Threshold of motion of bivalve and gastropod shells under oscillatory flow in flume experiments

Abstract

The threshold of motion of non‐fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat‐bottom setup under regular non‐breaking waves (swell) produced by a flap‐type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat‐concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current‐driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation.

Settling velocity and drag coefficient of platy shell fragments

“…() and Fick et al . () found that thresholds of motion of bioclastic particles are within or below the threshold envelope for siliciclastic sediment when considering particle sieve diameters. Because bioclastic particles have significantly lower settling velocities than siliciclastic sediments with similar entrainment thresholds, once such a platy shell fragment is initiated, it can be transported further than the sand particle with comparable critical bed shear stress.…”

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

“…The threshold of deposition of marine biota varies significantly between species, since bioclastic particles derived from them have varied densities and shapes (Rieux et al ., 2019). Previous flume tank experiments found the shear stresses at threshold of motion for both broken and non‐fragmented bioclastic particles were an order of magnitude lower or similar to siliciclastic particles of the same sieve diameter, due to their irregular shapes (Komar & Li, 1986; Flemming, 2017; Joshi et al ., 2017; Fick et al ., 2020). Threshold stresses are also typically larger by up to a factor of three for more poorly sorted sediments (McCarron et al ., 2019).…”

On the settling of marine carbonate grains: Review and challenges

“…Several types of density circulate in the literature (Fig. 5), some with contrasting definitions, including bulk density (Oehmig, 1993), dry bulk specific density (Weber et al, 1969), bulk skeletal density (Sadd, 1984), skeletal density (Morgan and Kench, 2014), solid density (Allen, 1984), mineral density (Milan et al, 1999), material density (Smith and Cheung, 2003), sediment density (Prager et al, 1996;Paphitis et al, 2002;Joshi et al, 2014;Fick et al, 2020), sediment grain density (LaBarbera, 1981), grain density (Kench and McLean, 1997), particle density (Kench and McLean, 1997;Cuttler et al, 2017), object density (Caromel et al, 2014), average sediment density, (Neumann and Land, 1975) sphere-equivalent density (Michels, 2000), average density (Diedericks et al, 2018;Li et al, 2020), apparent density (Jorry et al, 2006), relative density (Dey, 2003) effective density (Wanless et al, 1981;Fok-Pun and Komar, 1983;Kench and McLean, 1996) and specific gravity determination (Jell et al, 1965) (Table S1 in Supplementary Data). Some of these definitions are the same, others use the same term but wield different definitions.…”

“…Several studies have investigated the link between settling velocity and entrainment properties of carbonate grains. For example, taphonomic features, such as symmetry and concavo-convexity, control the settling mode and entrainment thresholds of single bivalve shells (Allen, 1984;Olivera and Wood, 1997;Rieux et al, 2019;Li et al, 2020;Fick et al, 2020). Weill et al (2010) emphasised the dual nature of shell fragments, which exhibit low settling velocities in combination with a high resistance to flow friction once deposited (bed roughness).…”

“…Once the simplified case of the settling of single carbonate grains in quiescent water is adequately understood, multiphase settling problems can be studied with numerous bioclastic particles falling together while interacting with one another (Wang et al, 2018). These issues are requirements for the ultimate future prospect of solving particle transportability problems of carbonate sediments, clarifying the relationship between settling velocity and shear velocity (Briguglio et al, 2017), suspension and saltation mechanisms (Fick et al, 2020), and the hydrodynamic behaviour of carbonate sediment concentrations in suspension (Hodson and Alexander, 2010).…”