An improved criterion for the threshold of motion of sand to gravel‐sized bioclastic carbonate sediments under steady unidirectional flow: Insights from coral skeletal grain experiments

ABSTRACT The threshold of motion of bioclastic carbonate sediments is fundamental to understanding sediment dynamics in coral reef systems, while there are currently few studies on its prediction. In this study, laboratory experiments were conducted on five sediment beds composed of coral skeletal grains coarser than 2 mm, subjected to steady unidirectional flow. Thresholds of coral grain motion, expressed as the critical Shields parameter and the critical Movability number, were compared with quartz‐based threshold curves and other carbonate bioclasts. By complementing existing datasets on fine carbonate grains with new experimental data on coarse coral skeletal grains, this study demonstrates that entrainment thresholds are consistently characterised by nominal diameter and particle density corrected for skeletal porosity, which reduces scatter in the Shields diagram. However, significant discrepancies remain, reflecting the limitations of Shields‐based criteria that neglect grain shape effects. In contrast, the critical Movability number, which incorporates settling velocity, produces closer fits to quartz‐based threshold curves than the Shields parameter and thus represents a more robust framework for threshold prediction. Building on this foundation, a new formula is proposed within the Movability number framework that explicitly incorporates a grain shape factor to estimate the critical shear velocity of sand to gravel‐sized bioclastic carbonate sediments. This criterion improves predictive accuracy and offers a mechanistic explanation for the dual hydrodynamic behaviour observed in carbonate bioclasts. This carbonate‐specific framework, grounded in coral skeletal grain experiments, advances understanding of sediment motion thresholds and provides a reliable basis for modelling reef‐island adjustment and carbonate shoreline dynamics under environmental change.