Threshold of motion and orientation of bivalve shells under current flow

Abstract

Disarticulated shells of three bivalve mollusk species (Anomalocardia brasiliana, Codakia orbicularis, and Divaricella quadrisulcata) were experimentally tested in laboratory flumes to determine the threshold of motion and final orientation of the valves. A total of 150 current flow experiments were conducted on single shells resting on a fixed sand bed. This study demonstrated that shells in the convex-up position are more resistant to flow when the umbo is pointing downstream rather than upstream. Moreover, species with higher frontal areas were more likely to be entrained at lower flow velocities. Results of dimensionless shear stress exhibited values far below the threshold of grains movement for beds of uniform roughness (Shields curve). It was observed that circular shells in convex-up positions were mostly orientated with the umbo pointing downstream. Conversely, elliptical shells in convex-up position tended to align their longer axis parallel to the flow with the posterior side of the valve pointing downstream. These results are not only directly applicable in interpretations of incipient shell motions and in paleocurrent analyses from field and sample data, but also support construction of accurate geological models.

A Comprehensive Criterion for Threshold of Motion of Bioclastic Sediments under Steady Unidirectional Flow

“…Figure S3. Correlation between the nominal diameter 𝑑 and the sieve dimeter 𝑑 of investigated coral particles, calcareous sand (Smith and Cheung, 2004), coral sediment (Bian et al, 2023), and bivalve shells (Silva et al, 2023). Coral debris (Present study) Calcareous sand (Smith and Cheung, 2004) Coral sediment (Bian et al, 2023) Bivalve shell (Silva et al, 2023) d n (mm)…”

“…An obtained linear relationship 𝑑 𝑛 = 1.34𝑑 𝑠𝑣 (R 2 =0.99, Figure S3) was used to estimate 𝑑 𝑛 of carbonate sand reported by Prager et al (1996). The 𝑑 𝑛 values of mollusc fragments (Paphitis et al, 2002;Weill et al, 2010;Rieux et al, 2019) were estimated using 𝑑 𝑛 = 0.62𝑑 𝑠𝑣 (R 2 =0.99, Figure S3) based on the shelly particle data of Silva et al (2023). As seen in the figure, 𝜃 cr of coral particles are plotted below the empirical curves (Shields, 1936;Paphitis, 2001) when 𝜑 = 0 is applied.…”