Field experiment on the dynamics of fine and coarse sediments over a gravel bar in an alpine river | Expérience de terrain sur la dynamique de sédiments fins et grossiers sur un banc de galets dans une rivière alpine

Alpine gravel-bed rivers are characterized by very poorly sorted sediments and significant grain sorting is generally observed on gravel bars. The grain size variability may significantly influence the inception of movement for each of the classes represented over a gravel bar. The purpose of this paper is to present some field measurements achieved on the Arc en Maurienne River, France. The main objective is to characterize the dynamics of fine and coarse sediments for conditions close to the incipient motion of gravels and for different degrees of clogging of the river bed. The field campaign was carried out on a gravel bar located 10 km downstream of St Jean de Maurienne during the dam flushing event on June 17th, 2014. Six patches including tagged pebbles (using PIT-tag) were built up with three different characteristics: clean patches formed with gravels and pebbles only, patches with a natural mixture where tagged pebbles were substituted to naturally arranged ones, and patches artificially clogged with fine sediments. These patches were located on the side of a secondary channel on the gravel bar. Surface grain size analyses of the gravel (using the Woolman method and photo analyses using BaseGrain) as well as topographic measurements were achieved the day before the event. During the flushing event, surface velocities over the gravel bar were measured thanks to video analysis (LSPIV). Water levels and mean slope in the secondary channel were measured thanks to pressure gauges positioned on the side of the secondary channel. An estimation of the bed shear stresses over the patches was thus possible all along the event. Intense water sampling was also carried out both upstream and downstream the secondary channel. During the postevent survey the day after, the PIT-tag search showed that the coarse particles did not move. On the other hand, a significant amount of fine sediments were deposited on the patches and infiltrated on the clean patches. Grain size analysis showed that deposited sediments were much coarser than sediments in suspension. An estimation of the fine sediment deposition rate and a discussion on the sediment dynamics is proposed from these measurements.