In this way, Chikuma River broke down.

The Chikuma River broke down in this way. Even at a distance from the ruptured place, it was inundated for 4 meters and at 20:20 on November 2.

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Simulation results have shown that evacuation after a levee “breaks” due to flooding of rivers caused by typhoon No. 19 in the various areas of the river is directly linked to life risks. On the Chikuma River in Nagano Prefecture, the inundation spread at once due to the “breakdown” of the embankment after the inundation, and the depth reached up to 4 meters even in an area about 2 km away from the breakup site.

Professor Yasuo Nibei of Tokyo University of Science, who is familiar with the mechanism of river inundation, explained the extent of inundation at that time, based on information such as "flooding" and "breakdown", surveys conducted in the field, and interviews with residents. Simulated depth.

According to this, when flooding occurs due to “overflow” where the water crosses the embankment at Hobo, Nagano City, Chikuma River, inundation begins in the area near the embankment.

Around 2am after about 2 hours, flooding due to "overflow" occurred in the range of about 1km north and south of the dike. The depth of inundation reaches a maximum of about 1 meter in the area along National Route 117, about 1 km away on the west side.

Then, at around 4 am, when the levee “breaks”, the inundation area spreads all at once, and at about 5 am one hour later, the inundation crosses the national highway and reaches the Shinkansen railroad tracks and vehicle bases. The maximum is over 2 meters.

Furthermore, at around 7:00 am, 3 hours after the breach, the inundation range greatly expanded beyond the Shinkansen line, reaching a maximum depth of 4 meters in an area about 2 km away from the breach. .

The inundation stopped almost at 8am, 4 hours after the breach, but eventually a large area of ​​950 hectares was inundated.

According to Prof. Niwabe, when the depth exceeds 2 meters, the number of people who lose their lives without being able to move in the building will increase, and when it reaches 4 meters, the first floor will be completely submerged. “The evacuation after that is extremely dangerous.

On that basis, Professor Nibe said, “If it breaks, the situation will change and the amount of water will increase by an order of magnitude. The momentum of the water will become much stronger, so the evacuation at that stage will be life-threatening. It ’s more important than anything. ”

Details of the simulation

In addition to the known inundation area, estimated flooding and rupture times, Professor Niwabe of Tokyo University of Science performed simulations based on local trace surveys and interviews with residents.

Flooding changes from blue to green to yellow as it deepens.

Blue is up to 1 meter, green is up to 3 meters, and yellow is up to 4 meters.

Inundation

Around 1 am, when flooding occurs due to “overflow” in Hoho, Nagano city, where the water crosses the embankment, the area along National Route 117, about 1 km away from the west, begins to flood.

Around 3:00 am before the break, flooding due to “overflow” occurred about 1 km north to south of the dike. The depth of flooding reaches up to 1 meter.

At this time, the Shinkansen vehicle center, which has been raised by 2 meters, has not been submerged.

4:00 am “Breakdown”

When the embankment “breaks” around 4am, the flooded area expands all at once.

Inundation deepens on the east side of National Route 117, and the inundation extends beyond the national road to the west.

At around 5:00 am, one hour after the crash, you will reach Shinkansen railroad tracks and vehicle bases further west of the national highway. The depth of flooding on the east and west sides of the national highway has exceeded 2 meters.

According to Professor Niebe, if the depth exceeds 2 meters, it will be difficult to move in the building, and more people will die.

3 hours after the break

Around 7:00 am, 3 hours after the “breakdown”, it spread beyond the Shinkansen line, and the depth of inundation reached about 4 meters at maximum in Toyono, 2 km away from the place where it broke.

If the depth of flooding is 4 meters, the first floor of the house will be completely submerged and it will be difficult to move on the second floor.

And at about 8:00 am, 4 hours after the “breakdown”, the inundation almost stopped, but eventually the area of ​​950 hectares was inundated.

Check hazard map

In the simulation, the inundation is deepest at a place away from the Chikuma River.

Professor Nippei said, “Even if you are away from the river, if a large river overflows like this time, water will flow into the low land and the inundation will deepen. Check the hazard map and check the risk of flooding in your place again. I wanted. "

The simulation does not include the influence of the tributaries and waterways that flow on the west side of the Chikuma River, so the actual inundation speed may have been even faster, so Professor Niwabe will proceed with the analysis.