At 2 a.m. on Sept. 21, Typhoon Roke, the 15th and biggest tropical storm yet to assault Japan this year, was over the Pacific 200 km south of Shikoku making its way slowly and ominously westward toward the main island of Honshu.

In Tokyo, where an atmosphere of public calm had reigned before the storm, Tetsuya Shimazu — who is in charge of disaster prevention in the Tokyo Metropolitan Government’s rivers department — was already on high alert in his 22nd-floor office in Building No. 2 of TMG’s twin towers in Shinjuku.

With him were many other TMG staff, some of whom had been summoned that night from the nearby public housing where they are billeted apart from their families whenever disaster looms or has struck the city — whether due to typhoons, earthquakes, tsunami or anything else.

Then, as dawn broke, things rapidly got busier for Shimazu and his colleagues. At 6:54 a.m., the Meteorological Agency upped its forecast for heavy rain and flooding in Tokyo’s central 23 wards and the suburban areas of Tama to keihō — the top-level alarm. That was followed by another keihō for mudslides in the western Tokyo city of Machida, and a lower-level chūihō (alert) for high tides in all Tokyo’s central wards.

Subsequently, in western Tokyo, which was hardest hit by the typhoon rains, 37 mm fell between 11:27 a.m. and 12:26 p.m.

Ever since that first keihō warning, Shimazu and his staff had been in a special operations room equipped with large monitors displaying real-time videos of the many rivers across Tokyo. Information from the Meteorological Agency was also being monitored there on a dedicated computer terminal that sounded an alert whenever new data arrived. With these inputs, the river management team was able to quickly assess flood risks, then use wireless emergency fax networks to issue warnings and advisories in the most timely fashion to any of Tokyo’s wards, cities, towns and villages.

At one point during that evening of Sept. 21, the Shiba and Shin-shiba rivers running through the city’s northeastern Adachi Ward rose to dangerously high levels, prompting Shimazu’s office, together with adjoining Saitama Prefecture and the Meteorological Agency, to issue a chuiho alert that is triggered when water levels in a river reach 40 to 60 percent of the embankment’s height. Next, if the levels kept rising to between 70 and 90 percent, there would be a keihō — an alarm that would quite likely be issued along with an evacuation order issued by municipalities to any residents in danger.

Besides the rainfall, river levels and mudslides, though, another serious concern that day was the high tide in Tokyo Bay. At the center of all cyclones the atmospheric pressure is low, and at the center of typhoons it is very low — so causing sea levels to rise to abnormal levels. Consequently, on Sept. 21 TMG’s river bureau started closing the 10 water gates facing the sea that it controls from 10:40 a.m. to prevent flooding in the city. Elsewhere, officials in TMG’s port and harbor bureau were also keenly attending to the 19 gates they oversee, and central government workers to the 10 for which they are responsible.

At the end of the day, Tokyo suffered minimal damage from Typhoon Roke, and only a handful of homes and other buildings were flooded. Although record winds of up to 40 meters per second blew over some big trees in central Tokyo and led to train services being suspended for hours — so stranding thousands of homebound commuters — nobody was killed or severely injured.

“Thankfully, the total rainfall (at Takao in western Tokyo, hit by the heaviest rain) remained at 232 mm,” Shimazu said. “And the rivers didn’t overflow.”

But what could have happened, if the storm had been more powerful, is a completely different story.

With global warming and the ongoing concretization and asphalting of Tokyo exacerbating the “heat-island” effect, according to experts the city’s chances are greater than ever of being swamped either by typhoons or one of the increasingly frequent “guerrilla storms” that strike suddenly with short but massive downpours in localized areas.

That bleak prognostication follows logically from the fact that Tokyo’s current flood-management systems — from river-improvement projects to underground runoff routes and its water gates — are designed to cope with downpours of up to 50 mm per hour. In recent years, however, they have often reached 100 mm per hour.

In July 2010, for instance, a guerrilla storm caused the Shakujii River, a tributary of the Arakawa, to overflow and flood parts of northeastern Kita Ward. That resulted in 90 staff at a warehouse run by a subsidiary of Japan Tobacco being temporarily trapped — and cigarette products worth hundreds of millions of yen being spoiled.

