From bedform evolution to channel slope fluctuations in steep-slope streams.
In mountain streams, bedload transport rates are prone to strong variability. Indeed an increasing number of laboratory and field studies in steep-slope conditions show fluctuations of bedload transport rate over several temporal and spatial scales. This behavior is even more pronounced for intermediate flow rates (i.e., small but frequent floods). Today, the characteristics and origins of these fluctuations are still poorly understood. One possible explanation of these fluctuations is that the formation and migration of bedforms greatly influence bedload transport.To accurately investigate bedload transport rates simultaneously with channel bed evolution, we designed an idealized laboratory experiment. A 3-m long, 8-cm wide, transparent flume was used with well-sorted natural gravel (d50=6.5 mm). A technique using accelerometers has been developed to record the arrival time of every particle at the outlet of the flume. An analysis of bedload transport rates on a wide range of time scales is thus possible. In addition, bed elevation is monitored using cameras from the side of the channel. First, we discuss the presence and the evolution of bedforms (antidunes in this study) for different sediment input rates, flume angles and flow rates. Particularly, in the stationary state, the larger the sediment input rate, the higher the mean migration velocity. Secondly, keeping steady flow rate and constant sediment input discharge, we observe various migration velocities and fluctuations of global channel slope. This celerity is faster when the channel slope is close to a critical angle. When channel slope is slightly above the angle of no-erosion, the local morphologies evolve slower. This shows that the channel bed behaves like a metastable system and bedform migration seems to be the key mechanism of erosion at the channel scale (like avalanches on a sandpile).