Dynamics of coral rubble on reef flats: from individual clast mobilisation to rubble strip morphological change

Rubble strips are shore-normal, elongated deposits on coral reef flats, composed of variably sized and shaped mobile coral clasts, which can contribute to the sediment supply on exposed low-lying coral reef islands. As climate change-driven ecological and physical disturbances increasingly impact coral reef systems and associated landforms, understanding the processes and controls governing their morphodynamics is critical. At an exposed intertidal rubble strip in Huvadhu Atoll, we examined: i) the morpho-sedimentological characteristics; ii) short-term (days-weeks) clast mobility using tracer experiments with in situ current and wave measurements; iii) intra-annual morphological changes using repeated UAV photogrammetric surveys; and iv) deposit age and rubble throughput using U Th dating to provide a chronological framework. This deposit is a modern reservoir of coral rubble (less than 250 years in age) which shows a lagoonward increase in clast age. Surface sediments displayed lagoonward surficial sorting with decreasing grain size ( D 50 = 5–45 mm) and a transition from tabular to branching clasts. Subsurface layers contained substantial sand fractions (>60%) indicating that these landforms are not exclusively coarse-grained. Tracer experiments revealed monthly clast mobility of 40–85% and displacements up to 16.6 m under modal hydrodynamic conditions (offshore H s = 0.48–2.09 m). Mobility increased with decreasing clast size, from tabular to massive to branching forms, and higher wave-current velocities. Morphological analysis indicated that 15–43% of the rubble strip experienced significant elevation changes bi-weekly to intra-annually, reflecting localised erosion and accretion. Although the net sediment volume of the strip remained stable, sediment was actively redistributed from the algal rim towards the lagoon. Overall, rubble strips are morphodynamically active landforms under moderate forcing conditions, with mobilisation controlled by size and shape attributes, and wave-current velocities.