Akira Kawamura, a professor of engineering in the Tokyo Metropolitan University’s school of environmental sciences, believes Tokyo’s flood risks are growing, due both to global warming, which has made typhoons more violent, and the aforementioned “heat island” phenomenon as heat-absorbing asphalt roads and concrete structures spark massive thermals leading to rain and thunderstorms in certain conditions.

In Tokyo, Kawamura added, most of the ground is paved, and therefore rain water is not absorbed by soil. Instead, it goes directly into drains that release it into rivers. Also, since Tokyo’s water-management system is structurally capable of dealing with only 50 mm of rain per hour, its sewers are at risk of overflowing — as seen in recent cases of manhole covers on the streets popping up and water gushing out like a fountain in areas hit by torrential rain.

In fact, the government has already mapped out a grim scenario for if one of the catastrophic downpours believed to occur once every 200 years destroys the embankment along the Arakawa River which flows into Tokyo Bay and is one of the nation’s biggest.

Based on this scenario, a January 2009 report released by the national government’s Central Disaster Prevention Panel concluded that 97 underground train stations on 17 railway lines — comprising 70 percent of Tokyo’s entire underground rail system — could be flooded if the river broke its banks.

Another of the panel’s worst-case scenarios, which sounds like a story from a horror movie, estimates that up to 6,300 people would be killed if Tone River, which winds through several prefectures in the Kanto region on its way to the Pacific, overflows at a particularly vulnerable point in Kazo, Saitama Prefecture.

Kawamura said that another factor contributing to the problem is the rampant property development in recent years that has taken place in traditionally flood-prone riverside areas of Tokyo, such as along the Sumida River.

“In addition to the environmental threat, we have this social problem of building houses far too close to urban rivers that are so vulnerable to floods,” he said.

So what can we do at an individual level?

Kawamura said that the most basic advice is just to avoid living in flood-prone districts. All the municipalities in Tokyo have their own flood-hazard maps, and these can be viewed (in Japanese only) at www.kensetsu.metro.tokyo.jp/suigai_taisaku/index/menu03.htm.

Those living in high-risk areas who are unable or unwilling to move should be sure to check out their local evacuation areas, Kawamura advised. However, when small neighborhood rivers overflow, in reality there’s very little time to react.

“So the safer bet might be to run up the stairs to the second floor of your house, instead of rushing outside,” he said.

Beyond such basic survival tips, TMG could acquire riverside areas over a span of 100 years and turn them into unused land that could be allowed to flood, Kawamura said. However, he stressed that the key to this strategy succeeding would be to ensure that the government bought plots in such areas as they come onto the market — and at market value — instead of resorting to the costly process of compulsory acquisition.

Otherwise, Kawamura explained, any government project in the name of river improvement could drive up the value of private property in such areas at the taxpayers’ expense.

Nevertheless, Frans H.M. van de Ven, a water management expert with the independent, Dutch-based research institute Deltares, said that Tokyo’s flood-management system, as it stands, is a good example for the world.

“Fluvial (river) and pluvial (rain) flood risks (in Tokyo) are high, but are very well controlled by the strong drainage infrastructure,” Van de Ven said via email after visiting Tokyo last month to give a presentation on water management at the Embassy of the Netherlands.

“Flood protection is further strengthened nowadays by on-site storm-water runoff-reduction facilities and water-resilient urban development/redevelopment — (meaning) constructing houses, other buildings, streets, power networks and other infrastructure in such a way that they are less vulnerable to flooding.”

As for future projects, Van de ven said Tokyo could look at further greening its riverside infrastructure to help reduce the heat stress by bringing more evaporation and shade to the city. This would also help the metropolis reduce its energy demand for air conditioning in summer, he noted.

“Greening the riverside infrastructure could also strengthen the urban landscape quality by making the water more visible and accessible for the public,” he said.

“In the long run, one could think of harvesting river- and storm-water for low-end purposes like irrigating urban green areas and flushing toilets.”

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