A method for evaluating population and infrastructure exposed to natural hazards: tests and results for two recent Tonga tsunamis

Abstract

Background Coastal communities are highly exposed to ocean- and -related hazards but often lack an accurate population and infrastructure database. On January 15, 2022 and for many days thereafter, the Kingdom of Tonga was cut off from the rest of the world by a destructive tsunami associated with the Hunga Tonga Hunga Ha’apai volcanic eruption. This situation was made worse by COVID-19-related lockdowns and no precise idea of the magnitude and pattern of destruction incurred, confirming Tonga’s position as second out of 172 countries ranked by the World Risk Index 2018. The occurrence of such events in remote island communities highlights the need for (1) precisely knowing the distribution of buildings, and (2) evaluating what proportion of those would be vulnerable to a tsunami. Methods and Results A GIS-based dasymetric mapping method, previously tested in New Caledonia for assessing and calibrating population distribution at high resolution, is improved and implemented in less than a day to jointly map population clusters and critical elevation contours based on runup scenarios, and is tested against destruction patterns independently recorded in Tonga after the two recent tsunamis of 2009 and 2022. Results show that ~ 62% of the population of Tonga lives in well-defined clusters between sea level and the 15 m elevation contour. The patterns of vulnerability thus obtained for each island of the archipelago allow exposure and potential for cumulative damage to be ranked as a function of tsunami magnitude and source area. Conclusions By relying on low-cost tools and incomplete datasets for rapid implementation in the context of natural disasters, this approach works for all types of natural hazards, is easily transferable to other insular settings, can assist in guiding emergency rescue targets, and can help to elaborate future land-use planning priorities for disaster risk reduction purposes.

Our Ohonua: Post-disaster Revitalization Urban Design Concept for Eua Island, Tonga

“…. These islands are volcanic formations resulting from the subduction of the Pacific Plate beneath the Australian Plate, exposing the island population to significant geological hazards such as subduction megathrust earthquakes, explosive volcanism, landslides, and tsunamis (Thomas et al, 2023). On January 15, 2022, an underwater volcano in the Southwestern Pacific Ocean violently erupted, causing devastating consequences for the nearby islands of the Tonga archipelago.…”

“…While the tsunami's maximum amplitude generally remained under 1 meter, some islands close to the volcano experienced higher and more destructive waves, suspected to be linked to submarine landslide events (Lynett et al, 2022). This is extremely worrying as 74% of the existing buildings in Tonga are located below the 15 m coastal tsunami hazard line, with 61% of the population in Tonga-tapu, 'Eua, and Ha'apai islands residing below that contour line (Thomas et al, 2023). The preliminary satellite-derived damage assessment of 'Eua Island by the United Nation's Office for the Coordination of Humanitarian Affairs (OCHA, 2022) revealed that 'Eua Island, located 100 km away from the Hunga Tonga-hunga a'apai volcano, suffered damage along its west coast, particularly on the shoreline of Ohonua Town.…”