There is a possibility that the embankment will collapse even in typhoon No. 19 "Arakawa" last year ... Expert analysis October 12, 18:29
Expert analysis revealed that in the worst case of typhoon No. 19 last year, the embankment could collapse even in the "Arakawa" flowing through the metropolitan area.
At that time, if the water level was calculated based on the multiple rain forecasts issued by the Japan Meteorological Agency and it broke, there was a risk of serious damage, and experts said, "We think that the flooding of Arakawa can occur sufficiently. We should proceed. "
Last year's Typhoon No. 19 reached a record level, such as observing the highest water level of 13 meters and 8 centimeters in the history of observation at the observatory in Nishi-ku, Saitama City, Arakawa, but no flooding occurred.
How to rain If it's even stronger ...
What would happen to the water level if it rained even harder?
Professor Yasuo Nihei of Tokyo University of Science, who specializes in river engineering, simulated a section of Arakawa about 40 km from Kumagaya City, Saitama Prefecture to Toda City, Saitama Prefecture.
For "How to rain", we used 21 forecasts actually announced by the Japan Meteorological Agency about 10 hours before the typhoon landed.
Of these, when we examined the worst case where the amount of rainfall in the main stream of Arakawa was 80 mm more than it actually was in two and a half days, the water of the river did not cross the embankment, but from the mouth of the river. On the right bank near "Hanekura Bridge" in Shiki City, Saitama Prefecture, which dates back 37.2 kilometers, the water level was only 26 cm until the water flowed into the residential area.
"Planned high water level" super 12 hours or more
Furthermore, it was found that the water level of 23 km, which is about half of the analyzed section, exceeds the "planned high water level", which is the upper limit water level that the embankment can withstand by design.
In some places, the water level exceeded this level for more than 12 hours, and according to Professor Nihei, if the water level exceeds the "planned high water level" for a long time, water permeates the embankment and there is a possibility of collapse. Is to increase.
Even in the vicinity of "Hanekura Bridge", it will continue to exceed for 6 hours, and if a collapse occurs here, the Ministry of Land, Infrastructure, Transport and Tourism estimates that in the worst case, there is a risk that a wide area to Tokyo Itabashi Ward and Kita Ward will be flooded. I will.
Expert "Flood of Arakawa may occur enough"
Professor Nihei pointed out that the flood did not occur because it actually rained less than expected, and said, "The flooding of Arakawa is not unrealistic and it is possible that it will occur sufficiently. I want it. "
If the embankment breaks ...
What if Arakawa broke the embankment on the right bank near "Hanekura Bridge" in Shiki City, Saitama Prefecture?
Looking at the website of the Ministry of Land, Infrastructure, Transport and Tourism, "Inundation Navi", in the worst case, the overflowing water is said to spread over about 50 square kilometers from Kawagoe City, Saitama Prefecture in the north to Akabane, Kita Ward, Tokyo in the south.
Near the collapse point, it is expected that the second floor of the building will be submerged by 6 meters.
Inundation spreading in Tokyo
Inundation has spread to Tokyo,
and it is expected that the first floor of
the
building will be submerged
, with
▽
3 meters around "Takashimadaira Station" in Itabashi Ward
and 4 meters in front of "Ukimafunado Station" in Kita Ward.
I will.
Inundation of about 1 meter is expected at Akabane Iwabuchi Station in Kita-ku, Tokyo, which is about 15 kilometers away from the collapse site.
In the worst case, "wide area evacuation" is necessary
About the flooding of Arakawa In the worst case, the country estimates that large-scale inundation will spread in the "Koto 5 Wards" of Edogawa Ward, Koto Ward, Katsushika Ward, Sumida Ward, and Adachi Ward.
According to this, about 2.5 million people live in the inundation area, and the capacity of evacuation centers in local governments is only 200,000.
For this reason, 930,000 people need to continue living on the upper floors of condominiums, and 1.37 million people need "wide area evacuation" outside the local government.
Residents around Ukimafunado Station
If Arakawa collapsed, we talked to residents around JR Ukimafunado Station in Kita-ku, Tokyo, which is expected to be inundated by about 4 meters.
A mother with a fourth grader and a three-year-old child said, "I knew the risk of flooding in this area, but there are few places to escape, so I would like to have a tower where I can evacuate."
In addition, a woman in her 60s said, "I think it is necessary to evacuate early because my husband is physically handicapped, but it is difficult for a couple alone, so it is difficult to find someone to help."