Rain could drive new disaster on Japan’s Mount Ontake

Rain has fueled fears of large-scale debris flows on Mount Ontake, which erupted last Saturday, leading to calls for precautions against a secondary disaster.

Concern has grown that forecast heavy rain could wash away volcanic ash at the summit of the mountain, which straddles the border of Nagano and Gifi prefectures, and trigger debris flows.

“We’ve seen nearly 50 centimeters of volcanic ash accumulate at one spot. Volcanic ash that has absorbed rainwater is as heavy as cement,” said Makoto Asaoka, head of a riot police unit from the Kanto region that left for Mount Ontake early Thursday to search for people still unaccounted for.

Rain halted the search for the day shortly after 11 a.m.

The area around Mount Ontake is expected to continue to see rain, and Typhoon No. 18 could approach the area next week, according to the Japan Meteorological Agency.

Saturday’s eruption is estimated to have generated about 1 million tons of volcanic ash. The soil covering the volcano’s surface normally allows water to permeate. However, if volcanic ash, which is made up of fine particles, accumulates on top of the soil and hardens like cement due to rain and other conditions, it will form a layer enclosing the soil so water cannot pass through.

If rain continues, water flowing down the surface of the mountain will wash away volcanic ash and the soil underneath, causing them to flow together while gaining further mass as they slide downward. This is the typical mechanism of debris flows following a volcanic eruption.

“A debris flow is likely to occur in a valley where volcanic ash accumulates. Conducting search operations in rain in such a valley carries the risk of a secondary disaster,” said Hiroshi Ikeya, a professor at the National Graduate Institute for Policy Studies who specializes in erosion control engineering.

Small-scale debris flows may have already occurred after volcanic ash absorbed moisture generated by the eruption. On Sunday, Asia Air Survey Co. took an aerial photograph that shows what appears to be traces of a landslide in a valley extending southwest from a crater.

After the 1991 eruption of Mount Fugen in the Unzen mountain range in Nagasaki Prefecture and the 2000 eruptions on Miyakejima island in Tokyo, rain fell on accumulated volcanic ash. It resulted in massive debris flows, damaging houses and other structures at the foot of the mountains.

Nobutomo Osanai, chief of the Public Works Research Institute’s erosion and sediment control division, said, “As the spewing of volcanic ash has halted around the crater at this time, debris flows that could cause damage at the foot of the mountain are unlikely to occur.”

A study by the Forestry and Forest Products Research Institute has shown that a debris flow will gain force and cause damage to a wider area when volcanic ash is merged into the mass. “According to experiments, the range of a debris flow is extended by about 30 percent. Volcanic ash appears to serve as a lubricant,” said Hirotaka Ochiai, head of of the institute’s research planning and coordination department.

If eruptions continue to hit the mountain over a long period of time and the volume of ash grows, the risk of debris flow will increase further. The risk is unlikely to be reduced in the snowy season, as heat from eruptions can thaw accumulated snow to cause a debris flow. This type of debris flow occurred after the 1926 eruption of Mount Tokachi in Hokkaido, leaving 144 people dead or missing.

“A major eruption could occur again on Mount Ontake,” Ikeya said. “Countermeasures, such as examining possible locations of debris flows, installing surveillance cameras and setting up erosion control dams, should be taken wherever possible.”