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