Clarification of Origin and Mechanism of Tsunami-generated Turbidity Current by a Flume Experiment

Abstract

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.