学会誌「自然災害科学」

海岸構造物の築造が海浜変形に及ぼす影響－高知県吉川海岸を例として－

Effect of Coastal Structures on Beach Topography Change - A Case Study of Yoshikawa Coast in Kochi Prefecture -

伊福 誠*・中田 正人**

Makoto IFUKU* and Masato NAKATA**

Abstract

The effect of decrease of sand supply from rivers and construction of coastal structures on the beach topography change is investigated by analyzing the results of bottom sounding over the past three decades. As part of Yoshikawa Fishery Port improvement work, the breakwater has been extended, resulting in the decrease of sand supply to coast and the shoreline receding by about 20-40m over the past three decades. Although the construction of the detached breakwater began in 1986 as a remedy against further eroding of the shoreline, the detached breakwater will likely not serve its purpose because of the interrupt on the littoral drift.

The deposit at Sugita, Yoshino and Nagase Dams in the Monobe river system amounts to approximately 11,365,000 cubic meters and it is equivalent to 8 times the reduced amount of sand at Yoshikawa Coast, which was obtained from the average amount of erosion over the past three decades. Close to the central part of coast investigated, about 150m offshore, there is a depression in the seabed which is gradually moving towards Akaoka Fishery Port. This movement is related to the high wave energy caused by frequent typhoons.

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