Applying Directional Filters to Satellite Imagery for the Assessment of Tropical Cyclone Impacts on Atoll Islands

摘要

This paper highlights the major insights provided by directional filters using ENVI (R) software for detecting and mapping cyclone-generated accretional features and analyzing postcyclone sediment reworking. It relies on the study of the impacts of category 5 tropical cyclone Fantala (April 2016) on Farquhar Atoll, Seychelles Islands. Using directional filters first allowed the detecting and mapping of terrestrial (sediment sheets, sediment lobes) and intertidal (shingle and rubble tongues, sediment sheets, sandbars) depositional features. It also helped with highlighting spatial-temporal variations in the postcyclone reorganization of reef flat deposits. One year after the cyclone, along the rectilinear shoreline of elongate islands (i.e. North Island and South Island), massive reef-to-island sediment transfer had already caused the dismantling of cyclone-generated features and beach readjustment; however, in places, newly formed cyclone features exhibited limited dismantling. In contrast, at the island tips, the reworking of postcyclone sediment deposits was still ongoing, explaining still rather limited beach readjustment. The results advocate for the use of remote sensing techniques that complement fieldwork and multidate shoreline change assessment to promote more comprehensive analyses of tropical cyclones' impacts.