1977年有珠山一の沢泥流の発生について

抄録

A series of eruptions of Mt. Usu since August 7, 1977, had brought ejecta of 8.3×10⁷ cubic meters over the vicinity (see Fig. 1). Due to rainfalls after the eruptions, mud-flows occurred on the piedmont of Mt.Usu. In September, the largest mud-flow occurred in Ichino-sawa, Izumi District of Abuta-cho, causing a gully erosion of 1, 100 meters long with transported boulders up to 1.7 meters in diameter.
Mud-flows are generally caused by the heavy rainfall through changes of runoff characteristics effected by the deposit of volcanic ejecta. The rate of runoff and the amount of peak discharge were probably increased by the deposition of the volcanic ejecta in Mt. Usu area due to low permeability of the ash. Since we did not know the real amount of the discharge at the time of the mud-flow occurrence, a maximum possible discharge was computed utilizing the rational formula:
_??_
where Q is peak discharge (m³/s), f is runoff coefficient, R is rainfall intensity(mm/h) and A is drainage area (km²) The values, A=0.66km², R=15mm/h estimated from the rainfall data and f=1 were used to obtain a value of 2.8m³/s for a maximum possible discharge. Because this amount of discharge is generally considered to be insufficient to cause mud-flows, other factors than the changes of runoff characteristics should have been responsible. In order to account for the enormous energy required to induce the mud-flow, the hypothesis that blocking and damming-up of the stream by slope failures on the valley sides and subsequent burst of the dam caused the mud-flow was postulated.
This hypothesis was investigated by geomorphological analyses comparing basin characteristics of Ichino-sawa (with mud-flow occurrence) and Kousu-gawa (without mud-flow occurrence) as shown in Figure 6. The slope maps of Figure 8 show that steep portion (_??_35°) of the valley sides, where slope failures are likely to occur, is much more extensive in the middle part of Ichino-sawa than in Kousu-gawa. Detailed analyses of aerial photo-graphs indicate the occurrence of many slope failures in Ichino-sawa and appear to support this hypothesis (see Fig. 9). Geological weakness of the Bunsei nuées ardente deposits and a small amount of pumice relative to the total amount of deposits are also thought to have contributed to the occurrence of the mud-flow (see Fig. 10).