Instability of coral limestone cliffs due to extreme waves

Abstract

Many boulders are located around the coastal cliffs with height of below 5 m made of coral limestone at Kuroshima, Okinawa, Japan. The origin of the boulders appears to be coral limestone cliffs which show developed notches. We undertook stability analysis, involving the wave pressure due to tsunamis, of wave-induced collapse of the cliffs. We fi nd that extreme waves are capable of inducing cliff collapse, as observed in circumstances where gravity is insuffi cient.

Threshold height of coastal cliffs for collapse due to tsunami: Theoretical analysis of the coral limestone cliffs of the Ryukyu Islands, Japan

“…Therefore, their definition of the "cliff height" includes both the height of cliff with and without a notch. However, the models based on stability analysis in Kogure and Matsukura (2010a) for gravity-induced collapses, and in Kogure and Matsukura (2010b) for wave-induced collapses, define the height of a notched cliff and that of the cliff above the notch as separate parameters (respectively H and h in Fig. 3b).…”

“…In Case 1, waves have no effect on collapse, because no stress acts on the notched roof. Models of wave-induced collapse based on stability analysis are derived for Cases 2, 3 and 4 based on the work of Kogure and Matsukura (2010b).…”

“…Kogure and Matsukura (2010b) showed that collapses of low coastal cliffs with a horizontal failure surface occurring in Kuro-shima Island, Japan, could have been induced by extreme waves such as tsunamis or bores occurring during a storm, as an alternative to gravitational collapse. In fact the stability analysis was undertaken only for collapses due to tsunamis, because of lack of information on the wave pressure of bores during storms or typhoons (see Kogure and Matsukura (2010b) for details). That paper considered only tsunami-induced collapses of low coastal cliffs with heights below 5 m, since Kuro-shima Island is a low-lying island.…”

“…To determine the critical notch depths for tsunami-induced collapse with a vertical failure surface, and to discuss threshold cliff heights for collapses due to tsunamis, we derive new equations based on stability analysis and apply them to the field data, following the analysis of Kogure and Matsukura (2010b). The equation presented by Kogure et al (2006) is also reconstructed in order to predict accurately the critical notch depths due to gravity-induced collapse.…”

“…Although Kogure and Matsukura (2010b) developed models that predict the critical notch depth for tsunami-induced collapses, application of their equations is limited to specific cliff collapses. These equations were derived for collapses with a horizontal failure surface which have a lower frequency of occurrence than collapses with a vertical failure surface, as is common at both high and low cliffs in the Ryukyu Islands (e.g.…”