Runup amplification of full-stroke and dam-break waves on a sloping beach in shadow zone by an offshore island

The Mentawais tsunami of 2010 was unusual in that it highlighted the effect of islands on the flood of beaches in the shadow zones behind them. Post-tsunami surveys of shorelines in the lee of offshore islands showed tsunami runup amplification (TRA), a higher degree of inundation relative to open coastal shorelines, in direct opposition to widely-held beliefs of the local population. Experiments of tsunami wave propagation around conical islands conducted at the Directional Wave Basin at Oregon State University revealed that, in general, there was an inverse relationship between island distance to the shoreline and the TRA with full-stroke waves. However, a noticeable phenomenon we found is that a dam-break wave (a tsunami propagating as a dissipative bore prior to reaching the island) results in a proportional relationship between the quantities in question. The non-hydrostatic wave model NHWAVE was used to help elucidate the reasons behind the relationship between TRA and island distance. To simulate the nonperiodic full-stroke and dam-break waves, a transfer function based on a continuous wavelet transform is used; this function relates a time series of free surface elevation to horizontal velocities, under the assumption of linear dispersion at the wave generating boundary. NHWAVE model results showed improvement in predicting TRA by full-stroke and dam-break waves in comparison with FUNWAVE-TVD Boussinesq model results. It is hypothesized that TRA is subject to the lateral (alongshore) velocity field formed by refraction and diffraction generated by the offshore island. It is believed that NHWAVE is more skillful in replicating these velocities over the steep slope of the island. Model validation and additional insights from the model are also discussed.