“More than 30 million tonnes of mud rushed down from nearby Mount Siandu [獻肚山] at a speed of 180kph when the mountain collapsed,” Chen Chien-chih (陳建志), a professor at National Central University’s Graduate Institute of Geophysics, told a press conference yesterday. “The massive mudslide was divided into two ... only 30 seconds after it began — the larger mudslide continued on until it hit the Cishan River [旗山溪] valley, wiping out the northern part of Siaolin. This explains why some survivors said they heard two large bangs that morning.”I posted on Siaolin last year, translating a presentation on the diversion project many villagers claim to be the cause of the disaster. The Public Construction Commission reported earlier this year, saying that rainfall, not construction, caused the disaster. To wit:
He said it was only 110 seconds from the beginning of the mudslide until the northern part of the village was wiped out.
The smaller mudslide went down the valley of a smaller, unnamed creek and formed a barrier lake, Chen said.
“As the mud, sand and stones built up, the barrier lake eventually collapsed 30 to 50 minutes later, burying the southern part of the village,” he said. “Most of the survivors came from the south of the village — they made it out in time, before the second mudslide buried the southern part of the village.”
National Taiwan University geosciences professor Hongey Chen (陳宏宇) said that long-term unstable geological composition was one of the causes of the tragedy.
“Field research found the geological composition of Mount Siandu to be quite unstable as it’s located on the border between shale and sandstone composition, and many geological layers there are actually leftover debris from ancient mountain collapses that occurred thousands of years ago,” Hongey Chen said. “If nothing happens, nothing happens — but if exceptionally heavy rainfall occurs, unstable layers can be washed down at any time.”
He said that as many as 92 other communities in Nantou, Chiayi, Tainan, Kaohsiung, Pingtung and Taitung counties are also under threat from unstable geological compositions.
Lee said the report’s conclusion was also based on the fact that the amount of explosives used to blast the tunnel was insufficient to have caused tremors large enough to lead to disaster.Commonwealth Magazine had an in-depth piece on the region's complex geology shortly after the disaster....
The largest amount of explosives used at any one time was 182kg, less than the tremor caused by the 2006 Hengchun Earthquake, the equivalent of 475 million kilograms of explosives.
And a warning for the future....
When there is too much precipitation, the environment's capacity to absorb it becomes overwhelmed.
"That's especially true in the Gaoping River's midstream and downstream areas, where the geological structure is a mix of sandstone, mudstone and shale. The terrain is not only easily loosened and vulnerable to collapse, it features dip slopes and multiple faults and is traversed by a fracture zone, conditions conducive to the entire side of a mountain sliding down," Chen explains. The result was widespread flooding, from the Cishan River Valley, encompassing Namasia Township and Siaolin Village, where hundreds were buried alive during Typhoon Morakot, to the Laonong River Valley, including Liouguei Township and the hot spring resort town of Baolai.
Dr. Huei-long Wu, the director general of the Soil and Water Conservation Bureau, estimates that the flooding last August created about 1.2 billion cubic meters of debris in the disaster area that stretched south of Jiayi County and into Taidong County. But only about a third of that, or 400 million cubic meters, actually made its way into rivers, with the rest remaining upstream or at midstream. Thus, the next big storm could trigger even bigger debris flows, Wu believes._______________
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