Threshold of Motion of Coral Reef Sediment under Currents in Flume Experiments

This study experimentally documented the thresholds of critical motion for coral sediments by conducting a total of 225 laboratory experiments that included the analysis of nine median sediment diameters and five water depths. A shape factor, Sf, which quantifies the shape of a single particle, was also extended to sediment as a whole and evaluated during the investigation. Both the critical Shields parameter and the incipient mean velocity were used to represent the critical thresholds of motion. The thresholds of motion for coral sediment were then compared with the results from quartz sand. The experimental results show that the critical threshold of motion is smaller for coral sediment than for quartz sand of the same median sieve diameter due to differences in particle shape in spite of the coral sediment being of higher density. For diameters higher than 1.425 mm, the critical Shields parameters and incipient mean velocities tend to have similar values to quartz sand. The differences between the two increased significantly for median sieve diameters <1.075 mm. Based on the experimental data, Shields curve bands and equations for the incipient mean velocity for sediments of differing shape factors were obtained for coral sediments as well as marine carbonate grains.