Kumamoto Kuma River flood flow 3 meters per second or more Evacuate to high ground early on August 3 22:38
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In the Kuma River, which flows through Kumamoto Prefecture that flooded at 13 locations, it is thought that the muddy stream overflowing from the river, called the "flood flow," reached a speed of more than 3 meters per second and washed away the houses even about 100 meters away from the embankment. That was revealed by analyzing the videos that were shot at the time. Experts point out that "vertical evacuation to the second floor or above of the building may be fatal, so it is important to move to a safe hill at an early stage."
In the Kuma River in Kumamoto Prefecture, the water level reached a record high at all six observation stations, and the dikes broke at two locations in Hitoyoshi City, as well as at 11 locations including Kuma Village and Yatsushiro City. It crossed over and overflowed from a place without a dike.
Professor Terunori Omoto of Kumamoto University, who specializes in river engineering, conducted a detailed analysis of the flow of water that overflowed from the river, which is known as the "flood flow" based on field surveys and images taken at the time.
Of these, an analysis of a video taken in the Kuma-murato district in the middle stream where the river width narrows sharply shows that the flow is 3 meters per second or more on a national road about 100 meters away from the embankment, and the water depth in the vicinity is 6.2 meters. It turned out that the power of the water was so large that it washed away the houses.
In addition, in the Arase area of Sakamoto-cho, Yatsushiro City, where the houses and shops along the river were completely destroyed, the water level on the left bank, which is outside the curve, is about 3 meters from the right bank from the traces left on both banks of the river. It was also confirmed that it had reached 4.8 meters high.
Professor Omoto analyzes that it is highly probable that the building was collapsed by hitting a village directly by suddenly flowing down a steep national road along which a flood flow, which was driven by centrifugal force, was running.
According to the survey, the area around the Hitoyoshi basin, where the flood flow occurred, and the downstream area became like a river in a wide area, and it is thought that there was a great deal of damage.
Professor Omoto pointed out that not only the Kuma River, but also mountainous areas with steep slopes, there is a risk of collapse and outflow of houses due to flood flow during heavy rainfall, and he added, ``Vertical evacuation to the second floor or above of the building. There is a risk of life, so it is important to move to a safe hill at an early stage.”
Residents "like a tsunami"
The Kuma River suffered a great deal of damage from the flood flow over a wide area of the basin.
Of these, in the teahouse area of Kuma Village, the residents were evacuated and were safe, but at least 21 of the 28 households were destroyed and destroyed.
Yumiko Nakagami (70), who lost her home, said, "The river flow was like a tsunami. We had a house flooded in the past due to flood damage, but when our house was washed away in front of us. I didn't think I would have been saved if I escaped to the second floor."
In addition, Kenji Fukushima, who was run away from an Ayu restaurant operating in Sakamoto-cho, Yatsushiro-shi, said, “The amount of water I have never experienced has increased. I felt that the destructive power of the flow was horrible as the house flowed in."
Depth of water and flow velocity
Damage to houses due to flooding of rivers has been repeated until now.
During the ``Kanto/Tohoku heavy rain'' in September 2015, the Kinugawa embankment broke in Ibaraki prefecture, and nearby houses were completely destroyed.In addition, Typhoon No. 19 in October last year also destroyed the Chikuma river embankment in Nagano prefecture. , The house was washed away by the flood.
According to the Ministry of Land, Infrastructure, Transport and Tourism, whether a building will collapse depends on the speed of flooded water and the depth of inundation.In a simulation, a typical wooden two-story house has a water depth of 2 meters and a flow rate of 4 meters per second. If it exceeds, the house will collapse or run out, so if the water is deep, there is a risk of collapse even if the flow does not reach 4 meters.
In a field survey conducted by Professor Terunori Omoto of Kumamoto University in the Wata district of Kuma-mura, Kumamoto Prefecture, inundation of a residential area reached 6.2 meters, and the analysis results of the video showed that the flooded water flow was 3 meters per second. It was confirmed that the housing was easily washed away.
Professor Omoto said, "Generally, residential areas have roads and houses in addition to levees, so resistance is large and the flow velocity is difficult to increase, but this time the river and surrounding areas are integrated, and the power of the flood flow is powerful. It is believed that the risk of river flooding is evaluated not only by the water depth but also by the flow velocity.”
Not reflected in the hazard map
The Ministry of Land, Infrastructure, Transport and Tourism has designated and announced areas with high risk, such as houses that may be washed away due to flood flow, as "expected flood areas for collapse of houses".
The “expected area for inundation such as house collapse” is particularly high in risk because the power of the flooded water flow is large and there is a risk that the house will be washed away or that the land may be eroded and washed away. The location is specified by the country or prefecture.
In this area, vertical evacuation that stays at home is dangerous, so local governments in the basin are considering evacuation sites and evacuation routes, amending the regional disaster prevention plan, and requesting that it be reflected in the hazard map.
In 2017, the Kuma River basin was designated as an area for flood disasters such as collapsed houses, but this time, there were a lot of damage in Hitoyoshi City, Kuma Village, and Yatsushiro City. It was not listed in.
Regarding the reason, Yatsushiro City said, "Although it reflected the range and depth of the inundation, there was no recognition that the house could be washed away, and it was not possible to inform the residents of the expected flood flow area", Hitoyoshi City "The estimated area covers a wide range, and adding information different from the inundation area may make the map difficult to understand," he said.
In addition, Toru Nakawata, Disaster Management Officer of Kuma Village, said, “In a small municipality such as Kuma Village, the budget is not sufficient enough to request a budget for updating the hazard map every year, and the lack of manpower was not enough to reflect it. However, in the future, I would like to rebuild it based on the lessons learned from this damage."
On the other hand, Hiroki Yamaguchi, Director of the Yatsushiro River National Highway Office of the Ministry of Land, Infrastructure, Transport and Tourism, said, ``We could not reflect the flood flow information because of our lack of information and support. In preparation for the next disaster, we would like to reduce the damage by letting local residents evacuate as much as possible by letting them know the expected flood flow area."
Land use restrictions in Switzerland
How to inform the residents about the danger of flood flow?
According to Professor Yukihiro Shimatani of Kyushu University, who is familiar with flood control measures, it is said that efforts are being made overseas to more specifically convey the danger by not only the depth of the inundation but also the flow velocity of the flood flow on the hazard map. ..
Professor Shimatani said, “In European countries, such as Switzerland, local governments are obliged to reflect on hazard maps and land-use planning. In this case, not only the depth of inundation but also the flow velocity of the inundation flow is taken into consideration. Areas have been set up, and measures have been taken to prohibit or limit the construction of houses and elementary schools according to the degree of danger."
In addition, Professor Shimatani said, “In the future, it will be necessary to consider limiting the use and development of land where there is a risk of flooding in Japan. To that end, cooperation between the national government, local governments and regions is important. It will become.”