Diagnosis of historical inundation events in the Marshall Islands to assist early warning systems

Inhabitants of low-lying coral atolls benefit from disaster risk reduction decision makers receiving early warnings of coastal inundation leading to heightened levels of alert and preparedness. Majuro, the capital of the Marshall Islands, is a coral atoll that experiences coastal inundation events on a near annual frequency and is likely to be exacerbated by sea-level rise, increasing the importance of early warning systems. However, current early warnings are not always provided for every inundation event. Inundation is driven by a combination of various oceanographic processes that contribute to sea level at the coastline, with the primary driver dependent on how extreme a particular process may be at the time. Incoming swell from distant storms and cyclones can trigger an inundation event, especially when coinciding with high spring tides and/or sea-level anomalies. Historical data from three directional scenarios were analysed to determine the critical values for offshore wave height, peak period, directional range, and sea level that had led to inundation in the past. Bulk wave statistics and static sea level were found to be sufficient information to identify the occurrence of an inundation event. These inundation thresholds serve as a reference to be used in conjunction with forecast models as an analogue for future events informing both the likelihood and impact. The analysis showed that inundation with a significant contributing swell factor propagates via three main routes, with approximately 50% occurring from the north-east. The two highest sea-level measurements on record both occurred during La Niña events, with both leading to inundation, suggesting that spring tides during La Niña events should exhibit a heightened level of alert for inundation at Majuro regardless of swell contribution.