Investigating coral rubble dynamics through tilting base and flume experiments

Coral rubble refers to fragments of coral skeleton or reef rock that have broken down through mechanical or chemical processes on reefs. Coral rubble motion under waves can cause larval mortality, bury live corals, and disrupt binding, hindering reef recovery after disturbances. This study integrates tilting base tests (previously applied in sediment transport studies) with wave flume experiments to advance understanding of rubble motion thresholds. Tilting tests on 17 rubble pieces, conducted both in air and underwater, showed no significant difference in critical angles, supporting the use of in-air tests as a practical alternative to more resource-intensive underwater trials. Additional tilting experiments on 24 rubble pieces across three substrate types revealed a mean critical angle of approximately 40° for a 50 % probability of motion, irrespective of substrate characteristics. This suggests that rubble morphology, particularly cylindrical shapes, plays a dominant role by bridging substrate roughness and maintaining stable contact points. Threshold velocities and probabilities of motion for individual rubble pieces were predicted from critical angles and validated against wave flume experiments. A sensitivity analysis of lift force coefficients indicated that a value of approximately 0.35 provided the best agreement between predicted and observed probabilities of motion. Additionally, the sheltering effect of the upwave rubble were examined, revealing that the probability of motion was reduced by 22 % and 50 % for a two-diameter and a one-diameter spacing, respectively. Overall, the findings of this study contribute to risk management frameworks for predicting thresholds of rubble mobilisation, identifying unstable reef areas, assessing recovery potential, and designing targeted restoration interventions. • Rubble morphology dominates motion thresholds over substrate effects. • Sheltering can reduce the probability of rubble motion by up to 50%. • Improves rubble stability estimation for reef-management. • Extends the tilting-base method to investigate coral rubble dynamics. • Validates in-air tilting tests as a practical alternative to underwater testing.