Experimental reproduction of tsunami deposit

Abstract:

Understanding the process of sediment transport and deposition under a tsunami inundation is essential to provide the credible information about potential tsunamis from tsunami deposits. Detections of tsunami deposit has contributed to reveal centuries-old record of tsunami incursions. However, our knowledge is still not enough for evaluating the scale of past tsunamis using deposits.

In this study, a laboratory experiment was conducted to investigate the relationship between the hydraulic condition and sedimentological features of tsunami deposit. The large wave flume in CRIEPI, one of the largest wave flume in the world, which has 205 m length, 3.4 m width and 6 m depth was used. The sandy beach with uniform slope (1/50) were made in the flume. Sand dune of 0.2 high was placed near the shoreline. The tsunami was made by the wave generator which has 2.2 m stroke. The wave at the shore line has 0.6 m depth and the horizontal velocity reached up to 3.5 m/s. The incursion of the wave and its return flow completely washed out the dune and resulted in the deposition especially near the dune. The thickness of deposit shows landward thinning and fining, which has been widely confirmed by field observations.

In addition, sedimentary structures of the deposit was investigated using the method similar to that used in geological survey such as core sampling and relief peel sampling. The obtained samples were investigated using a X-ray computed tomography. The obtained CT-images shows that most part of deposition consists two or more subsections divided by horizontal lamination although the deposition near the dune has drastic and complex change thickness and grain size. The subsections shows upward-fining and upward-coarsening which are been reported as common sedimentary structures of tsunami deposit from field surveys.

Considering the similarity of sedimentary structures in the deposit reconstructed in this experiment and actual tsunami deposits, this experiment succeeded in reproducing tsunami deposit and would show typical pattern of topography change and deposition caused by the tsunami incursion. The experimental method used in this study and the obtained deposition patterns will help understanding the relationship between hydraulic condition and resulting tsunami deposition.