日南海岸・青島の弥生橋橋脚砂岩塊の窪み深さと日射の関係

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Four masonry piers of Yayoi Bridge, connecting Aoshima Island with Kyusyu Island in Japan, are composed of Miocene sandstone blocks. The shape of the piers is the frustum of a pyramid and they are called as P1, P2, P3 and P4 in this order from the Aoshima Island side. Most of the sandstone blocks have a tafoni-like depression on their surface. For all sandstone blocks, the maximum depth of depression was measured. The averaged maximum depression depth of the four side walls of each pier is the largest on the sunny south-facing wall. The distribution of the maximum depression depth on the east- and west-facing walls of the piers is similar to the pattern of shadow made by the bridge girder. Considering that insolation plays an important role in the formation of depressions, the amount of insolation absorbed by each sandstone block was evaluated. The results are summarized as follows: (1) On the east- and west-facing walls of the piers, the maximum depression depth on each sandstone block is positively proportional to the global insolation absorbed by the block. (2) The altitude-averaged maximum depression depth for each side wall of the piers is generally proportional to the average global insolation for the wall. The depth of the north-facing wall of P1, P2 and P3, however, is smaller than the value anticipated from the global insolation, while the depths of the west- and northfacing wall of P4 are larger. (3) In the intertidal zone, the averaged maximum depression depth for each side wall is the smallest on P4 and becomes larger from P3 to P1. Difference in the depth between P1 and P4 is proportional to the global insolation in the intertidal zone. (4) In the coastal spray zone, the space-averaged maximum depression depth each side wall of the piers is the largest on P4 and becomes smaller from P3 to P1. This tendency cannot be explained by the difference in insolation.
日本地形学連合の論文
2006-04-25

日南海岸・青島の弥生橋橋脚砂岩塊の窪み深さと日射の関係

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