Remote Sensing of a Shallow, Fringing Reef Platform for Analysis of Island Sector Susceptibility and Development of a Coastal Vulnerability Index

Like many islands in middle latitudes, Pacific islands are often fringed by protective coral reefs that attenuate wave energy impacting the coastline. Measurement of the extent and distribution of shallow reef platforms by remote sensing of bathymetry enables an assessment of the relative exposure or protection of each part of an island. Fringing reefs are one of a number of physical variables relevant to the coastline's susceptibility to erosion and inundation resulting from extreme weather and conditions exacerbated by climate change and rising sea levels. In this research the extent of shallow reef platform around Lifuka Island, Tonga, was determined using a modified Stumpf band ratio method applied to Landsat 8 and high-resolution Quickbird satellite imagery, with aerial LIDAR data available for ground-truth purposes. The results indicate that both sets of satellite imagery provide a means of measuring the bathymetry to at least 15 m with a correlation (R²) value of 0.87 for Landsat 8 and to 15 m with a correlation (R²) value of 0.86 for Quickbird, when compared with the LIDAR survey. This article forms part of the research into the preparation of a coastal vulnerability index to inform decision-making and risk assessment for Pacific Island nations at the subisland scale. The mapping of the fringing reef by remote sensing allows its spatial distribution to be considered in the assessment of risk to coastal regions to inundation and erosion processes. Further, it demonstrates the value of freely available Landsat data for planning adaptation to climate change in developing nations at a level pertinent to local governance of villages and communities.