Short-term morphological response of rubble coral islands to the impact of a small tropical cyclone

Claudia Le Quesne, Thomas Edmund Fellowes , Lara Talavera, Ana Paula da Silva, Lachlan Perris, Ratneel Deo, Ana Vila-Concejo 

Tropical cyclones (TCs) are extreme storm events with the potential to cause significant damage to coral reef and island ecosystems. The evolution of coral rubble (shingle) islands within these ecosystems relies on the complex eco-morphodynamic relationship between the supply of biogenic sediment from the reef and subsequent transport by hydrodynamic forces. Storms have the potential to alter this relationship, posing a potential threat to rubble islands structure and stability with the forecasted increase of the intensity of storms in the Southwest Pacific with climate change. Traditional methods of monitoring the impacts of storm focus on long-term effects, often utilising field-based approaches. Here we use unpiloted aerial vehicles (UAVs) and remotely sensed data as a cost-effective method to provide high-resolution spatial data to understand short-term effects of tropical cyclones, and the resulting storm generated waves. We observed the impacts of TC Gabrielle (2023) on One Tree Reef within the Southern Great Barrier Reef and quantified the changes of One Tree Island (OTI), a well-developed rubble island (~5.9 Ha), and Two Tree Island (TTI), a developing rubble island (~0.1 Ha). At both islands, shoreline length decreased (-7.3% TTI, -0.5% OTI) and total rubble volume decreased (-1.8% TTI, -0.3% OTI). The rubble tracts attached to OTI prograded towards the island as result of the TC and distributed sediment along its shore. This study presents high-resolution data for remote rubble islands and incorporates a detailed volumetric analysis complementing traditional planimetric methods for the monitoring of remote islands.