Flume experiments test grain-size distribution of onshore tsunami deposits

Abstract

We conducted flume experiments to examine the effects of grain size on the distribution of tsunami deposits, using well-sorted quartz sand with median diameters (D50) of 0.064, 0.134, 0.215, and 0.250 mm. In these experiments, we investigated deposits transported onto a flat terrestrial area by a single tsunami-like bore without a strong backwash flow. The post-tsunami distributions of the coarser (D50 = 0.215 and 0.250 mm) and finer (D50 = 0.064 and 0.134 mm) sands differed markedly. The amount of the deposited coarser sand decreased landward across the entire terrestrial area, whereas that of the deposited finer sand was either approximately uniform across the terrestrial area or increased gradually landward. Time-series of flow depth and sediment transport observed via composited video frame images showed that suspension of the coarser sand did not reach the height of the maximum flow depth, and decreased with distance from the sediment source, and that the sediment settled gradually across the terrestrial surface with the amount deposited decreasing landward. Conversely, the finer sand was densely suspended up to the maximum flow depth and was transported over longer distances across the terrestrial area with the amount deposited being relatively uniform or increasing slightly landward. Advection lengths that we calculated on the basis of our experimental parameters supported the dependence on grain size of the distribution patterns of onshore deposits shown by our flume experiments. Moreover, advection lengths that we calculated for the 2011 Tohoku-oki tsunami from previously reported tsunami parameters were consistent with a deposit distribution similar to that observed in the field after the tsunami. Although we ignored some key factors (e.g., geomorphology and sediment availability), our flume experiments showed that the distribution patterns of identical tsunami flows differ for sediments of different grain size.