Numerical simulations to account for boulder movements on Lanyu Island, Taiwan: tsunami or storm?

Abstract

Boulders that originated from the Holocene coral terrace and coral reef are distributed on the coral terrace or talus on the shore of Lanyu Island, Taiwan. We employed numerical simulation of storm waves and estimated whether the boulders could be moved by storm waves with a return period of 50 years, by larger storm waves with return periods of hundreds of years, or by tsunamis. The coral boulders are distributed between 36 and 128 m from the shoreline at elevations of 2.7 to 9.3 m. The sizes of the boulders are in the range 0.3 to 6.4 m. The boulder volume and a-axis length versus distance from the shore show weak and moderate correlation (r = 0.21 and 0.48), respectively. We reproduced the runup of waves using the equation of continuity for a two-dimensional non-compressive fluid and the Navier-Stokes formula. In order to handle the free surface of the fluid, the volume of fluid method was applied. For the flow velocity to move the boulder, we employed the theoretical formula for the transport of boulders. We used the wave height for a return period of 50 years and set the height of the input wave to 13.1 m. The results show that all the large boulders on Lanyu Island could be moved by storm waves with a return period of 50 years. Moreover, the computations show that most of the boulders could not have been transported by tsunamis generated by the Mw8.7 earthquakes. The ages of two boulders indicate that they were transported after 340 and 6,330 years ago. If these boulders were moved by tsunamis several hundred years ago, they would have been relocated subsequently by storm waves.

Untangling boulder dislodgement in storms and tsunamis: Is it possible with simple theories?

“…Other simple models of boulder transport have been put forward by Nakamura et al . [], Buckley et al . [], Nandasena et al .…”

“…Nott's equations are widely used [e.g., Kelletat et al, 2007;Nott, 2003b, and references therein] and have also been modified [Barbano et al, 2010] and extended [Paris et al, 2010]. Other simple models of boulder transport have been put forward by Nakamura et al [2014], Buckley et al [2012], Nandasena et al [2011] and Benner et al [2010]. RF5D1L1B2W > 0 (1) Drag and lift forces are functions of velocity, which is retrieved from numerical models, predefined theoretical wave representations or assumed to be the peak velocity at the wave crest in the studies mentioned above.…”

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

“…This method is useful if the maximum storm waves in the region can be obtained. In a similar manner, Nakamura et al () also applied the method to boulders in Taiwan, clarifying that all the large boulders can be moved by storm waves with a return period of 50 years. Barbano et al () conducted a similar approach to differentiate tsunami and storm boulders on the Ionian coast of Sicily using wave transport equations jointly with statistical analysis of boulders.…”

Prehistorical frequency of high‐energy marine inundation events driven by typhoons in the Bay of Bangkok (Thailand), interpreted from coastal carbonate boulders

“…For comparison, on Ludao Island in eastern Taiwan, where typhoons are common but tsunamis rare (Lau et al , ), transported slabs of broken beachrock provide evidence for MFVs between 3.2 and 5.9 m/s (Lau et al , ). On the neighbouring island of Lanyu, Nakamura et al () showed from coral boulders deposited up to 9.3 m a.m.s.l. that MFVs of 1.4 to 16.9 m/s are required.…”