Multiple scale analysis of bed mobility in a highly modified system (Durance River, South-Eastern France)

International audienceThe Durance River is a gravel-bed river deeply impacted in its downstream part by gravel mining and flow diversion. The modification of the flow regime and the gravel extractions have led to a significant reduction of the bankfull channel width (from 50 to 80% for the 1960-2000 period), a river bed degradation and, above all, a vegetation encroachment associated to an important overbank sedimentation. These changes result in a modification of flowing conditions during floods and an increase in flood risk because the lateral mobility is not high enough to rework regularly the vegetated landforms. Therefore lateral mobility processes and rate have to be better understood and quantified.We work on three different spatial scales (bar, bend and reach scales) in order to characterize and quantify the lateral mobility. First of all, at fine scale, we use PIT Tags and an event topographic survey to study the entrainment threshold variability of the particles of the bar and the bank toe. Then the link between discharge, lateral bank mobility and vertical channel mobility is investigated at bend scale thanks to event topographic survey and physical characterization of the bed material (size and shape). Finally, the lateral mobility quantification at reach scale requires both a diachronic study on orthophoto and an in situ topographic survey. The first results highlight not only a coupled mobility between bank and channel but also a high spatial variability of lateral mobility linked to both local hydrodynamic conditions and geographical position of sediment sources