三保半島海浜から内側陸棚における砕屑物の移動と堆積

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タイトル別名
  • Migration and sedimentation of clastic sediments from beach to inner shelf off Miho Peninsula
  • ミホ ハントウ カイヒン カラ ウチガワ リクダナ ニ オケル サイセツブツ ノ イドウ ト タイセキ

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The coastal erosion is a serious problem in Japan, especially along a coast of the Miho Peninsula, Shizuoka. Since 1980's, numerous coastal structures have been built and nourishments have been performed for protection and preservation of the coastline, however, it has not become a fundamental solution so far. The mechanisms of clastic sediment transport, timing of the coastal erosion, and an influence of the erosional debris to a seafloor are not clearly understood. It is important to examine the problem of the coastal erosion from the beach to the continental shelf comprehensively. This paper describes topographic changes from the beach to the inner shelf to see seasonal variation and examines the relation of the coastal erosion and clastic sediment transport. The following results are discussed; 1) A timing and amount of the coastal erosion are examined in wave-dominated zone. The coastal erosion occurs dominantly at summer and autumn seasons when several typhoons hit the coast, and the coastal geographical feature change in winter and spring seasons is not significant. The seafloor becomes depositional zone when the coastal erosion occurs. 2) Several bulges of the sea floor (Coastal Swells) are found just seaward side of the coastal structures. Numerous linear and positive topographic features (Sand Ridges) are found off these Coastal Swells. The topographic characteristics determined by narrow multi-beam system and grain size analysis on dredged samples together suggest bottom materials of these positive topographic features are results from masstransportation from the coast to shelf. 3) Configurations of subaqueous dunes distributed at the northern part (the Hagoromo Spur) of the study area were clearly changed for about half a year. These changes reflect the influence of the storm, typhoons during summer and autumn seasons. 4) Sedimentary environment is greatly different between the northern and southern parts of the study area. In the northern part (the Hagoromo Spur), clastic sediments are chiefly transported by the strong bottom current. In the southern part (off Komagoe), clastic sediments appear to be migrated by mass-transportation.

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