Characteristics of rainfall-induced landslides in Miocene formations: A case study of the Shenmu watershed, Central Taiwan
Introduction
Sediment-related disasters, including debris flow and landslides, have frequently occurred in Taiwan during the past two decades, particularly following the 1999 Chichi earthquake (ML = 7.3). The most well-documented recent debris-flow events were those caused by typhoons, including Typhoon Toraji in 2001 (Cheng et al., 2005), Typhoon Mindulle in 2004 (Chang et al., 2007), and Typhoon Morakot in 2009 (Wu et al., 2011). The average annual number of deaths attributed to typhoons and heavy rainfall in Taiwan was 134 persons (including missing persons) from 2001 to 2010, and this figure was higher compared with the reports of past decades (Directorate General of Budget, Accounting and Statistics, 2012). The Shenmu watershed was approximately 36 km from the epicenter of the 1999 Chichi earthquake, during which the peak ground acceleration (PGA) reached 250–400 gal (Chung, 1999). The frequency of debris-flow events in the watershed was approximately 0.75 per year during the past 16 years (Table 1) and approximately one per year in the past decade; thus, the Shenmu watershed is a representative debris flow-prone watershed in Taiwan.
Table 2 shows the geomorphological characteristics and deposition materials of the three most debris flow-prone watersheds in the world, namely the Jiangjia Valley watershed in Southwestern China (Liu et al., 2009), the Moscardo Torrent in Northeastern Italy (Marchi et al., 2002), and the Shenmu watershed in Taiwan. The typical geomorphological characteristics of debris flow-prone watersheds have been widely described in the literature (Spalletta et al., 1979, Marchi et al., 2002, Cui et al., 2005, Liu et al., 2009, Lenzi et al., 2011, Yu et al., 2012). Debris flow-prone watersheds are typically small watersheds exhibiting average slopes close to or greater than 25°, high reliefs (> 1000 m), highly concentrated rainfall during the wet season, numerous and frequent landslides, fragile strata composed primarily of highly fractured and weathered sandstone and shale, and abundant colluvium on the upstream hillslopes.
Long-term debris flow observation systems have been established in mountainous areas in several countries over the past 50 years to analyze the characteristics of debris-flow or serve as early warning systems for possible catastrophic debris-flow events; such systems have been established in Southwestern Canada (VanDine, 1985), Southwestern China (Hsiao et al., 2007), Northeastern Italy (Berti et al., 1999), Japan (Kamei and Nowa, 1985), and Taiwan. Currently, 22 long-term debris flow observatories are located in Taiwan, including one in the Shenmu watershed that was established in 2002 and equipped with digital video cameras, geophones, wire sensors, and rainfall gauges. Monitoring data from the Shenmu long-term observation station has primarily been used by the Taiwanese government to develop a “Debris Flow Disaster Prevention and Evacuation System,” and protect the lives and properties of the residents in the Shenmu watershed. Whereas 14 casualties were associated with Typhoon Toraji in 2001, zero was caused by Typhoon Morakot in 2009, indicating the effectiveness of the long-term observation station.
The frequent debris flow events in the Shenmu watershed result from the frequent landslides that have occurred in the past two decades; thus, it is worth discussing why this area has been prone to landslides during that period. The frequent landslide events can be analyzed on the basis of long-term observation data, rainfall data, landslide distribution, and field survey data. How the landslide characteristics of the watershed and macrosediments deposited in the Shenmu watershed affect the morphology of the downstream region can provide insightful morphological information for evaluating other watersheds in which copious sediment is deposited in the upstream region.
Section snippets
Study area
The Shenmu watershed, including the Aiyuzi (area 4.0 km2, average slope 39.3°), Housha (area 26.2 km2, average slope 35.1°), and Chushui (area 8.6 km2, average slope 35.8°) watersheds, is located in the upstream basin of the Chenyoulan River in Central Taiwan, and encompasses an area of 38.8 km2 (Figure 1). The elevation of the Shenmu watershed ranges from 1153 to 2859 m, exhibiting steep mountainous terrain and an average slope of 36°. Land sloped > 30° comprises approximately 70.8% of the watershed
Rainfall type
Rainfall events were classified based on their hourly rainfall intensity and rainfall duration. An average hourly rainfall intensity of > 30 mm/h was considered a high-intensity rainfall event, and rainfall durations of > 24 h, 12–24 h, and < 12 h were considered long-duration, mid-duration, and short-duration rainfall events, respectively. For example, a rainfall event with an average rainfall intensity of 35 mm/h and a duration of 36 h was considered a long-duration, high-intensity rainfall event. In
Landslide proneness of the Shenmu watershed hillslopes
The landslide distribution in the Shenmu watershed is closely related to the geomorphological and geological characteristics of the watershed, particularly the slope and formation lithology. In this study, five landslide inventories were adopted for further analysis, namely the inventories generated before Typhoon Herb (1996) and after Typhoon Herb, Typhoon Mindulle (2004), Typhoon Kamelgi (2008), and Typhoon Morakot (2009).
Fig. 4 shows the distribution of each landslide inventory regarding the
Discussion
The aforementioned analysis provides a clear framework for landslides in the Shenmu watershed. A detailed landslide classification can elucidate the causative factors of these landslides. In this study, the landslides that occurred in the Shenmu watershed were classified as bank-erosion and nonbank-erosion landslides. Based on our field survey experiences, bank-erosion landslides in the Shenmu watershed should demonstrate three characteristics: the lower boundary of the landslide is extremely
Conclusion
The frequent landslides in the Shenmu watershed located in Central Taiwan can be attributed to seismic forcing caused by the 1999 Chichi earthquake, the increasing amount of heavy rainfall and flooding events, steep hillslopes, fractured strata exhibiting a high fault density, and the large-scale sediment input affecting the rivers. These landslides caused severe bank-erosion landslides downstream. Small landslides located in the lower area of the watershed dominated the landslide distribution
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