Thresholds of motion of shell debris under unidirectional flow: influence of faunal composition
Résumé
Understanding bottom sediment erodibility 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 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, Mutilus sp., Ruditapes philippinarum) and one introduced (Crepidula fornicata). A set of experiments have been performed in a small recirculating flume. Threshold of motion of the eight species under unidirectional current for several debris 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”. Depending on the species, the evolution of taucr with increasing grain diameters follows either an asymptotic or a more linear trend. Significant disparity of entrainment threshold is observed between species, which increases with particle sizes. Three groups can be discerned: Anomia ephippium, Crassostrea gigas and Ostrea edulis show the smallest tau_cr, Crepidula fornicata, Scrobicularia plana, Mutilus sp. for an intermediate tau_cr and Cerastoderma edule; Ruditapes philippinarum for the highest taucr. For example, for the sieve fraction 2-3.15 mm, Anomia ephippium is the first to move with a tau_cr of 0.38 N.m-2, and Cerastoderma edule shows the highest critical shear stress (tau_cr = 2.13 N.m-2). Oyster shells (Ostrea and Crassostrea sp.) are mainly composed of foliated sheets of calcite, sometimes interstratified with soft, porous and chalky material. These two structures present very different thresholds of motion (e.g. for 2-3.15 mm sieve fraction: 0.54 N.m-2 for the chalky material, and 1.08 N.m-2 for the foliated calcite sheets). Globally, the interspecies variations of hydrodynamic behaviour can be attributed to differences in shell density, shell structure, grain shape, or to a combination of these three parameters.