The flooding caused by Typhoon No. 19, which inundated many areas of eastern Japan in October 2019, occurs about once every 400 years, according to a Kyoto University research team.
However, if climate change continues as is, flooding on the same scale is expected to become more frequent and occur about once every 25 years by the end of the century.
Levees collapsed at six river systems because of the typhoon. These include the Abukumagawa, Arakawa and Shinanogawa river systems, which flow into the Tohoku, Kanto and Koshinetsu regions. They are all considered important river systems -- or Class 1 river systems -- and the main channels are managed by the government.
There are 31 Class 1 river systems in these regions. The research team analyzed the probability of having six or more Class 1 river systems overflow in the same year, or a Typhoon No. 19-grade flood. Based on meteorological and other data observed over a period of 60 years between 1951 and 2010, the team estimated the probability to be 0.23%-0.27%, or about once every 400 years.
The team made another estimation under the condition that the average temperature will be 4 C higher than pre-industrial temperatures in the 18th century, accounting for a lack of progress in reducing greenhouse gas emissions. The results showed it will likely increase to 3%-5%, meaning similar flooding will occur about once every 20 to 30 years on average. It also became clear that up to 13 river systems could collapse with just one typhoon.
According to the research team, massive flooding occurs because there is an increased amount of water vapor in the atmosphere, which increases the amount of rainfall, as a result of rising temperatures.
"Simultaneous flooding could occur with unbelievable frequency," said Tomohiro Tanaka, an assistant professor at the university who specializes in hydrology. "Disaster prevention measures must be strengthened with this assumption in mind."
Read more from The Japan News at https://japannews.yomiuri.co.jp/
