Analysis of water and sediment flows over an alternate bar in a gravel bed river

Bars are present in most gravel bed rivers and exert a strong influence on flow and sediment processes. We focused on topographic changes of an alternate bar in a gravel bed river with high suspended sediment concentration. This phenomenon was studied on the river Arc (French Alps). The area of interest was approximately 600-meter long and quite straight with a gravel bar along the left side of the channel. Bar bed material was gravel and, due to high suspended sediment concentration in this river, fine sediments were found on the bed surface and included in the bed. To understand how water and sediment flows organized around this alternate bar, a series of measurements was performed in the proximity of the gravel bar during a flushing operation from upstream dams. Topographic surveys helps to identify deposition and erosion areas. During the operation that was equivalent to a one year period return flood, suspended sediment concentration, flow depth and water velocity were monitored. Field measurements were compared with two-dimensional hydrodynamic modeling which permitted to identify zones and times of high/low velocities.