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Effects of horizontal pressure gradients on bed destabilization under waves

Published online by Cambridge University Press:  05 January 2017

C. Berni ,
H. Michallet  and
E. Barthélemy
C. Berni
Affiliation:
University Grenoble Alpes, CNRS, LEGI, CS40700, 38058 Grenoble, France Irstea, UR HHLY, 5 rue de la Doua, CS 70077, 69626 Villeurbanne CEDEX, France
H. Michallet*
Affiliation:
University Grenoble Alpes, CNRS, LEGI, CS40700, 38058 Grenoble, France
E. Barthélemy
Affiliation:
University Grenoble Alpes, CNRS, LEGI, CS40700, 38058 Grenoble, France
*
†Email address for correspondence: herve.michallet@legi.cnrs.fr
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Abstract

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.

JFM classification

Boundary Layers: Boundary Layers Geophysical and Geological Flows: Coastal engineering Geophysical and Geological Flows: Sediment transport
Type
Papers
Information
Journal of Fluid Mechanics , Volume 812 , 10 February 2017 , pp. 721 - 751
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Copyright
© 2017 Cambridge University Press 

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