Experts have found that the debris flow in Atami City, Shizuoka Prefecture, which contains a large amount of embankment, has a higher proportion of water than the debris flow that occurs in the natural ground.
Experts point out that the embankment has the characteristic of containing more water, and that a large amount of water may have increased the speed and power of the debris flow.
This was revealed on the 15th by Professor Akihisa Kitamura of the Shizuoka University Disaster Prevention Research Center, who specializes in geology and conducted a field survey.
According to this, as a result of analyzing the earth and sand collected at the debris flow site, the proportion of water contained was 31% at the top of the debris flow and 36.2% at the bottom.
In most cases, the proportion of water in debris flows that occur in naturally deposited ground is about 10% to 25%, and since the debris flow this time contained a lot of water, the flow speed became faster and the power increased, and the damage range. Also seems to have expanded.
Regarding the reason why the debris flow contained a lot of water, he points out that the embankment had more gaps in the earth and sand than the natural ground, and absorbed a large amount of water and flowed down at once.
Professor Kitamura said, "The debris flow seems to have increased energy not only due to the topography but also due to the water contained in it. The embankment may store an irresistible amount of water in nature, so be careful about construction methods and drainage facilities in various places. I need to do it. "
Unsuccessful search and removal of earth and sand
The large-scale debris flow that occurred in Atami City took 15 days and 12 days after the outbreak.
The search for the missing person is difficult due to the large amount of earth and sand and the geographical condition of steep slopes.
In the Izusan area, the debris flow reached the sea about 2 km away, involving houses along the river.
How far has the search activity progressed?
By the 14th, heavy equipment could be introduced only in the central part of the damaged area.
Heavy machinery cannot enter the mountain side and the sea side due to steep slopes and a large amount of earth and sand.
This makes the search difficult.
At a rapid pace to overcome this situation is the development of routes for passing heavy machinery.
Of these, in the area on the mountain side, emergency slopes were prepared by laying logs and wooden boards on the slopes, and on the afternoon of the 15th, heavy machinery could be put in for the first time.
On the other hand, in the area on the sea side, a large amount of earth and sand is piled up along the Shinkansen track, blocking the entry to the search area beyond that.
Removal work using heavy machinery is continuing at the site, and the Self-Defense Forces and others are hoping to open a new route on the 17th and accelerate the difficult search.