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- Title
A Numerical Modeling Approach for Better Differentiation of Boulders Transported by a Tsunami, Storm, and Storm‐Induced Energetic Infragravity Waves.
- Authors
Watanabe, Masashi; Goto, Kazuhisa; Roeber, Volker; Kan, Hironobu; Imamura, Fumihiko
- 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. Plain Language Summary: Coastal boulders transported by the tsunami and storm waves have been used to reconstruct the sizes and recurrence intervals of past extreme waves. Differentiating tsunamis from storm boulders is essential for revealing the characteristics of past coastal disasters. However, as the hydraulic characteristics of the tsunami and infragravity‐dominated storm waves (energetic IG waves) are similar, the distribution characteristics of these boulders are also likely to be similar. This makes identifying tsunami and storm boulders difficult. We reveal that the conditions that can easily generate energetic IG waves are not identical to those affecting the boulders' transport distances. We also show that dimensionless transport distances (=transport distance of a boulder/offshore wave height) shorter than 4.0 x 10 over planar topography and 3.0 x 10 over reef topography are characteristic of storm boulders, even storms dominated by energetic IG waves. Thus, a longer transport distance generally points to tsunami boulders even if energetic IG waves were generated during an event. Additionally, we have demonstrated that the offshore wave height and wavelength can be derived without numerical modeling if the relationship between tsunami boulders' dimensionless transport distance and the Iribarren number was used. Key Points: Even when infragravity waves dominate, coastal boulders are transported to shorter distances compared to transport induced by tsunami wavesSeabed slope angle significantly affects the transport distance of storm bouldersThe Iribarren number can be derived from coastal boulder distribution and then used to estimate offshore tsunami conditions
- Subjects
TSUNAMI warning systems; TSUNAMIS; BOULDERS; ROGUE waves; STORM surges; TOPOGRAPHY
- Publication
Journal of Geophysical Research. Earth Surface, 2023, Vol 128, Issue 9, p1
- ISSN
2169-9003
- Publication type
Article
- DOI
10.1029/2023JF007083