[H-CG25] Earth surface processes related to deposition, erosion and sediment transport

[HCG25-01] Numerical estimation of the maximum size of paleo-storm waves at Amami Oshima Island, Japan inferred from coastal boulder deposits

*Kenta Minamidate^1,2, Kazuhisa Goto², Hironobu Kan³, Takashi Ishizawa⁴, Naoto Koiwa⁵, Akifumi Ito⁶ (1.Graduate School of Science, Tohoku University, 2.Department of Earth and Planetary Science, The University of Tokyo, 3.Graduate School of Integrated Sciences for Global Society, Kyushu University, 4.International Research Institute of Disaster Science, Tohoku University, 5.Faculty of Education, Hirosaki University, 6.Faculty of Humanities and Social Sciences, Yamagata University)

Keywords:typhoon, coastal boulder deposit, numerical simulation, coral reef, hazard evaluation, sediment transport simulation

In the subtropical region, storm wave events are one of the most influential factors on coastal landform formation, ecosystems, and human society. However, the scarce of the observational record makes it difficult to determine the upper limit of high-energy but low-frequency events. Coastal boulder deposits on the reef at Cape Ayamaru, Amami Oshima Island, Japan, may serve as proxies to record the local size of high-energy wave events over the past several thousand years. Here, we describe the distribution of the boulders at Cape Ayamaru and estimate the wave condition that can explain the boulders' distribution by numerical simulation. Field survey results show that the boulders' distribution tends to be inland fining trend according to the wave attenuation on the reef flat. Some boulders exhibit imbrication feature. Importantly, the maximum wave that hit Cape Ayamaru, estimated by numerical simulation, exceeded the largest wave ever observed in Naze tidal observatory of Amami Oshima Island. On the other hand, the estimated maximum wave at Cape Ayamaru does not exceed the maximum wave observed instrumentally at Nakagusuku Bay near Okinawa Island, which is the largest wave ever observed in Japan. Our results suggest that the long-term hazard risk of storm waves might have been different between the northern (Amami Oshima Islands) and the central parts (Okinawa Islands) of the Ryukyu Islands.