琉球石灰岩の海食崖にみられるノッチの限界深さ : 八重山諸島・黒島での事例 [in Japanese] Critical Depth of Notch of Coastal Cliffs Made of Ryukyu Limestone in Kuroshima, Yaeyama Islands [in Japanese]
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A wave-cut notch is formed at the base of vertical coastal cliffs made of Ryukyu Limestone in the Ryukyus, Japan. Many rectangular blocks, which appear to have originated in cliff failures, are observed on shore platforms in front of the cliffs. This fact suggests that the development of a notch increases the cliff instability and eventually results in a cliff failure. In order to determine the critical depth of a notch, the two limestone coasts were selected in Koroshima, Yaeyama Islands. We measured the horizontal depth of a notch, cliff height (h) and block length though field investigations. Notch depth, cliff height and block length range from 2.0-4.0 m. 1.5-2.4 m and 3.0-4.7 m, respectively. Assuming that the occurrence of cliff failure follows the principle of a cantilever, the critical depth of notch (l_c) should be given by: l_c=√^B<hS_1/pg> where S_1 is the tensile strength in rock mass, g is the acceleration due to gravity, and B is a nondimensional constant. An analysis of the field data applying to this equation indicated that a boundary condition for delimiting the presence of notched cliffs and fallen blocks given by B = 1.5x 10^3.
沖縄地理 (7), 25-32, 2006-12
The Okinawa Geographical Society