in Japan Time
https://www.japantimes.co.jp/news/2019/10/16/national/flooding-tama-river-tokyo-crisis-typhoon-hagibis/#.XajgDPZuLIU"Flooding of Tama River put capital on the brink of crisis during Typhoon Hagibis
by Reiji Yoshida
Staff Writer
Oct 16, 2019
Tokyo faced crisis last Saturday, with water levels in the Tama River quickly climbing as heavy rains and winds from Typhoon Hagibis inundated the Kanto region on an unprecedented scale.
Hakone, in Kanagawa Prefecture, saw a staggering 922.5 mm of rain that day alone — three times as much as the total for the month of October of an average year.
Levees all along the Tama, which stretches over 138 kilometers between Tokyo and Kanagawa Prefecture, were designed to withstand precipitation levels seen only once in every 200 years. But at the Ishihara observation station in the capital’s Chofu area, water levels had hit their highest-ever record of 6.24 meters by 11 p.m. on Saturday, far exceeding the 5.9 meters threshold the levees were built to withstand.
Since the levees were designed to have a safety margin of 1.5 meters, making their total height 7.4 meters at the Ishihara observation station, the riverbanks withstood the storm, but only barely.
Any failure of levees along the Tama River could have brought devastating flooding to areas of Tokyo and Kanagawa. For the first time ever, the city of Kawasaki issued an urgent warning, for 915,770 local residents to evacuate by 7 p.m. that night.
“Yes, the situation was very tense,” said Kenichi Ito, who heads the initial crisis management response team at Kawasaki Municipal Government.
In the age of climate change
That tense night for Tokyo and Kanagawa residents has underscored the risks Japan faces in the age of climate change, predicted to increase the number of powerful typhoons like Hagibis.
“This time, the (levees of the) Tama River withstood the typhoon well,” said Nobuyuki Tsuchiya, a senior civil engineering expert for the Tokyo-based Japan Riverfront Research Center.
But given the progression of climate change, stronger typhoons are more likely to strike Tokyo and the metropolitan area, which are “not in any way ready yet (to handle such storms),” he said.
In March 2018, the Tokyo Metropolitan Government released the results of a flood simulation based on a worst-case scenario involving a massive typhoon simultaneously causing heavy rains and tidal flooding.
The results were shocking: Waters would submerge about one-third of the 23 wards of central Tokyo, including 90 percent of Sumida, Katsushika and Edogawa wards, as well as parts of the Marunouchi, Shimbashi and Ginza downtown business districts — the heart of the nation’s capital.
The three wards in eastern Tokyo are particularly vulnerable because many of them are so-called “zero-meter zones,” meaning they are lower than sea level.
According to the metropolitan government, the simulation was based on a worst-case scenario that could happen only once every 1,000 to 5,000 years. But experts warn that powerful typhoons are likely to hit Tokyo more frequently than in the past as the climate continues to warm.
Last Saturday, the Arakawa River also rose to an alarming level, prompting the Edogawa Ward Office to issue an advisory for 432,000 local residents to evacuate."
