Abstract
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.
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.
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.
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.
