Debris flow What happened locally at that time?
Risks hidden in the “fill” July 5, 19:14
A debris flow that occurred on the 3rd of this month in Izusan, Atami City, Shizuoka Prefecture.
The water, which contained a large amount of earth and sand and gained momentum, went down the slope with tremendous force and swallowed buildings and cars.
"When I looked out from the window, the tree broke and rushed closer to me, making a popping noise."
The situation at that time gradually became clear from the testimony of the residents.
Experts who analyzed the local situation point out the risk of "filling" that collapsed this time.
About 15 minutes after the first debris flow
A man rescued by a firefighter from his home near the Aizome River in Izusan, Atami City, where a debris flow occurred, talked about the situation at that time.
Mamoru Okubo
"I heard a rattling sound of trees, and after about 10 minutes, the earth and sand suddenly flowed."
"About 15 minutes after the first debris flow, the debris flow that gained momentum rushed in again. I had a firefighter drop a ladder from the top of the cliff behind my house, and about 10 people nearby climbed about 10 meters to evacuate. After that, another debris flow came and I didn't feel comfortable living. "
Make a popping noise ...
A man in his thirties who lives in the upper part of a village located upstream of the Aihatsu River and is currently evacuating to a shelter.
At the beginning of the debris flow, I was at home.
"At around 10:27 am, when I felt an earthquake-like tremor, there was a power outage. Then, I heard a go, and when I looked out the window, I heard a popping noise and the tree broke and rushed to the vicinity. I saw the earth and sand collapsing, but it looked like earth and sand that did not contain much water. "
"At around 10:40, when I went outside and checked the surroundings, earth and sand containing water was flowing on the ground more than before. The flow speed became slower or faster, and it flowed. The width of the earth and sand was widening and narrowing. The smell of the earth was in the area. "
"I took it out with my clothes on"
Yutaka Kawase, president of Atami City's welfare taxi company "Izu Ohana," hurriedly found a couple in their 80s with weak legs staying at home over the elderly housing in the area immediately after the debris flow. I took it to the evacuation center.
Aimi, a wife who also serves as a driver, searched for an elderly person who was left behind in the area by acting differently from her husband.
In cooperation with the fire brigade I met there, I rescued three people one after another, including the elderly who are deaf and seem to have missed the disaster prevention radio, and the elderly who live in wheelchairs and cannot evacuate on their own. I evacuated.
Aimi Kawase
"I called out to the elderly, but they didn't seem to know what was happening because they had bad ears, so I took them out with their clothes on."
Debris flow is a phenomenon in which soil and stones that have collapsed due to heavy rain flow down at once together with water, and occur in places called alluvial fans at the exit of valleys where there are steep rivers and valleys.
The speed at which earth and sand flow down with a large amount of water is as fast as a car, at several tens of kilometers, and it becomes difficult to evacuate after a debris flow occurs.
Shizuoka Prefecture analyzed the situation of the collapsed site on the upstream side based on detailed data on the topography of the country and prefecture.
According to this, comparing the data of the Ministry of Land, Infrastructure, Transport and Tourism around 2010 with the data of the prefecture last year, the so-called embankment that fills the soil is applied over about 200 meters east-west and 60 meters north-south of the place of the terrain that seems to have been a valley. It means that it was.
In addition, since the data with different accuracy are compared, the amount of embankment may reach about 54,000 cubic meters, based on the analysis value.
In addition, depending on the location, there were places where soil was piled up more than 10 meters from the topography around 2010, and the site survey conducted on the 3rd confirmed that the slope before it was piled up was visible.
The place that collapsed due to the debris flow
On the 3rd and 4th, the Geospatial Information Authority of Japan compiled a distribution map of places where debris flow seems to have collapsed or accumulated earth and sand from images of drones taken by the Ministry of Land, Infrastructure, Transport and Tourism and Shizuoka Prefecture.
According to the distribution map, about 2 km from the coastline, the mountain surface is about 300 meters long and up to about 100 meters wide, and it is thought that the debris flow started due to the collapse of earth and sand.
In the residential area that extends about 1 km downstream from there, the place where the earth and sand was piled up was about 1 km to the coastline, and the width was about 100 meters.
The Geospatial Information Authority of Japan says, "It is a interpretation of the range that can be seen from the sky, and it does not reflect everything, but I would like it to be used as a reference for disaster response and recovery activities."
The Geospatial Information Authority of Japan will take aerial photographs with surveying aircraft as soon as the weather recovers, and will summarize the comparison of aerial photographs before and after the disaster.
Risk of "filling"
Regarding the cause of the debris flow, geotechnical experts point out that the place where the embankment was located is a place where groundwater tends to collect, and this place may have been the starting point of the debris flow.
Susumu Yasuda, an emeritus professor at Tokyo Denki University, who specializes in geotechnical engineering, commented on the image of the upstream part of the debris flow taken by Shizuoka Prefecture. It is analyzed that the water pressure increased due to the rain and pushed out the earth and sand.
Professor Emeritus Susumu Yasuda of Tokyo Denki University
"The most upstream site is a place where water easily collects, and as far as the state of the scooped ground is seen, this place may have been the starting point of the debris flow. It is conceivable that the soil on both slopes was scraped as it went down, and the amount of earth and sand increased at once, causing more damage. "
" When filling the valley and filling it, measures such as improving drainage through a drain pipe at the bottom are taken. It is necessary, but it is necessary to examine in detail what kind of measures were taken and what the original terrain was like. There are cases where there are debris and industrial waste disposal sites in the mountains in other areas. I want you to think that there is a risk. "
Also, if you look at the photographs taken by the Geospatial Information Authority of Japan from the sky around the embankment that collapsed this time, you can see the situation before and after the embankment.
In the photos taken in the 1970s, the trees are overgrown and the ground is invisible, but in the photos taken in 2012 and 2017, you can see how the soil was shaved and piled up in steps.
According to the Ministry of Land, Infrastructure, Transport and Tourism, regarding the collapsed embankment on the upstream side, in 2007, a contractor in Odawara City submitted a notification to Atami City to bring in the soil.
On the other hand, it is not known what the soil was brought in or where it came from.
Professor Noriyuki Daiyoshi of Kanto Gakuin University, who is familiar with embankment construction
"Although measures are to be taken when embankment is carried out, water tends to collect at the connection between the original ground and the embankment, and it becomes unstable when heavy rain falls. It becomes easy to collapse. I want you to think that the places where such embankments are carried out are spread all over the country and there is a similar risk everywhere. "
Governor Kawakatsu of Shizuoka Prefecture said at a press conference after announcing the results of the analysis on the 4th, "This time, all the embankment part was taken, and there was a way to modify the mountain that would be very dangerous. I think there are some references in other areas, so I will thoroughly verify it. "The prefecture will investigate the relationship between embankment and earth and stone flow.