IMPACT OF OFFSHORE DREDGING ON BEACHES ALONG THE GENKAI SEA, JAPAN

Haruyuki Kojima, Takeshi Ijima, Takaaki Nakamuta

Abstract


Offshore dredging has been taking place in the coast of the Genkai Sea, Japan, since the early 1970's, which totaled more than 20 million cubic meters. In order to evaluate the impact of offshore dredging on shorelines as well as the adjacent sea bed, comprehensive coastal engineering studies were carried out over the last four years. The results obtained are not sufficient to establish a direct cause-andeffect relationship between offshore dredging and beach erosion; however, the correlation is sufficient to serve as a warning of a potentially serious problem. Moreover, the dredged holes above 30 meter depth are found to trap the sand from the neighboring bed, and considerable movement of sediment by wave action was observed above 3 5 meter depth. Thus, offshore mining that would minimize interruption of beach littoral system should be operated below 35 meter depth in the study area.

Keywords


Genkai Sea; offshore dredging; dredging

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