音響解析による気仙沼湾における津波襲来後の海中環境と堆積物分布特性の把握  [in Japanese] IMPACT OF THE 2011 TSUNAMI ON THE GEOGRAPHY AND SEDIMENT DISTRIBUTION IN KESENNUMA BAY, MIYAGI, JAPAN  [in Japanese]

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Abstract

津波のリスクが高い沿岸域における防災減災対策の強化に向けた基礎資料として,津波による海底地形および底質の変化の情報は有用である.気仙沼湾も,2011年3月11日に襲来した津波によって,海底が削剥され,多くの瓦礫が流入した.津波襲来後の地形変化と底質分布特性を明らかにするために,音響解析装置を用いて,水深と反射強度の情報を取得した.反射強度と採集した底質試料の物性の関係を基に,底質分布を把握した.震災前後の地形を比較した結果,急激に湾の幅が狭くなる奥部(狭窄部および気仙沼漁港)で海底が削剥されていた.サイドスキャンイメージによると,狭窄部と湾南東部には瓦礫が密集してデューンが形成されていることが判明した.このことは,津波による海底の削剥が局所的であり,陸域から流入した瓦礫が偏在することを示唆している.

 In Kesennuma Bay, the study area, the sea bottom was scoured, the geography and sediments were altered, and much debris, oil, and chemical materials flowed into the sea from land as a result of the 2011 0311 tsunami. Highly precise information for the change of geography and sediment distribution by this tsunami applicable to the reduction of tsunami damage can be obtained by this investigation. We collected data on water depth, refraction intensity by acoustic systems, and four sediment samples, interpreted the intensity related to the physical properties of the sediment, made a three-dimensional topographical map and distribution map of the sediment and debris, and evaluated the marine environmental change based on a comparison with a chart published before this earthqueick.<br> The geographical changes by impact of the tsunami restricted to the inner bay, such as a north-south geographical rise (< 8 m water depth) on the east side and a depression (> 16 m water depth) on the west side.<br> Acoustic reaction is strong in the uneven geography present in the dune field which intersects perpendicularly with the bay axis in the joint area between the inner and central area of the bay. Coarse sediment and woody material are present in the area.<br> Three clusters composed of many dunes are also distributed in the southeast area of the bay. The reflective intensity is strong at the top of the dune and is weak at its bottom. Fishermen stated that muddy sediment was widely distributed in the bay before this earthquake and that the sea bottom in the southwest area of the bay was exposed at the time that the water surface reached its lowest point during the tsunami. Thus, the evidence suggests that much debris and clastic particles were transported and the dune was formed by the tsunami's backwash.

Journal

  • Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering)

    Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) 68(2), I_180-I_185, 2012

    Japan Society of Civil Engineers

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