Kumamoto It is difficult to evacuate due to the sudden rise in the water level at the inundation confluence of the Kuma River Expert 4th August, 15:03
A heavy flood on the river caused by the heavy rain on the 4th of last month caused flood damage to elderly people's facilities, etc. Experts' analysis showed that the rate of rise of the water level was as high as 1 meter per hour, making evacuation difficult. The area is located at the “joint” of the two rivers, and experts point out that early evacuation is important as similar risks exist across the country.
Due to the heavy rain on the 4th of last month, the Kuma River that flowed through Kumamoto Prefecture was inundated and the nursing home for the elderly in the Wata district of Kuma village was flooded, killing 14 residents.
How was the situation at that time? Professor Yasuo Nihei of Tokyo University of Science, who is familiar with the mechanism of river flooding, simulated the inundation situation based on topographic and water level data, and images from river cameras.
Part of the sudden rise in water level was also taken by local residents, and it is said that it is consistent with the results of this analysis.
Professor Nihei said that in addition to the fact that a large amount of water gathered at the confluence of the Kuma River and its tributaries in the Watari district, the fact that there was no escape for water due to the mountain behind was also a factor in the rapid inundation. I am analyzing.
On top of that, Prof. Nihei said, "There is a high level of risk at "Juyubu," which occurs every year. There are many "Juyubu" in various parts of Japan. Please try to evacuate."
Inundation situation simulation
Professor Nihei simulated the inundation situation based on the topography of the Watari area, water level gauge data, and images from river cameras.
6am-flooding begins
As a result, in the Watari district, inundation will begin around 6 am on the 4th of last month.
Flooding began to overflow from the rising water level of the Kuma River and tributaries flowing through the district, and around the nursing home, the rising rate of the water level from 6 am to 7 am was about 27 cm.
Even in the image taken by the residents, at around 5:48 am, the muddy brown water spreads in the school grounds of the elementary school near the nursing home, but the roads and pedestrian crossings are visible.
However, in the image of the elementary school taken from a high place around 6:55 am, the first floor of the gymnasium was flooded, and the schoolyard could not be seen.
7 am-Expansion of flooding
According to Professor Nihei's analysis, the water level of the Kuma River rises further around 7 am, and it seems that the river has spread to the vicinity of JR Hisatsu Line and National Route 219.
The water of the tributary river will not be able to flow into the Kuma River, and will flow directly into the area from around the elementary school, increasing the depth of inundation.
The video taken by the residents around 7:52 am also shows that there was a considerable flow due to the muddy water up to the first floor of the house near the Kuma River.
Inundation depth increases rapidly
After that, a large amount of water continued to flow in and the depth of inundation increased, and it seems that the nursing home reached a depth of 3 meters and 70 centimeters at 10 am.
The rise in the water level due to the inundation from 7 am to 10 am was extremely fast at about 1 meter and 15 centimeters per hour, and it is thought that evacuation was difficult.
The video taken by the residents around 9:38 am also shows that National Highway 219 was flooded with water, and a place near the Kuma River was flooded to a height close to the roof.
Damage occurs one after another at the junction with the tributary
Along the Kuma River, inundation damage occurred one after another at "merging points" with tributaries, even in areas other than the Watari district.
A private surveying company, Pasco, analyzed images of satellites taken on the 4th of last month, and found that along the Kuma River, inundation damage had occurred at 19 confluences where only the photos could be taken. It was
Confluence damage all over the country
Even in the past disasters, the damage at the "merging area" has continued.
In Mabi-machi, Kurashiki City, Okayama Prefecture, where large-scale inundation damage occurred due to heavy rain in western Japan, the embankment broke near the confluence of the Odagawa and the tributaries Takamagawa and Suemasa However, a lot of water flowed into the residential area.
Last year's Typhoon No. 19 caused major damage near the confluence of the Abukuma River and the tributary Adachi Tara River in Motomiya City, Fukushima Prefecture, and the Tama River in Takatsu Ward, Kawasaki City, Kanagawa Prefecture. Inundation damage has also occurred near the confluence of the Hirase River.
Expert "Emergence of confluence early in every part of the country"
Professor Yasuo Nihei of Tokyo University of Science, who analyzed the Kuma River, said that the cause of the rapid inundation that occurred in the Watari district was that it was the "merging part" of the river and the surrounding "topography". I'm pointing out.
The "merging part" of the two rivers
According to Professor Nihei, the Watari district is located in the mainstream of the Kuma River and the “merging part” of the tributary that flows into it, making it easy for water to collect.
Terrain surrounded by mountains
In addition to that, it is analyzed that it was surrounded by mountains in the back, and it was difficult to drain the water accumulated in the lower part, which caused a rapid inundation.
Professor Nihei said, "At the confluence part, once flooding occurs, there are many places where there is no escape area and the water level rises rapidly. Recently, remarkable damage has occurred every year, so special attention is required. I point out.
In addition, “Japan has rivers all over the country, and there are many places where large rivers and small tributaries meet. Especially, those who live near the confluence should be careful about the risk of inundation. It is very dangerous to evacuate, so it is important to evacuate to a safer place farther away at an early stage, "horizontal evacuation."