Variability of shoreline water levels on an atoll in the Maldives

Many atolls in tropical oceans are under the threat of sea-level rise. Mitigation planning requires reliable estimates of increasing wave runup in response to sea-level rise and changes in wave climate. However, shoreline dynamics on reef-lined coasts, particularly the effect of low-frequency waves, are not yet fully understood. Herein, we present a field study on the southernmost atoll of the Maldives to clarify the variability of shoreline water levels driven by waves and tides. The atoll is located just south of the equator and has a moderate wave climate; it is occasionally impacted by long-distance swells from the South Indian Ocean. Wave evolution on a reef-lined coast was observed in situ for a period of seven months. The shallow water depth tightly constrains reef-flat waves under normal wave conditions; however, this constraint is broken under extreme conditions by low-frequency waves generated from long-period swells. Particularly, very low-frequency waves (<0.005 Hz) are amplified via wave resonance over the reef platform and significantly contribute to shoreline motions. Our observation of shoreline motions confirmed that, under energetic conditions, extreme wave run-ups are characterized by swell run-ups riding on crests of low-frequency waves. A combined approach of observations and analytical modeling revealed the long-term variability of shoreline water levels, highlighting the stochastic nature of extreme water levels influenced by a joint probability of far-field swells and high tides. Our results suggested that extreme shoreline water levels would increase at a significantly higher rate than sea-level rise in the future.