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

Abstract

Clifftop coastal boulders transported by storm waves or tsunamis have been reported around the world. Although numerical calculation of boulder transport is a strong tool for the identification of tsunami or storm boulders, and for estimation of the wave size emplacing boulders, models which can reasonably solve boulder transport from below a cliff or from a cliff‐edge onto a cliff‐top do not yet exist. In this study, we developed a new numerical formulation for cliff‐top deposition of boulders from the cliff edge or below the cliff, with validation from laboratory tests. We then applied the model using storm and tsunami wave forcing to simulate the observed boulder deposits at the northwest coast of Hachijo Island, Japan. Using the model, the actual distribution of boulders was explained well using a reasonable storm wave height without assumption of anomalously high‐water level by storm surge. Results show that boulder transport from the cliff edge or under the cliff onto the cliff‐top was possible from a tsunami with periods of 5~10 min or storm waves with no storm surge. However, the actual distribution of boulders on the cliff was explained only by storm waves, but not by tsunami. Therefore, the boulders distributed at this site are likely of storm wave origin. Our developed model for the boulder transport calculation can be useful for identifying a boulder's origin and can reasonably calculate cliff‐top deposition of boulders by tsunami and storm waves. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

The origins of marine and non-marine boulder deposits: a brief review

“…Initial debate often focused on the mass of the largest boulders or other megaclastic rocks. However, recent studies and model simulations (e.g., Dewey and Ryan 2017;Goff et al 2018;Watanabe et al 2019;Cox et al 2020) suggest that this alone is not a sufficient criterion. It is also impossible, now, to distinguish such deposits based on cumulative histograms of size (Dewey and Ryan 2017) or shape factor which is probably controlled more by bedrock jointing or reef morphology than transport mechanism.…”

“…Care must be exercised in making this comparison because sampling methods were not standardized, for example the Irish data are from a longer controlled profile at one location (see Dewey and Ryan 2017 for details), whereas that of Sicily was across three locations. Watanabe et al (2019) argue that as storm waves have periods of 10 s of seconds their hydraulic force is rapidly attenuated inland. Accumulations of storm transported boulders in a cliff-top setting require that the site is subject to either many storm waves in one event or frequent repeated storms.…”

Significance of boulder shape, shoreline configuration and pre‐transport setting for the transport of boulders by tsunamis

“…This and other comparable approaches have been widely used since then (e.g. Noormets et al, 2004; Pignatelli et al, 2009; Paris et al, 2010; Etienne et al, 2011; Nandasena et al, 2011; Engel and May, 2012; Sugawara et al, 2014; Watanabe et al, 2019), even though it is commonly understood that their simplification of the transport process leads to significant uncertainty (Sugawara et al, 2014; Oetjen et al, 2017; Bressan et al, 2018; Nandasena, 2020). These efforts are today complemented by physical experiments on boulder transport (e.g.…”

Modern coastal tempestite deposition by a non‐local storm: Swell‐generated transport of sand and boulders on Eleuthera, The Bahamas

“…Angular boulders and mega‐clasts scattered on rocky cliff‐tops, or aligned in shore‐parallel boulder ridges, have often been interpreted as evidence of wave transport by both tsunamis (Scheffers & Kelletat, 2003) and storms (Hearty, 1997; Williams & Hall, 2004; Hall et␣al ., 2019; Cox et␣al ., 2012). Physical (Cox et␣al ., 2019) and numerical models (Nandasena & Tanaka, 2013; Zainali & Weiss, 2015; Watanabe et␣al ., 2019) have been developed to constrain the transport potential of waves, in order to refine palaeo‐event interpretations. It has been argued that storm waves are a more likely mechanism for tempestite deposition than tsunamis in tectonically inactive basins, particularly where the coastline and bathymetry is characterized by modest (<20 m) cliff heights and steep, narrow continental shelfs.…”