Abstract
Reef islands are some of the most highly sensitive landforms to the impacts of future environmental change. Previous assessments of island morphodynamics primarily relied on historical aerial and satellite imagery. These approaches limit analysis to two-dimensional parameters, with no ability to assess long-term changes to island volume or elevation. Here, we use high-resolution airborne LiDAR data to assess three-dimensional reef island features for 22 islands along the north-western coast of Australia. Our primary objective was to utilize two regional LiDAR datasets to identify characteristics indicative of island sensitivity and future vulnerability. Results show reef platform area to be an accurate predictor of island area and volume suggesting larger island volumes may reflect (1) increased carbonate production and supply from the reef platform and/or (2) enhanced shoreline protection by larger reef platforms. Locations of foredune scarping (an erosional signature) and island orientations were aligned to the regional wind and wave climate. Reef island characteristics (island area, volume, elevation, scarping, and platform area) were used to rank islands according to sensitivity, using a new Island Sensitivity Characteristics Index (ISCi) where low ISCi indicates stable islands (large areas and volumes, high elevations, and fewer scarped areas) and high ISCi indicates unstable islands (small areas and volumes, low elevations, and more scarped areas). Comparison of two LiDAR surveys from 2016 and 2018 validates the use of 3D morphometrics as important (direct) measurements of island landform change, and can complement the use of 2D parameters (e.g., area) moving forward. Results demonstrate that ongoing use of airborne LiDAR and other 3D technology for monitoring coral reef islands at regional scales will enable more accurate quantification of their sensitivity to future impacts of global environmental change.
Highlights
Coral reef islands are low-lying sedimentary structures formed from the accumulation of unconsolidated carbonate sediments deposited onto or adjacent to atoll rims and coral reef platforms [1,2]
In the 2D Principle components analysis (PCA), island area and platform area were strongly correlated to the Island Sensitivity Characteristics Index (ISCi) values (0.9)
This understanding will be crucial for forecasting short and long-term trajectories of reef-island stability and a better quantification of island sensitivity thresholds to changing climate. The benefit of this approach is a rapid, broad spatial scale assessment of island sensitivity, and with further development, may be applied alongside ecological and ethnographic datasets to prioritize management outcomes of both island landforms, human economics, and the ecological value of their terrestrial and benthic coral reef ecosystems. This was the first systematic study to assess the geomorphic sensitivity of reef islands to changing metocean conditions using regional-scale LiDAR
Summary
Coral reef islands are low-lying sedimentary structures formed from the accumulation of unconsolidated carbonate sediments deposited onto or adjacent to atoll rims and coral reef platforms [1,2]. Photogrammetric analysis of unmanned aerial vehicle (UAV) imagery has been used to generate high-resolution digital surface models (DSMs) and orthophoto mosaics that provide detailed characterization of landscape topography, elevation, and volumetric change [6,7,8,9]. This allows small, discrete landforms, such as individual islands, to be readily mapped, and changes in island volume to be detected [7]. It is difficult to scale up these methods to cover larger regional areas
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