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 × 10⁶ m³, 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.

Introduction

A catastrophic landslide is a rapid, large gravitational mass movement, which changes the topography and remains for a long time. Guthrie and Evans (2007) regarded a landslide as catastrophic when it is individually formative and persists more than 10 times longer than moderate sized landslides. Many catastrophic landslides accompany avalanches—events that in their post-failure stage involve rapid runout and emplacement of relatively thin sheets of crushed, pulverized, and dry rock (Hewitt et al., 2008). Rock avalanche, which is also called sturzstrom (Heim, 1932, Hsü, 1975), may be complex as rockslide-avalanche (Mudge, 1965), or rockfall avalanche (Schuster and Krizek, 1978). Previous research on catastrophic landslides has been reviewed several times (Voight, 1978, Evans and DeGraff, 2002, Hewitt, 2006) and has shown that many are induced by earthquakes. Fewer cases have been reported of catastrophic landslides induced by rainstorms (Sidle and Chigira, 2004, Catane et al., 2007, Catane et al., 2008, Evans et al., 2007, Guthrie et al., 2009). In such cases, rock debris may be saturated with water (classified as debris flows) or not saturated (classified as debris avalanches) (Takahashi, 2010). One of the most recent catastrophic landslides occurred at Mt. Canabag in southern Leyte Island, Philippines, in 2006, which followed a continuous heavy rainfall and two recorded small earthquakes (Catane et al., 2007, Catane et al., 2008, Evans et al., 2007, Guthrie et al., 2009).

Catastrophic landslides are commonly preceded by gravitational deformation (Voight, 1978, Chigira, 1992, Chigira, 2001, Chigira and Kiho, 1994, Evans and DeGraff, 2002, Crosta et al., 2006), although this by itself does not necessarily transform into a catastrophic failure. The failure of the slope is controlled by internal factors, such as fracture development in rock mass and external factors (Kilburn and Petley, 2003, Korup, 2004, Petley et al., 2005). Gravitational deformation creates a variety of internal structures and new materials (Hutchinson, 1988, Chigira, 1993a), some of which are more susceptible to earthquakes and others to rainstorms. Therefore, gravitational deformation may provide a clue in predicting potential sites of catastrophic landslides, and it must be interpreted in the context of slope development.

Progress in understanding these issues depends on the accumulation of case histories, particularly for contemporary landslides, as prehistoric landslides provide no direct information on their behavior and pre-event conditions. Here, we present the results of a geological and geomorphological investigation of the most recent rain-induced catastrophic landslide, which was triggered by Typhoon Morakot in Taiwan on 9 August, 2009.

Typhoon Morakot swept Taiwan on 7 August 2009, resulting in 619 deaths, 76 missing persons, the temporary evacuation of 24,950 residents, flooding, and more than US$ 5 billion in economic losses (National Disasters Prevention and Protection Commission, 2009). It also isolated numerous villages in southern mountain areas of Taiwan. It was the worst typhoon disaster in Taiwan for 50 years. The typhoon landed at Hualian in eastern Taiwan, crossed the island in a northwestward direction, and left at Taoyuan on the northwest coast (Fig. 1). The typhoon, which was a medium-strength event according to the classification system of the Taiwan Central Weather Bureau, with a maximum wind speed of 40 m s^− 1, set a precipitation record of 2749 mm for a single rainfall event at Alishan, where the previous rainfall record of 1749 mm was set by Typhoon Herb in 1996 (Lin and Jeng, 2000).

The catastrophic landslide at Shiaolin Village, Kaohsiung County, was the largest landslide induced by Typhoon Morakot (Fig. 2). It occurred at 6:16 AM (local time) on 9 August, when the cumulative rainfall had reached 1676.5 mm, about three days after the start of the rainfall according to the record at the Jiasian station (C0V250) of the Taiwan Central Weather Bureau, located 11.4 km SSW of the village (Fig. 3). The landslide dammed the rain-swollen Chishan River, but the dam was breached at about 7:40 AM on 9 August, flooding the downstream area (Feng, 2010). Abrupt river water level changes were recorded at 27.8 km downstream of Shiaolin Village (Feng, 2010): 2.75 m drop during the period from 7:10 to 7:50 AM and 7.88 m rise during the period from 8:40 to 9:30 AM. Total casualties in Shiaolin Village were more than 400 people dead and missing. The village itself no longer exists.

We investigated the Shiaolin landslide through fieldwork and analyses of digital elevation models (DEMs), topographic maps, rainfall data and seismic records, as well as analyses of mineralogy and particle size distribution of the landslide material. The main purposes of this study are: (i) to characterize the geological and geomorphological features of the Shiaolin landslide, (ii) to describe the role of rainfall in this landslide and (iii) to discuss whether it would have been possible to predict this landslide in advance.