Chikuma River Breakup Main cause of water level rise upstream of narrow river | NHK News

Chikuma River broke down The main factor is the rise of the water level upstream of the narrow river area October 16th 6:00

Regarding the collapse of the bank of the Chikuma River flowing through Nagano Prefecture, the Ministry of Land, Infrastructure, Transport and Tourism is characterized by the fact that there are alternating locations where the river is wide and narrow, and the main factor is that the water level has risen upstream of the location where the river is narrow. It is said.

According to the Ministry of Land, Infrastructure, Transport and Tourism, the river width at the place where the bank of the Chikuma River broke was about 1 km, but the river width was about 200 meters in the section about 7 km downstream, which means that the river width has narrowed abruptly.

For this reason, when the river increases, the flow becomes worse in the narrower river, and the water level is stagnated and the water level rises so that it can be blocked in the upstream, leading to the breakage of the embankment.

The Chikuma River has been flooded for the same reason in the past, and in 1983, the levee in Iiyama City, Nagano Prefecture, broke down due to heavy rain caused by a typhoon, causing massive inundation damage.

In order to prevent damage, the national government has formulated a river improvement plan for about 30 years since 2014, and is proceeding with construction to reinforce the embankment and excavate the riverbed.

A place where the river narrows suddenly

Regarding the flooding of the Chikuma River, Professor Yasuo Nibei of Tokyo University of Science, who is familiar with river disasters, said that the flooding occurred upstream from a place called the “narrow part” where the river width suddenly narrowed, and there has been repeated flood damage since ancient times. It points out that it was happening.

Prof. Niwae says, “I think that the flood control measures were taken, but it was raining more than that, and it was thought that it brought about such flooding.”

Furthermore, after pointing out that there are places in the country where the river width suddenly narrows, such as the Chikuma River, Professor Nibe said, “I think it is difficult for local residents to judge where the width is narrow and dangerous. In the hazard map created by the local government, the inundation area is drawn, so please understand it first, and even if the disaster happened is not where you live, I want you to think about it instead. ”