Dynamics of Forced Alternate Gravel Bars Under Unsteady Flow: an Experimental Investigation

We investigated in this study the dynamics of forced alternate gravel bars under unsteady flow by laboratory experiments. We developed a reference case of two forced alternate gravel bars with the water worked methodology in an18 m-long and 0.7 m-wide inclined flume. Forced alternate bars were formed through sequential transport, erosion, and sedimentation cycles, which is a way to reproduce the surface and subsurface properties of gravel-bed rivers. The first unsteady flow experiment was performed over the reference case with a constant sediment supply rate maintained throughout the experiment. The second unsteady flow experiment was performed without sediment supply over the bed topography obtained at the end of the first unsteady flow experiment to test the impact of sediment supply. In both unsteady flow experiments, the first forced gravel bar became flatter and shorter, and the second forced bar disappeared when the water discharge approached its peak. The two forced alternate gravel bars redevelopedduring the falling limb of the hydrograph with different shapes. Bar height and wavelength evolutions under the hydrograph are well predicted by a theoretical model for steady flow.