A numerical modeling approach for better differentiation of boulders transported by a tsunami, storm, and storm-induced energetic infragravity waves

Abstract

Coastal boulders are often indicators of past extreme wave events. In fact, the coastal boulder distribution induced by infragravity-dominated storm waves (energetic IG waves) may be similar to that induced by tsunamis; however, this assumption is yet to be investigated. We show that the factors responsible for generating energetic IG waves under storm scenarios are not identical to those affecting the boulders' transport distances. Our results indicate that the storm waves typically only transport boulders over short distances as compared to boulders deposited by tsunamis, even when energetic IG waves are being generated. When the dimensionless transport distance of a boulder (=transport distance of a boulder/offshore wave height) is less than 4.0 × 10 over planar topography and 3.0 × 10 over reef topography, both waves can potentially be responsible for the transport distance. In this case, whether a reasonably-sized storm or tsunami can explain a boulder location in a study area should be investigated through detailed numerical modeling. We found a clear relationship between the dimensionless transport distance of tsunami boulders and the Iribarren number, and it is plausible to directly estimate offshore wave height or wavelength from the tsunami boulder distribution and beach slope without numerical simulation.