Risk of flood high in Japan in wake of Typhoon Noru

Even with modern flood-control structures, the risk of flood damage remains high in Japan in the aftermath of Typhoon Noru, which made its first landfall in Yakushima, an island in Kagoshima Prefecture, Japan, as a Category 1-equivalent storm, before making a second landfall near Kainan City in Wakayama Prefecture, Japan, according to catastrophe modelling firm AIR Worldwide.

Noru was the fifth tropical cyclone to form in the West Pacific Basin this typhoon season, developing first as a tropical depression on July 20 and then intensifying between July 29 and 30 from a tropical storm to a Category 5-equivalent storm—the strongest this year to date—after engulfing Tropical Cyclone Kulap in a rare instance of what is known as the Fujiwhara effect.

Noru then weakened to the equivalent of a Category 3 storm on August 3 as it passed over colder waters and moved northwestward toward southern Japan. It continued to weaken as it tracked farther northward through August 4, reaching Category 2 equivalency. Noru weakened to a Category 1-equivalent storm before making its first landfall in Kagoshima Prefecture on the island of Yakushima, after which it turned northeastward. The storm experienced slow and steady weakening before making its second landfall in Wakayama Prefecture, near the city of Kainan, on the main island of Japan.

More than 400 flights were canceled as Noru approached central Japan and West Japan Railway Company halted operations of more than 70 express trains. Three oil companies reportedly held back shipments because of the storm. Noru’s slow forward movement of 20 km/h is raising the risk of prolonged rainfall. Residents should be prepared for landslides, swollen and overflowing rivers, and high waves, according to the Japan Meteorological Agency (JMA).

According to AIR, Japan has strict and well-enforced construction codes, although many existing structures predate the existence of these codes. Residential exposures in Japan are dominated by wood construction; non-wood residences primarily consist of steel and concrete. Modern wood construction typically demonstrates the best performance in typhoons among all wood constructions in Japan. However, damage to roof coverings and windows can allow wind-driven rain to enter and cause extensive damage to contents. Furthermore, dislodged external components can become wind-borne debris and cause damage to surrounding structures and glazing.

AIR also noted that even with modern flood-control structures, the risk of flood damage remains high in Japan. For a given flood depth/effective surge depth, a residential wood frame building generally will 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. Water damage to machinery and contents drives most flood-related loss; also, because damage is usually limited to the lower stories of a building, high-rise buildings will experience a lower damage ratio than low-rise buildings because a smaller proportion of the building is affected.

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