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

Abstract

The threshold of motion of bioclastic sediments is the fundamental aspect for understanding of sediment dynamics in coral reef systems while there are currently few studies on its prediction. We conducted laboratory experiments, and showed that the threshold of motion of coral skeletal grains is more appropriately characterized by the nominal diameter and particle density that is defined as the density of grains with its skeletal void filled by the fluid. Distinctions in threshold of motion of observed coral particles and other bioclastic sediments arise from the influences of grain density and shape, resulting in a notable departure from existing empirical thresholds based on quartz sand. We then propose a new formula for estimating critical shear velocity of bioclastic sediments by introducing a grain shape parameter. The new comprehensive criterion improves the understanding of threshold of motion of bioclastic sediments with highly heterogeneous properties under steady unidirectional flow.