The Reef Island Geomorphic Activity Assessment: A new approach to quantify cay geomorphic change

Cays (low-lying reef islands) are dynamic, unconsolidated sedimentary landforms which adjust their shape and position on a reef in response to hydrodynamic conditions and sediment supply. Quantifying meaningful cay geomorphic change is necessary to understand their natural variability and detect change patterns and trajectories. Shoreline movements on cays have been quantified globally using the Digital Shoreline Analysis System (DSAS) which records shoreline movements at regularly spaced shore-normal transects. However, DSAS was developed for relatively straight coasts and is less suited to cays which have 360° shorelines. Here we introduce the Reef Island Geomorphic Activity Assessment (RIGAA), an alternative approach to quantify changes to cay area and shape, orientation, and position that uses the entire cay footprint. To compare the RIGAA to the DSAS, we used digitised cay shorelines captured monthly for three cays on the Great Barrier Reef to quantify cay geomorphic change over almost a decade (2015–2023). Both approaches yield comparable assessments of net cay shoreline change but the DSAS overall Net Shoreline Movement metric suggests progradation (4.14–13.12 m) dominates at all cays whereas the RIGAA indicates more diverse behaviours, including shoreline contraction (Taylor Cay −32.8 %), expansion (Bushy Islet +22.9 %) and stability (Masthead Island +2.9 %). The RIGAA approach accounts for the frequency and magnitude of shoreline perturbations and provides a comprehensive assessment of cay morphodynamic behaviour applied to the entire cay footprint. The outputs provide meaningful metrics for a range of users, including key information about overall cay area and morphological change, and movement.