Sea-Level Rise Effects on Changing Hazard Exposure to Far-Field Tsunamis in a Volcanic Pacific Island

Coastal flooding exacerbated by climate change is recognised as a major global threat which is expected to impact more than a quarter of all people currently residing in Pacific Island countries. While most research in the last decade has focused on understanding the dynamics and impacts of future coastal flooding from extreme sea levels, the effects of relative sea level rise (RSLR) on exacerbating tsunami hazards are not well understood. Far-field or distant sourced tsunamis tend to have relatively lower impacts in Pacific Island states compared with locally sourced events, but there is limited understanding of how the impact of far-field tsunamis changes over time due to RSLR. Using the hydrodynamics software BG-Flood, we modelled the Tōhoku-oki tsunami from propagation to inundation in Samoa under incremental SLR to examine the effects that RSLR has on changing the exposure of the built environment (e.g., buildings) to a far-field tsunami. Outputs of maximum tsunami inundation and flow depth intensities which incorporate incremental SLR were then combined with digital representations of buildings and depth-damage functions in the RiskScape multi-hazard risk modelling software to assess the changes in building exposure over time. Results suggest that the impacts of Tōhoku-oki-type far-field tsunamis become significant once RSLR reaches 1 m above present levels. Present-day building exposure will increase by approx. 500% with 1 m RSLR by 2080–2130, and approx. 2350% with 2 m RSLR by as early as 2130–2140. These findings provide useful insights for application to tsunami hazard risk assessments under changing sea level conditions in analogous island environments.