Grain Size Distribution of the 1926 Volcanic Mudflow at Mt. Tokachi, Japan

Tomoyuki NANRI, Takashi YAMADA, Mio KASAI, Tomomi MARUTANI, Shigenori TAKASHIMA and Takayuki YAMAHIRO

In May 1926, a volcanic mudflow triggered by the eruption of Mt. Tokachi, referred to as  the 1926 mudflow, ran through the Furano and Biei rivers in Hokkaido, Japan, killing 144 residents. This study analyzed the grain size distribution of this mudflow to determine the reason why it sustained such a large force across a gentle plain. The mudflow deposits were sampled and analyzed along the Furano River. To compare the results with those of previous studies, the grain size distribution of deposits were identified in eight different sections. Then the distribution was categorized into three groups: very fine (<0.1 mm in diameter), fine (0.1 2 mm in diameter), and coarse (e"2 mm in diameter). In each section, the proportion of grains in each group were multiplied by the net volume of the mudflow deposition provided by Nanri et al. [2009] to estimate the volume of each grain group. At the downstream end of the source and scouring zones, the very fine group comprised 54% of the transported sediment. This increased to 61% at the end of the transport zone, and reached 85% at the downstream end of the deposition zone. Hence, the 1926 mudflow contained a large amount of very fine materials and the resulting high gap density of the fluid in the mudflow enabled it to extend for more than 20 km from its point of origin across a gentle plain. The large fluid dynamic force resulted in extensive damage to buildings on the plain.

2018/1 163-169