Typhoon Roke Threatens Japan: AIR Report

September 21, 2011

According to catastrophe modeling firm AIR Worldwide more than a million people were evacuated in central and western Japan in advance of Typhoon Roke’s arrival. The storm formed on September 10 and is the 15th named storm of the 2011 Northwest Pacific Typhoon Season. Japan is still recovering from the effects of Typhoon Talas.

AIR said the storm is currently “being steered around the western boundary of a nearly stationary subtropical ridge. The eye of Roke, which has become less organized over the last few hours, was located about 500 km [312.5 miles] southwest of Osaka and is moving 16 km/h [10 mph] in a northeastward direction according to the Japan Meteorological Agency’s 1400 UTC advisory.”

Typhoon-force winds extend outward up to 80 kilometers [50 miles] from its center, while tropical-storm force winds extend outward up to 260 kilometers [162.5 miles]. Maximum 1-minute sustained wind speeds are currently 200 km/h [125 mph], making Roke a Category 3 typhoon. Severe weather warnings are in effect for central and western regions of Honshu.

“The storm is expected to accelerate and maintain its present northeastward movement toward Japan’s coast,” explained Dr. Peter Sousounis, principal scientist at AIR Worldwide. “Roke is projected to slowly weaken as it approaches Japan. Roke is currently forecast to make landfall in central Honshu, about 200 km southwest of Tokyo, and then move northeastward across central Japan on Wednesday.”

He also noted thatn there, “is some concern that this current path could take the storm towards Japan’s earthquake-devastated Tohoku region. Roke will undergo extratropical transitioning and begin weakening after landfall, and its rapid forward speed should carry the storm across Japan in approximately 12 hours.”

“Japan’s mountainous coast will enhance precipitation on the north and east sides of the storm, creating flood and landslide hazards. Additional precipitation is estimated on the order of 200-300 mm [app ¾ to 1 ¼ inches] in low-lying coastal regions. However, the increased forward speed should limit the most extreme precipitation scenarios.”

AIR said the JMA has “warned of flooding and landslide threat in the western prefecture of Wakayama after it was badly affected by Typhoon Talas. Authorities are now concerned that rain from Roke will cause the collapse of mud dams that formed in the aftermath of Typhoon Talas, which would cause downstream flooding. The water level behind a mud dam in the city of Tanabe in Wakayama had already risen to 60 cm [app. 2 feet] below full capacity in the afternoon, compared with 2.5 meters [app.8 feet] just a few hours prior.”

According to AIR, “Japan has strict and well-enforced construction codes; modern structures are expected to withstand Roke’s forecast wind speeds with minimal damage, again, making the primary concern from Roke flood damage. Flood damage in Japan is not automatically included in wind policies.
“The vulnerability of buildings to flood damage varies by construction type. For a given flood depth, a residential wood-frame building is expected to sustain more damage than a residential masonry building. Concrete construction is less vulnerable to flood than steel or masonry. Commercial and apartment buildings usually have stronger foundations than residential buildings, and are thus better able to resist flood loads.

“Flood vulnerability also varies by building height. Because damage is usually limited to the lower stories of a building, high-rise buildings will experience a lower damage ratio—the ratio of the repair cost and the total replacement value of the building—than low-rise buildings because a smaller proportion of the building is affected.”

Source: AIR Worldwide.

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