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Conference Papers Year : 2019

Hydrodynamic behaviour of mollusc shell debris: influence of faunal composition


Understanding the hydrodynamic behaviour of bioclastic particles is necessary to refine our interpretation of depositional environments in the fossil record and to improve predictive numerical models for coastal zone management. Many coastal sediments are partly composed of biogenic particles, which shapes and densities differ strongly from classic rounded quartz grains. This results in particular hydrodynamic behaviours. Characteristics of biogenic particles can also vary significantly between species. If numerous studies have investigated the hydrodynamic behaviour of bioclastic sediments derived from reef-dwelling organisms, there is a paucity of research focusing on “cool-water carbonate” bioclastic particles (i.e. mollusc shell debris, calcareous algae,...). The aim of the present research is to characterize the influence of faunal composition on the settling velocities and the entrainment threshold of mollusc shell debris from temperate regions. Shells have been sampled in the southern coast of Mont-Saint-Michel bay (Brittany, France) which is bordered by a coarse, shelly coastal barrier, before being ground and separated into individual sieve fractions. Eight species representative of the faunal composition in the area have been studied: four wild species (Cerastoderma edule, Scrobicularia plana, Anomia ephippium, Ostrea edulis) three reared species (Crassostrea gigas, Mutilussp., Ruditapes philippinarum) and one introduced (Crepidula fornicata). Settling velocities of the eight species have been measured in a settling tube for several debris sizes. A set of experiments have been performed in a small recirculating flume. Threshold of motion of the species under unidirectional current for different grain sizes have been characterized using an Acoustic Doppler Velocimeter Profiler. Critical bed shear stress values (tau_cr) were derived from velocity profiles in the boundary layer, by a logarithmic regression of the “law of the wall”. Results show consistent interspecific differences in settling velocity and critical bed shear stress, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Mollusc shells, according to their composition (calcite and/or aragonite), have either compact, foliated or a more complex structure. Debris derived from foliated shells (Anomia for example) moves earlier and settles slower than debris derived from compact shells (Scrobicularia and Crepidula for example). Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. Debris from species with dense compact shells behave like siliciclastic particles while particles from foliated (or chalky) shells show lower critical shear stress values. The results of the Movability number suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris.
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Dates and versions

hal-02166657 , version 1 (27-06-2019)


  • HAL Id : hal-02166657 , version 1


Alissia Rieux, Pierre Weill, Dominique Mouazé, Bernadette Tessier. Hydrodynamic behaviour of mollusc shell debris: influence of faunal composition. 34th IAS Meeting of Sedimentology, Sep 2019, Rome, Italy. ⟨hal-02166657⟩
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