Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition

Alissia Rieux (1) , Pierre Weill (1) , Dominique Mouazé (1) , Clément Poirier (1) , Farid Nechenache (1) , Laurent Perez (1) , Bernadette Tessier (1)

1 M2C - Morphodynamique Continentale et Côtière

Bioclastic particles derived from mollusc shell debris can represent a significant fraction of sandy to gravelly sediments in temperate and cool‐water regions with high carbonate productivity. Their reworking and subsequent transport and deposition by waves and currents is highly dependent on the shape and density of the particles. In this study, the hydrodynamic behaviour of shell debris produced by eight mollusc species is investigated for several grain sizes in terms of settling velocity (measurements in a settling tube) and threshold of motion under unidirectional current (flume experiments using an acoustic profiler). Consistent interspecific differences in settling velocity and critical bed shear stress are found, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Drag coefficients are proposed for each mollusc species, based on an interpolation of settling velocity data. 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. The results 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. The influence of the flat shape of bioclastic particles on the initiation of motion under oscillatory flows and during bedload and saltation transport is yet to be elucidated.

Mots clés: Bioclastic sand; critical bed shear stress; shell density; scanning electron microscopy images (SEM); flume experiments; unidirectional flow

Domaines: Sciences de la Terre; Interfaces continentales, environnement; Stratigraphie

Alissia Rieux, Pierre Weill, Dominique Mouazé, Clément Poirier, Farid Nechenache, et al.. Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition. Sedimentology, 2019, 66 (3), pp.895-916. ⟨10.1111/sed.12521⟩. ⟨hal-01897637⟩