In Tsuruoka City, Yamagata Prefecture, on the 31st of last month, a mountain behind a house collapsed and two people were killed in a landslide disaster. It is highly likely that snowmelt water and rainwater seeped into the stratum, and that it occurred."
In the early hours of the 31st of last month, a landslide occurred in Nishime, Tsuruoka City, Yamagata Prefecture, killing two people and collapsing about 10 buildings.
On the 5th, Professor Emeritus Koji Yagi of Yamagata University, who specializes in geomorphology, conducted an emergency field survey together with the person in charge of the prefecture, etc., and confirmed the state of the collapsed slope and the condition of the soil.
After the survey, Professor Emeritus Yagi responded to the interview and explained that the rocks in the collapsed back mountain had been weathered and had become brittle and had become clay-like.
In addition, he indicated that the heavy snow in the middle of last month suddenly melted due to rain, etc., and the snowmelt water and rainwater seeped underground and accumulated in large amounts in the stratum, raising the water pressure.
The slopes are still moving at the site, and the amount of snow is expected to increase until the middle of next month. It is said that it is necessary to judge
Professor Emeritus Yagi said, "The residents are in a situation where they can't go home with peace of mind. Considering the snow melting, we have to keep a close eye on them until early spring."
Tsuruoka City intends to indicate future policies regarding evacuation orders issued to residents based on the views of Professor Emeritus Yagi.
There is still movement on the slope.
After the field survey, Mr. Mitsuhiro Isokino, Director of the River and Sabo Section of the Construction Department of the Shonai General Branch Office of Yamagata Prefecture, responded to the interview and revealed that a device will be installed at the site to observe the movement of the slope.
Iso Kimino said, "We received a report from an expert who accompanied the survey that 'a part of the slope is still moving and it is necessary to continue to observe it carefully.' In order to determine whether it is possible to return home, we will install a device that measures the movement of the slope.In the future, we will consider specific schedules and respond as quickly as possible."
The local weather conditions until the landslide disaster occurred
According to the Japan Meteorological Agency, at an observation point in the center of Tsuruoka City, Yamagata Prefecture, about 10 kilometers away from the site of the landslide, the amount of rainfall reached 500.5 mm from the 1st to the 30th of last month, which was the highest in the history of observation for December. rice field.
In Katsura Aramata, Tsuruoka City, which is about 15 kilometers away from the landslide site, 88 centimeters of snow was observed as of the 20th of last month, and 34 centimeters of snow fell on the 23rd of last month.
After that, it turned to rain, and on the 23rd, it rained 46.5 mm in one day in Tsuruoka City, and on the 28th, the maximum temperature was 9.8 degrees, which was nearly 4 degrees higher than the average year's 6.1 degrees. During this period, the temperature was high.
The amount of snow accumulated in Katsura Aramata was 34 centimeters at dawn on the 31st of last month, when the landslide disaster occurred, 54 centimeters less than on the 20th. I think so.
Sediment-related disasters caused by successive snowmelts and accumulated snow 30 cases a year
In Itoigawa City, Niigata Prefecture, in March of the year before last, the snow melted at once as the temperature rose, loosening the ground and causing a landslide about 100 meters wide and about 1 kilometer long.
Thirteen houses and other buildings were damaged, and evacuation orders have been issued for six people in two households, and residents continue to live in evacuation shelters.
Looking at the number of landslide disasters caused by snowmelt and accumulated snow that occurred in the year before by region, Niigata prefecture had 18 cases, Ishikawa prefecture had 4 cases, Yamagata prefecture, Nagano prefecture, and Fukui prefecture each had 2 cases. It is
According to the Ministry of Land, Infrastructure, Transport and Tourism, there is a high possibility of landslide disasters due to large amounts of snow melting in areas where the ground is fragile and the slopes are unstable.
Also, looking at the number of occurrences by month in the year before last, 20 cases, or nearly 70% of the total, occurred from March to May, and tend to increase in spring when the snow melts.
Sediment-related disasters caused by melting snow Be careful in areas with heavy snowfall
Sediment-related disasters caused by the melting of accumulated snow have occurred in the past, mainly in areas with heavy snowfall.
Experts point out that if the temperature rises or rains in areas with increased snowfall, the risk of landslide disasters may increase, so pay attention to changes in slopes.
According to Toshiyuki Kurahashi, a senior researcher at the Civil Engineering Research Institute for Cold Region in Sapporo, landslide disasters caused by melted snow occur almost every year in Hokkaido, Tohoku, and Hokuriku. about it.
Regarding the mechanism, senior researcher Kurahashi says, "Snow melts and water seeps into the soil, loosening the ground and causing collapse. In addition to steep slopes, it is also likely to occur on topography of 'valleys' where surrounding water tends to collect." and
On top of that, he points out, ``If the temperature suddenly rises or it rains, the amount of water that seeps into the ground increases when the melted snow and rain come together, so you need to be especially vigilant.'' .
Furthermore, it is difficult to predict when a landslide disaster will occur because the amount of water that seeps into the ground changes depending on not only the depth of the snow cover but also the density of the snow, and it is also affected by topography and geology.
He also said that in areas with increased snowfall, we should pay attention to rain and temperature conditions, and said, ``If cracks, steps, bulges appear on the slopes, or if pebbles fall, it is a sign that the ground is moving, so immediately We need to evacuate. If there is snow on the surface, it will be harder to notice changes in the slope than usual, so please be careful."