Effects of horizontal pressure gradients on bed destabilization under waves

International audienceWe report on new experiments designed to investigate bed destabilization processes ina two-dimensional wave flume physical model of a beach. The mobile bed consists ofnon-cohesive granular material of low density. The wave conditions are provided byrepeating a cycle of waves made of two bichromatic groups of different period. Thehorizontal and vertical velocities are acoustically profiled vertically from free-streamelevation down to the still bed level in the surf zone. Additional measurementsof the fluid pressure at positions closely aligned horizontally and vertically in andslightly above the sediment bed are undertaken. Mobile bed interfaces, still bedand top interface, are detected via acoustic and optical methods. Both methods arecross-compared and give similar results. Flow turbulence over the bed is analysed,the Reynolds turbulent shear stress is found negligible compared to the orbital flowinduced momentum diffusion. The shear stress and the horizontal pressure gradientare computed at near-bed elevation and used in the bed incipient plug flow model ofSleath (Cont. Shelf Res., vol. 19 (13), 1999, pp. 1643–1664). Both the model and themeasurements confirm that destabilization occurs when the non-dimensional pressuregradient (or Sleath number) exceeds the threshold value of 0.3 which is simultaneouswith strong flow acceleration. The near-bottom fluid shear stress detected during theseflow accelerations at steep wave fronts is found experimentally to be negative, whichis retrieved with an unsteady plug flow model. This is suggesting that the fluid abovethe bed resists the sediment layer motion at these particular phases