Settling velocity and drag coefficient of platy shell fragments

Settling velocity of bioclastic particles in coastal and shallow marine environments is essential for interpreting depositional facies and processes. There is, however, a paucity of accurate formulae for predicting the settling velocities and drag coefficients of platy biogenic particles in particular. This study provides experimental settling data based on 320 platy shell fragments from a sediment core recovered in Li'an Lagoon, south-eastern Hainan Island, China. The results indicate that the settling velocities of platy shell fragments are strongly correlated with nominal diameters and Corey shape factors (ranging from 0.02 to 0.20 in this study). On this basis, a practical equation of acceptable accuracy was established for platy particles, relating dimensionless settling velocities to dimensionless diameters and Corey shape factors. Similarly, another empirical formula for quickly calculating the equivalent diameter of platy shell fragments in practice was proposed as well. Regarding the strong dependence of the drag coefficients using equivalent spherical diameters to Corey shape factors, the drag coefficient based on the diameter of the equivalent maximum projected area remains almost constant and is hence physically well-suited for the definition of grain drag coefficients. The settling data of this study has extended the lower Corey shape factors limit of bioclastic particles, and the equations presented here can be used for quantitative interpretations of sedimentary records, modelling of depositional processes and investigations of other platy particles.