Nearly 70 percent of underground railway tracks in Tokyo could be flooded if a riverbank burst following catastrophic rains, a government panel said Friday.
The worst-case scenario based on a once-every-200-years flood, was raised in a report by the government's Central Disaster Prevention Council, which is also working on guidelines for a passenger evacuation manual before March 2010, it said.
According to the report, if the capital were hit by disastrous rains, as many as 97 underground railway stations on 17 railway lines could be flooded. The damage could expand much faster below than on the surface, since railway tunnels would act like water pipes, it said.
Train services would be suspended, paralyzing the capital for a long time as repairs are made.
The panel's scenario assumes 550 mm of rain falling in three days along parts of the Arakawa River, which flows along the Tokyo's border with Saitama Prefecture, overcoming riverbanks in Kita, Adachi or Sumida wards.
Experts think that kind of rainfall only hits once every 200 years.
The worst damage would be in Kita Ward, where floodwaters would inundate the underground just 10 minutes after the riverbanks burst and hit Tokyo station in 12 hours, and Ginza and Kasumigaseki stations in 15 hours.
The flood damage could cover as much as 147 km of subway track by the sixth day, submerging 121 km of rail and filling 81 stations to their ceilings.
In the event a riverbank breaks in Adachi Ward, 138 km of track covering 16 lines and 89 stations could be flooded. In Sumida Ward, 37 km on nine lines and 27 stations would be at risk.
The panel proposes covering station gateways with plastic sheets, steel sheets and wooden boards, on top of the water bars.