Examining the impact of the Great Barrier Reef on tsunami propagation using numerical simulations

Abstract

Coral reefs may provide a beneficial first line of defence against tsunami hazards, though this is currently debated. Using a fully nonlinear, Boussinesq propagation model, we examine the buffering capacity of the Great Barrier Reef against tsunamis triggered by several hypothetical sources: a series of far-field, Solomon Islands earthquake sources of various magnitudes (Mw 8.0, Mw 8.5, and Mw 9.0), a submarine landslide source that has previously been documented in the offshore geological record (i.e. the Gloria Knolls Slide), and a potential future landslide source (i.e. the Noggin Block). We show that overall, the Great Barrier Reef acts as a large-scale regional buffer due to the roughness of coral cover and the complex bathymetric features (i.e. platforms, shoals, terraces, etc.) that corals construct over thousands of years. However, the buffering effect of coral cover is much stronger for tsunamis that are higher in amplitude. When coral cover is removed, the largest earthquake scenario (Mw 9.0) exhibits up to a 31% increase in offshore wave amplitude and estimated run-up. These metrics increase even more for landslide scenarios, where they tend to double. These discrepancies can be explained by the higher bed particle velocities incited by higher-amplitude waves, which leads to greater frictional dissipation at a seabed covered by coral. At a site-specific level, shoreline orientation relative to the reef platforms also determines the degree of protectiveness against both types of tsunamis, where areas situated behind broad, shallow, coral-covered platforms benefit the most. Additionally, we find that the platforms, rather than gaps in the offshore reef structure, tend to amplify wave trains through wave focussing when coral cover is removed from simulations. Our findings have implications for future tsunami hazards along the northeastern Australian coastline, particularly as the physiological stressors imposed by anthropogenic climate change further exacerbate coral die-off and reductions in ecosystem complexity. Therefore, areas that experience a protective benefit by the Great Barrier Reef’s platforms could be disproportionately more vulnerable in the future.

Numerical simulation of submarine landslides and generated tsunamis: application to the on-going Mayotte seismo-volcanic crisis

“…wave propagation was simulated in mean high water springs (+1.92 m at Dzaoudzi to the Mayotte vertical datum IGN1950 following the RAM 2017), which, in most cases, reduces the protective effect of the reef[Thran et al, 2021], compared to the other reference tide levels (e.g. mean high water neaps or mean tide level with values of 1.02 and 0.35 m/IGN1950 respectively),• a global island subsidence of 0.15 m linked to the deflation phenomenon that has been observed since summer 2018[Cesca et al, 2020, Lemoine et al, 2020a, without taking into account the west-east differential visible in GNS measurements, • two DTM (Digital Terrain Model) with spatial resolutions of 50 m and 10 m were used.…”