When monster Typhoon Haiyan slammed the Philippines in early November, it inflicted especially severe damage on Leyte Island, where the city of Tacloban was engulfed by a massive storm surge.
Experts said the devastation was caused mainly by two factors — the powerful gusts of the super typhoon and an elongated shallow bay. Since Japan has similarly shaped areas, including Osaka and Tokyo bays, experts warn that what happened in the Philippines could strike at any time in the Japanese archipelago.
Haiyan’s storm surge washed away hundreds of buildings, leaving mountains of rubble throughout Tacloban and wiping out other cities and towns in the area.
“Waves of about 3 meters suddenly struck with the storm,” a man in his 50s said at the spot where his house used to stand.
Many are believed to have drowned or gotten crushed to death by collapsing buildings.
Storm surges emerge when water near the surface of the ocean is “sucked up” by the low atmospheric pressure at the center of typhoons and is blown toward land by their powerful rotating winds. The phenomenon is more likely to occur in shallower waters.
At one point, Haiyan’s pressure had sunk to as low as 895 hectopascals at its center. The waters off Tacloban, meanwhile, are only about 20 meters deep. The sea may have risen by up to a meter due to the sucking effect alone, with gusts up to 360 kph pushing it ashore.
Japan has many shallow bays that are only about a few dozen meters deep, but not all are protected. “Some bays are equipped with measures for disasters, but there still can be unexpected events,” said Shinji Sato, a coastal engineering professor at the University of Tokyo.
In 1999, high tides caused by a powerful typhoon hit the town of Shiranui, now known as the city of Uki, in Kumamoto Prefecture, killing 12 people. Kiyoshi Takikawa, a professor in coastal environment engineering at Kumamoto University, said the deaths were the result of multiple factors — including that the typhoon took the worst possible course and that the town’s bay was shallow.
The bay, which has an elongated shape, is around 20 km long and has a 7-km-wide opening. The terrain around it is similar to Tacloban’s.
Takikawa said that, in theory, the narrower the bay, the more energetic the surge. “Bays that are shaped like this are everywhere, such as Tokyo Bay, Ise Bay and Osaka Bay,” he said.
The term “super typhoon” is used by the Joint Typhoon Warning Center run in Hawaii by the U.S. Armed Forces to designate storms that have a minimum wind velocity of around 240 kph.
Since typhoons grow more powerful when ocean temperatures rise, global warming has been viewed as a factor in the development of super typhoons. Some experts believe that if global warming continues, typhoons of a similar magnitude to Haiyan could strike Japan in the future.
In 2010, the Cabinet Office’s Central Disaster Prevention Council unveiled a worst-case scenario that projected 7,600 people would die if a storm about the size of the 1934 Muroto Typhoon hit Tokyo Bay, assuming global warming causes ocean levels to rise.
As for Osaka Bay, the Land, Infrastructure, Transport and Tourism Ministry said 1.8 million people might be at risk of getting caught up in a future weather disaster. Japan has been repeatedly hit by fatal tidal surges, including one caused when a massive typhoon barreled into Ise Bay in 1959, leaving 5,000 people dead or missing.
“The fear (of high tides) is not communicated well,” Takikawa said. He said the authorities need to work on measures to upgrade breakwaters and other infrastructure and to ensure proper evacuation measures can be taken.