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.