Accommodating Spatially Varying Tidal Planes in Tsunami Hazard Assessments

Tsunami hazard assessment is commonly performed using a conservatively high sea level, typically the highest astronomical tide (HAT). However, HAT is not constant but varies from location to location. This is especially true in northern Australia such as along Queensland’s Coast where HAT varies from 1.4 m at Lady Elliot Island to over 5.4 m near St Lawrence. Picking any single tidal plane leads to over or under-estimation of the HAT tsunami inundation at most sites. We propose a simple yet effective technique to model tsunamis with a spatially varying HAT. It does not require separate simulations at multiple tidal levels and does not induce spurious flows due to an uneven sea surface. We demonstrate the accuracy of this method using the Gladstone region as a case study. The tests include comparisons with tsunami scenarios at different HAT levels, and comparisons of “mini” probabilistic tsunami hazard assessments at three sites with markedly different HAT levels: Lady Elliot Island, South Trees, and Rosslyn Bay. The “mini” probabilistic tsunami hazard assessment considers four major earthquake source zones and uses importance sampling to efficiently leverage offshore scenarios and return periods from the 2018 Australian Probabilistic Tsunami Hazard Assessment. Tsunamis are simulated globally from the earthquake source to the Gladstone region using nested grids with resolution down to a 10 m to resolve inundation and marine hazards. The proposed tidal adjustment technique allows tsunami hazard assessments to be conducted in regions where the chosen tidal plane is spatially inhomogeneous. In Gladstone, these methods produce inundation that is typically much smaller than currently used heuristics that are not derived from hydrodynamic models. These support tsunami evacuation plans and enable emergency management planners to refine the tsunami procedures for mariners and better focus procedures for onshore evacuation in the event of a land-warning tsunami. Presented at the 2025 Australasian Coasts & Ports Conference