Disaster map of the Taisho volcanic mudflow informing the arrival time and the level of damage after the eruption of Mt. Tokachi in 1926

Abstract

In 1926 a volcanic mudflow triggered by the eruption of Mt. Tokachi, named “Taisho mudflow”, ran through the Furano River in Hokkaido and caused destructive damage along the course. Several disaster maps presenting the path have been produced since then, while additional information of the arrival time and the levels of damage on the map was considered helpful to establish evacuation plans for the future eruption. In this study, the travelling time of the flow was estimated by interviewing locals who experienced the disaster (Nanri et al., 2004). The damage level was also classified into three groups, High (Ⅰ) : 90% of houses were completely destroyed and 30% of residents were killed, Moderate (Ⅱ) : 30-90% of houses were completely destroyed and 10-30% of residents were killed, and Low (Ⅲ) : houses were partly destroyed and less than 10% of residents were killed. Estimating fluid dynamic force produced by the flow, more than 10³ KN/m for the level Ⅰ, 10²-10³ KN/m for the level Ⅱ, and 10¹-10² KN/m for the level Ⅲ were required during the travel respectively. Applying the relationship between average river bed slope and the force given, the distribution of the damage level was estimated and demonstrated through the path. In summary, it took the flow 20 minutes to travel from the crater through the valley to 0.5 km downstream of the outlet after the eruption, giving a high level of damage. Subsequently, another 10 minutes were required to travel over a flood plain as damaging the area to a moderate degree. Finally, after 40 minutes from the eruption the flow arrived at the Kami-Furano Township, as yielding a low degree of damage.