The origin and mechanisms of a tsunami-generated turbidity current are examined in this study by flume experiments. A reduced scale model of coastal topography including beach, shoreface, and shelf using fine to very fine sands was situated in an experimental flume, and the oscillatory flows were generated by the tsunamis in the flume. As a result of the experiments, turbidity currents occurred in two regions: upper shoreface and slope break on the shelf. In the upper shoreface region, the turbidity current was generated by the hydraulic jump which was resulted from a collision of the surging run-up and the return ebb flows. The hydraulic jump caused significant amounts of bottom erosion, and downslope-migrating sediment-gravity flow occurred subsequently from the suspended sediment on the shoreface. On the other hand, velocity of the run-up flow of the tsunami increased at the slope break on the shelf, resulting in the entrainment of sediment on the sea floor. As a result, another turbidity current occurred from the suspension cloud of entrained sediment at the slope break on the shelf, and was accelerated by the returning ebb flow of the tsunami. These two turbidity currents are different in velocity and size. The current generated on the upper shoreface is larger in flow height and velocity than the one generated on the shelf-slope break region. These results of experiments give insights to interpretation and further investigation of tsunami-generated turbidity currents and their deposits.
We studied the relationship between estuary-fill process and formation of tsunami deposits in the southern part of Minamisoma City, Fukushima Prefecture, Northeast Japan. We took a 26.5 m core of the Holocene estuary-fill sediments and reached underlying Pleistocene sandy siltstone. We described the lithology of the core and identified sedimentary facies, and carried out soft X-ray photograph, dry bulk density measurement, grain size analysis and dating of radiometric 14C ages. The Holocene estuary-fill is composed of the deposits of backshore, sandy to muddy tidal flat, subtidal estuary and salt marsh with intercalated event sand beds, and artificial fill and paddy soil in ascending order. The event sand beds yield many marine shell fragments. They indicate the sands were transported by landward flows. The beds contain rip-up clasts and gravels, and represent grading structures, basal reverse grading, multiple grading and periodic sedimentation. These characters suggest the possible tsunami deposits.
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