Numerical analysis of boulder transport by the 2004 Indian Ocean tsunami at Pakarang Cape, Thailand

At Pakarang Cape, Thailand, we conducted field observations and numerical modeling of the spatial and grain size distributions and sources of boulders deposited by the 2004 Indian Ocean tsunami; the boulders are a very rare example of boulders for which a tsunami origin is well confirmed. Numerical modeling of the tsunamiʹs boulder transport revealed that the grain size and spatial distributions of boulders are explainable by the initial distribution of boulders at the source and the tsunamiʹs local behavior, which is controlled strongly by the tsunami hydrodynamic features and local topography. Soon after the bore front reached at the beach, the tsunamiʹs hydraulic force on the tidal bench decreased suddenly below the critical force necessary to displace the boulder because of the seaward propagation of the reflected wave that had been generated at the beach slope. Consequently, all boulders were deposited below the high-tide line and subsequently concentrated at the western coast of the cape. Absence of boulders that had been originally deposited deeper than 10 m water depth is also explained by this reflected wave because the wave passed over boulders that had been originally deposited at 10 m water depth before the boulder crossed the reef edge. Our well-tuned model elucidates how the tsunami was able to deposit large boulders on the tidal bench at the cape. The results demonstrate that, theoretically, up to 250 tons of boulders might have been emplaced on the tidal bench by the 2004 Indian Ocean tsunami at the cape (approximately 7 m wave height). The calculated maximum weight is of comparable order to that of boulders around the world that are inferred to be of tsunami origin, although we infer that the weight is determined based on the sensitive balance of the local topography, hydrodynamic features of the tsunami, and the initial distribution of boulders at the source.