvial channel upstream of the bedload monitoring station. Both datasets revealed a seasonal counterclockwise bedload hysteresis which can be attributed to a yearly sediment pulse reaching the catchment outlet during autumn and early winter. We believe that these seasonal bedload pulses could be typical of small alpine catchments with active gully erosion on bedrock sensitive to frost-cracking processes.