Koichi Yakita

Submarine topography and sediment distribution after the 11 March 2001 tsunami impact in Kesen-numa Bay, Miyagi, Japan

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 photographs by AUV and video films by ROV. We evaluated the marine environmental change based on a comparison with a chart published before this earthquake. The data suggest that the geographical changes by impact of the tsunami restricted to the inner bay and Ohshima strait, and much debris and clastic particles were transported and the dune was formed by the tsunamis backwash.

Evaluation of the natural recovery in an artificially modified geography

The purposes of this study were to obtain geographical and geological information before the dyke of the Isahaya Land Reclamation Project is opened, and to evaluate the natural recovery in the excavated area in Isahaya Bay. The presence of no new sediment within the ditches suggests that the natural return to the original geography is difficult after being artificially modified. However, the presence of many fish in the ditches suggests that the hollows are not necessarily a cause of the occurrence of low oxygen water, and that the hollows do not need any special measures to recovery geographically.

Autonomous underwater vehicle (AUV) investigations to protect valuable invertebrates

The lancelet is an important animal in evolutionary studies. Artificial proliferation is necessary to secure individuals for experiments. However, the number of individuals and their habitats are decreasing remarkably in the Ariake Sea. Despite the transplantation of frys, the number of adults has not increased. In addition, basic data such as water parameters and bottom conditions in the habit have not been clarified. The geography, oceanography and geology of the largest habitat in the Ariake Sea were investigated using acoustic machines and an AUV. The cause of the decrease in individuals is presumed not to be a result of an increase of mud in the sediments, but due to lancelets being preyed upon by rays based on CCD camera and side scan sonar images taken by the AUV. These results show the utility of AUVs in important discoveries of the biology of the hydrosphere.