Chikuma River Breakup Topographical factors that narrow the river width will also be affected? Expert pointed out October 13 19:09

Prof. Yasuo Niwabe, a professor at Tokyo University of Science, who is familiar with river disasters about the damming of the Chikuma River flowing through Nagano Prefecture and flooding a wide area, in addition to the record heavy rain in the basin, It is pointed out that the levee may have collapsed due to topographical factors that suddenly narrowed.

According to the Hokuriku Regional Development Bureau, the Ministry of Land, Infrastructure, Transport and Tourism, Chikuma River was a hobo district in Nagano City. According to Professor Niwabe's analysis of the video, the depth of the inundation was about 3 meters because there was a place where the first floor of the house was almost submerged in the vicinity of the broken site.

In the vicinity of the place where the embankment broke down, the river width is close to 1 km, but as it goes downstream, it narrows rapidly and the downstream river width is about 200 meters narrow.

According to Professor Niebe, it is difficult for water to flow in such a narrow river area, and in the upstream part, water accumulates so that it can be dammed and the water level tends to rise.

For this reason, it has been pointed out that even in the vicinity of the Hobo area, the high water level continued for a long time, and the water overflowing from the river crushed the levee, leading to the failure.

“After the heavy rain in western Japan last year, flooding occurred in a wide range of rivers at the same time. It was difficult to prevent flooding with only hardware, and it was difficult to prevent flooding. It is important to improve the preparations for evacuation of the local residents. "

Chikuma River Water level rises about 12 meters in 16 hours

According to the Chikumagawa River Office of the Hokuriku Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, the “Chikuma River” flowing through Nagano Prefecture had a levee break for about 70 meters in the Hoho district of Nagano City.

In the data of the water level observatory in Nakano City Tachibana, about 6 km downstream from the site where the breakdown was confirmed, the water level began to rise gradually from the morning of the 12th.

The change in the water level was about 10 centimeters per hour until around 1 pm on the 12th, but it rose about 50 centimeters per hour between 2:00 pm and 7:00 pm.

Furthermore, it rises by about 90 centimeters by 1 hour until 8 pm, by 1 meter by 1 hour until 9 pm and by 1 pm, and by 1 hour until 11 pm The water level suddenly rose to about 1 centimeter and 70 centimeters.

As a result, we observed 10 meters and 14 centimeters at midnight on the 13th, and exceeded the 9 meter and 60 centimeters of the “flooding dangerous water level” where the risk of flooding is very high.

About one hour later, at around 0:55 am, the river was flooded and the dike started to be cut, and the inundation was confirmed with a surveillance camera.

After that, the water level continued to rise and reached a maximum of 12 meters and 44 centimeters around 4 am. At noon on the 12th, the water level was 40 centimeters, which means that it rose about 12 meters in 16 hours.

The Chikuma River office does not know the exact time when the embankment broke down, but from around 2 am when the video of the surveillance camera was interrupted to around 5:30 am when the staff visited the site. The embankment is believed to have been destroyed.