Inundation area of ​​Typhoon No. 19 matches much with the inundation assumption in the hazard map

Inundation area of ​​Typhoon No. 19 matches much with the inundation assumption in the hazard map October 25, 18:40

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River inundation due to dyke breaks caused by typhoon 19 heavy rain.
NHK analyzed the flooded areas of the eight river basins such as the Abukuma River in Fukushima Prefecture, where the damage was particularly severe, and found that most of them were supposed to be flooded in the “hazard map” of the local government.

(If you look at the video, you can compare "Inundation assumption" with "Actual inundation area")

NHK compares the actual inundation range estimated by the Geospatial Information Authority of Japan with respect to the eight river basins that have been inundated due to the breach of the embankment, etc. Analyzed.

The river basin compared is
▽ Abukuma River in Fukushima Prefecture and Miyagi Prefecture,
▽ Yoshida River in Miyagi Prefecture
▽ Chikuma River in Nagano Prefecture,
▽ Iruma River, Koshibe River, Izu River in Saitama Prefecture,
▽ Naka River and Kuji River in Ibaraki Prefecture.

The map overlap analysis used was the “inundation assumption” of the Ministry of Land, Infrastructure, Transport and Tourism, which is the basis of the hazard map of the local government, and the electronic data of the “inundation area” estimated by the Geospatial Information Authority of Japan based on aerial photographs.

In the vicinity of Yoshida River, Ogo-cho, Miyagi Prefecture ...

Overlapping and analyzing in detail with the map information visualization system, the actual inundation area agreed with the area that was supposed to be inundated in a wide area, which was Ogi-cho, Miyagi Prefecture, in the basin of the Yoshida River. It is near.

In the area near the Yoshida River, the inundation situation was almost the same, and inundation occurred in the area around the tributaries that flowed into the Yoshida River.

Downstream of the Abukuma River In Kakuda City, Miyagi Prefecture and the surrounding area ...

In Kakuda City, Miyagi Prefecture, which is located downstream of the Abukuma River, and the surrounding area, the estimated inundation area spreads radially along the tributaries, but inundation spread along these places.

Chikumagawa In Hoho Nagano City and the surrounding area ...

Around 950 hectares, which were estimated to be flooded, were assumed to be flooded in Hobo and its surroundings, where two people died after the bank of the Chikuma River broke down and many houses were damaged by water. It was completely included in the range.

In 4 rivers, all inundation areas are “expected inundation area”.

Of the 8 rivers analyzed this time,
▽ The four rivers of the Chikuma River, the Iruma River, the Koshibe River, and the Naka River
It was confirmed that all flooded areas were included in the estimated flood area.

Some places were damaged outside the scope of flooding assumptions ...

On the other hand, in the heavy rain of Typhoon No.19, there were places where damage occurred outside the range where flooding was supposed.

In Marumori Town, Miyagi Prefecture, inundation damage also occurred in residential areas and fields outside the areas that were assumed to be flooded by the Abukuma River in hazard maps.

According to the town, the inundation in this area was not caused by the Abukuma River itself, but was caused by the flooding of its tributaries, and at least 70 houses were flooded on the floor.

Why wasn't flooding assumed in the hazard map in this area?

The country and town explained that the tributary of the Abukuma River where the flooding occurred this time was not included in the assumption because the river was small and was not the target river for inundation. It is.

Hazard maps are not well understood

On the other hand, local residents are not fully aware of the hazard maps of local governments.

Four years ago, Joso city where Kinugawa embankment in Ibaraki Prefecture broke down due to heavy rain in Kanto and Tohoku in 2015, and many houses were washed away. As a result of the survey, 65% responded that they did not know or had never seen the hazard map, while only 10% confirmed the hazard map and understood the degree of inundation. .

The Ministry of Land, Infrastructure, Transport and Tourism says that the number of people who know hazard maps is increasing due to floods such as heavy rains in the West of Japan last year, but there are not many people who understand the contents and connect to actions such as evacuation.

Expert “Knowing local risks with hazard maps”

Associate professor Naoya Sekiya of the University of Tokyo, who is familiar with evacuation in the event of a disaster, said that most of the flooded areas were in the range assumed in the hazard map. The importance of the hazard map was shown again in this heavy rain.Since the area where flooding occurs is determined to some extent, it is important to look at the hazard map and know the risks in the area where you live. " It was.

On the other hand, regarding the occurrence of inundation in some areas that were not expected, “The flood hazard map is based on the assumption of inundation mainly in large rivers, and there are places that are not subject to assumptions along small rivers. This is not to say that areas that are not supposed to be inundated are safe.Since small rivers can flood and the surrounding areas can be inundated, make sure that you are careful about being close to the river. " Pointed out.