Abstract
Abstract The Japanese islands, situated in a tectonically unstable region with a highly variable geology, are exposed to high wave energy and microtidal environments in most locations. Rocky coasts are common, most having a steep cliff with coastal recession being primarily driven by wave erosion. A fundamental relationship between recession and wave force is obtained through reanalysis of previous laboratory data. On the basis of this relation a model is constructed for the development of type B platforms, that is, horizontal or subhorizontal platforms that have a steep scarp at the seaward edge. The process of wave attenuation on this type of platform and weathering-induced strength reduction of rocks are incorporated into the model. The model is applied to the southwestern coast of the Kii Peninsula and a platform at Ebisu-jima of the Izu Peninsula. Long-term development rates of platforms in the former area are examined: the model indicates that the rate of erosion when platforms were initiated at 6000 years BP is two orders of magnitude greater than present. At the Ebisu-jima platform, wave-induced erosion processes are explored on a daily basis: the model provides a description of temporal variations in platform growth, although the result is not fully satisfactory.
Highlights
Rocky coast landforms, from the global perspective, differ considerably in morphology reflecting variations in wave climate, tidal range, local tectonics and glacio-hydro isostatic sea-level changes as well as lithological factors
Results of numerical modelling by Madin et al (2006) emphasize the effect of water depth on wave attenuation over a coral reef having a horizontal surface just like a type B platform; their results strongly suggest that (a) wave height tends to decay exponentially with the distance from the reef edge; and (b) the degree of decay depends on water depth – the more gradual attenuation occurs with greater depth
Some scatter of data points is seen, a general trend can be represented by equation (17): slower wave attenuation occurs with increasing relative water depth
Summary
From the global perspective, differ considerably in morphology reflecting variations in wave climate, tidal range, local tectonics and glacio-hydro isostatic sea-level changes as well as lithological factors. Horizontal platforms on the other hand have a horizontal or subhorizontal erosion surface that terminates seawards in a marked scarp Platforms of this type are well developed on the coast in microtidal environments (spring tidal range: ,2 m). The three types of landforms – sloping platforms, horizontal platforms, and plunging cliffs – are developed along the coastline of Japan, most of which is microtidal and subject to high wave energy. Considering the wave and rock factors with the latter having not been weathered, we will attempt to obtain a fundamental equation that can describe wave-induced cliff erosion which is a key process for rocky coast evolution. Based on the new relationship developed in the present study, we will construct a model for this type of platform considering the wave attenuation process and introducing the role of weathering in reducing the strength
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