Catastrophic landslide induced by Typhoon Morakot, Shiaolin, Taiwan

Abstract

Typhoon Morakot induced the catastrophic and deadly Shiaolin landslide in southern Taiwan on 9 August 2009, resulting in more than 400 casualties. We undertook a geological and geomorphological investigation with the aim of reconstructing the events leading up to this landslide and to clarify factors that contributed to its development. Cumulative rainfall reached up to 1676.5 mm in about three days under the influence of the typhoon, and the Shiaolin landslide, with a volume of 25 × 10 6 m 3, occurred one day after the peak in rainfall intensity. The landslide occurred on a dip slope overlying late Miocene to early Pliocene sedimentary rocks consisting of silty shale, massive mudstone, and sandstone. It started as a rockslide in the upper third of the landslide area and transformed into a rock avalanche that crossed a series of terraces and displaced or buried the village below. It buried the riverbed of the Chishan River and ran up the opposite slope, creating a landslide dam 60 m high, which was breached about 1 h and 24 min later, flooding the village. The velocity of the landslide is estimated to have been 20.4 to 33.7 m s − 1 and its apparent friction angle was 14°, which indicates its high mobility. The detachments in the source area consist of combinations of bedding planes and joints or faults. The landslide was preceded by gravitational deformation, which appeared as hummocky landforms before the landslide and as buckle folds exposed after the event. The landslide deposits consist of fragments of mudstone, shale, and sandstone, as well as clayey material at its base. This clayey material, consisting of illite, chlorite, quartz, feldspar, and calcite, is assumed to have strongly influenced the long, rapid runout.