How should large-scale flood damage that hits urban areas be coped with?
It is important for the central and local governments, as well as residents, to cooperate in preparing measures against such a situation.
Five ward governments in the Koto area of eastern Tokyo -- Sumida, Koto, Adachi, Katsushika and Edogawa -- have unveiled a hazard map and resident evacuation plan, in expectation of a maximum-scale flood.
The latest move presupposes a situation in which there are floods along such local rivers as the Arakawa, which runs through the area, following a storm surge and torrential rains caused by an extremely powerful typhoon. When there is an increased risk of flood, the five ward governments would jointly issue a wide-area evacuation advisory. The advisory would urge about 2.5 million local residents -- a figure comparable to more than 90 percent of the area's population -- to evacuate to locations outside of the area by themselves.
The plan is unusual, as it calls on a large number of people to evacuate in a limited span of time. But given the high risk of flood damage in the area, it is safe to say such an evacuation would be a necessary measure.
The area has always been low-lying. Land subsidence has advanced because of the active extraction of groundwater and natural gas that had been carried out since the Meiji era (1868-1912). Consequently, large parts of the area are now at sea level. In addition to the Arakawa, the Edogawa and many other rivers run through the area.
The area was widely flooded when powerful Typhoon Kathleen struck in 1947, leaving more than 1,900 people dead or missing.
The area is usually protected by embankments. However, in preparation for typhoons with increased strength and torrential rains, the five ward governments set up a consultative council and have been discussing necessary responses with the central and Tokyo metropolitan governments.
Set precedent for other cities
The severity of a possible scenario is unimaginable. Densely populated zones in the area would be mostly submerged, with some places expected to remain flooded for more than two weeks. They could be submerged up to 10 meters deep. If people were to remain on the upper floors of a condominium under such conditions, their daily activities would be constrained because of suspended electricity and water supplies.
It would be difficult to evacuate after the area is submerged. If people were left stranded in the order of hundreds of thousands or millions, relief activities could not reach them all.
The problem is how to ensure people evacuate on their own.
Many people have nowhere they can expect to evacuate to. The five ward governments are studying how to secure public evacuation areas. However, they are making no headway in negotiations with local governments outside the area, as their evacuation plan is extremely large-scale. The central government should play a role in coordination with neighboring prefectural governments and other entities, instead of leaving the local governments to do the work on their own.
Another difficult problem is how to secure means for an evacuation. If automobiles are used for evacuation, it would cause huge congestion at bridges, as many rivers run through the area. There is no guarantee that trains will be running smoothly at a time when a typhoon is approaching.
Possible measures include opening expressways to pedestrians so people can walk along them during an evacuation. Another method is for people in each community to evacuate as one, taking advantage of the relatively strong social bonds among residents in eastern Tokyo's traditional neighborhoods. If residents have to evacuate before their homes are flooded, they would be given subsidies for necessary expenses.
These efforts are unprecedented. A bold way of thinking is required.
A similar situation could also be expected elsewhere -- not only in the five Tokyo wards. The evacuation plan devised by the five ward governments will serve as a useful reference for Osaka, Nagoya and other cities.
(From The Yomiuri Shimbun, Aug. 29, 2018)
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