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Article Dans Une Revue Earth Surface Processes and Landforms Année : 2010

Hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers

Résumé

Cheniers from Mont-Saint-Michel bay (France) are coarse shelly sand ridges migrating on the mudfl at up to the salt marshes where they accumulate and merge in a littoral barrier. In this macrotidal setting and low wave forcing, the cheniers are rarely submerged. However, they are found to move up to several metres during coincidence of spring tide and wave activity. Their processes of migration, morphology and internal structure (composition of the beddings, grain size, sorting and grain arrange- ment) are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment. This paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers: settling velocities of the shell fragments were measured using a 2 m long sedimentation tube. Thresholds of motion under unidirectional current, velocity and turbulence vertical profi les were characterized in a small recirculating fl ume using Laser Doppler Anemometry (LDA). The fl at-shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re-suspension effect of the fl ow when imbricated in a sediment bed. The shear stress in the bottom boundary layer has been measured in the viscous and log sub-layers. Nikuradse roughness heights (ks) for shell debris beds of different sizes have been quantifi ed. It is found that ks ≈ 2*56d50. This value is close to the ones used for classic rounded sand grains despite their major differences of shape. The dual behaviour of the shell fragments (low settling velocity, good resistance to unidirectional fl ow) should be considered as a key to understanding how this coarse material is transported across the tidal fl at, and fi nally accumulated as cheniers. Further fl ume experiments including wave activity and tidal fl uctuations are necessary to better quantify these complex processes.

Dates et versions

hal-00663244 , version 1 (19-01-2018)

Identifiants

Citer

Pierre Weill, Dominique Mouazé, Bernadette Tessier, J.C. Brun-Cottan. Hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers. Earth Surface Processes and Landforms, 2010, 35 (14), pp.1642-1654. ⟨10.1002/esp.2004⟩. ⟨hal-00663244⟩
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