Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan

Abstract

Tsunami boulders deposited along the coast constitute important geological evidence for paleotsunami activity. However, boulders can also be deposited by large storm waves. Although several sedimentological and theoretical methods have been proposed to differentiate tsunami and storm wave affected boulders, no appropriate numerical method exists for their differentiation. Therefore, we developed a new numerical scheme to differentiate tsunami and storm wave boulders for coastal boulders on Ishigaki Island, Japan. In this area, tsunami and storm waves have emplaced numerous boulders on the reef and the coast. By conducting numerical calculations of storm waves in this region, we estimated the size of a storm wave that can explain the maximum clast size distribution of boulders on the reef. Consequently, we showed that a wave with a combination of 8 m in initial wave height and 10 s period can satisfy the above conditions when we assume mean sea level. In contrast to the boulders on the reef, all boulders deposited along the shore are heavier than the calculated possible maximum clast size distribution by the storm wave. Therefore, we confirmed these boulders as being of tsunami origin. Results of previous studies showed that they were most likely deposited or reworked by the 1771 Meiwa tsunami. Then, using the tsunami boulders, we numerically estimated the wave period and amplitude of the 1771 Meiwa tsunami, which should have had a 4–5 min period and 5.6–5.9, 6.3–7.0 m amplitude, respectively. Using the proposed scheme, it is possible to differentiate tsunami and storm wave boulders and estimate the size of past storm waves and tsunami waves, although it is noteworthy that there are exceptions for which the scheme cannot be applied.

Modeling boulder transport by coastal waves on cliff topography: Case study at Hachijo Island, Japan

“…Accumulations of cliff‐top boulders have been reported by many researchers (e.g., Williams and Hall ; Fichaut and Suanez ; Hall et al, , ; Cox et al, ). Accumulations at one location can be explained because the period of a storm wave is only several tens of seconds, hence the hydraulic force of storm wave is quickly attenuated toward inland (Egashira et al, ; Watanabe et al, ); therefore, boulders accumulate in a narrow range. In contrast, a tsunami period is generally long compared to storm waves, so the hydraulic force of tsunami is not as quickly attenuated inland (Goto et al, ).…”

“…For boulder transport beginning on the cliff, the eight boulders described in Table were also used, then the motion of each boulder was calculated. For this calculation, we used the method by Imamura et al () and Watanabe et al () (see Appendix 3).…”

“…In this study, a two-dimensional (horizontal-vertical) calculation was used to study the relationship between boulder transport and input wave conditions such as wave height and wave period. Indeed, the accuracy of using a cross-section to calculate boulder transport by storm waves has previously been validated by Watanabe et al, (2016).…”

“…After that, distribution limit of boulders due to storm waves was calculated by estimating the position where friction force of boulder and the maximum wave force were balanced. The accuracy of this method has been validated in the study of Watanabe et al (2016).…”

Projecting the risk of damage to reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100

“…porous media. The time resolution was 0.01 s and the calculation time was 3600 s. We used data for 1801-3600 s. The model has been successfully applied to waves breaking at coral reefs under TC conditions (Kawasaki et al 2007;Hongo et al 2012;Nakamura et al 2014;Watanabe et al 2016). The model calculates wave characteristics (wave velocity and water level) using four input parameters: (1) incident significant wave height at the outer ocean (SWH o ); (2) incident significant wave period at the outer ocean (SWP o ); (3) incident water level at the outer ocean (WL o ); and (4) topography.…”