**2. General information of landslide triggers in Cishan River watershed, Taiwan**

The island of Taiwan is located at the obliquely convergent boundary of the Eurasian continent and the Philippine Sea plate and separated from Eurasia continent by the 175 km wide Taiwan Strait. This collision of the Luzon arc results in two-thirds of its area being covered by rugged mountains and hills, steep topography, young (3 million years) and weak geological formulations, active earthquakes, and loose soils [5]. The average elevation is 765 m and about 31% of the total island area has an elevation of exceeding 1000 m. Most mountains are very steep with slope gradients of 25o and local relief of 1000 m or more.

Taiwan is periodically disturbed by typhoons and local storms and experiences a 2500 mm mean annual rainfall depth equivalent to a triple of global mean values. Majority of rainfall occurs between June and October when trade wind and typhoons strongly provide the sources of moisture. During the rainy seasons, the geological and climatic regimes usually combine to generate severe hillslope denudation by soil erosion, landslides, and debris flows.

Dadson et al. [6] indicates that the rivers of Taiwan transfer suspended sediment of 384 Mt/y to the ocean based on the data measured between 1970 and 1999. This represents 1.9% of estimated global river-borne suspended-sediment discharges but is only derived form 0.024% of Earth's subaerial surface. The Erosion rate were quite high in the eastern Central Range and southwestern region of Taiwan, especially on the Western Foothills with a rate of up to 60 mm/y [6].

#### **2.1. Cishan River watershed and its lithologic complexes**

by typhoons that usually brought heavy rains in Taiwan. In recently decades, the most severe landslides triggered by heavy rains in Taiwan are those by Typhoon Morakot in the period of August 5 to 10, 2009. Typhoon Morakot brought more than 2000 mm rainfall during 6 days, leading to a large number of landslides, especially in southern Taiwan. The understanding of landslide hazards has been still limited until Typhoon Morakot hit in Taiwan that caused a total of 45,125 of landslides and the catastrophic Hsiaolin landslide in the Cishan River watershed (CRW). The Hsiaolin landslide generated a huge debris dam that blocked the Cishan River and its consequent dam break caused more than 400 people dead and missing; also, the

After Typhoon Morakot, the natural environmental conditions of watershed systems were significantly changed due to severe landslides and associated damages. For developing costeffective landslide hazards mitigation strategies and measures, we should have better understanding on the causes and effects of landslides. Basing on the data taken from the study area of CRW, we devote this chapter to: (1) address the causes and effects of landslides in this watershed (i.e., CRW) accompanied with the primary factors of landslide triggering such as the geologic and topographic settings and rainfall characteristics; (2) evaluate the devastation of landslides caused by Typhoon Morakot and its aftermath; (3) assess the present status of landslide hazards mitigation strategies in Taiwan. Results from landslide research in this chapter lays the foundation and establishes the guidelines for developing possible effective

The island of Taiwan is located at the obliquely convergent boundary of the Eurasian continent and the Philippine Sea plate and separated from Eurasia continent by the 175 km wide Taiwan Strait. This collision of the Luzon arc results in two-thirds of its area being covered by rugged mountains and hills, steep topography, young (3 million years) and weak geological formulations, active earthquakes, and loose soils [5]. The average elevation is 765 m and about 31% of the total island area has an elevation of exceeding 1000 m. Most mountains are very

and local relief of 1000 m or more.

Taiwan is periodically disturbed by typhoons and local storms and experiences a 2500 mm mean annual rainfall depth equivalent to a triple of global mean values. Majority of rainfall occurs between June and October when trade wind and typhoons strongly provide the sources of moisture. During the rainy seasons, the geological and climatic regimes usually combine to generate severe hillslope denudation by soil erosion, landslides, and debris flows. Dadson et al. [6] indicates that the rivers of Taiwan transfer suspended sediment of 384 Mt/y to the ocean based on the data measured between 1970 and 1999. This represents 1.9% of estimated global river-borne suspended-sediment discharges but is only derived form 0.024% of Earth's subaerial surface. The Erosion rate were quite high in the eastern Central Range and southwestern region of Taiwan, especially on the Western Foothills with a rate of up to

village itself no longer exists [44].

14 Environmental Risks

**watershed, Taiwan**

steep with slope gradients of 25o

60 mm/y [6].

landslide hazards mitigation strategies and measures.

**2. General information of landslide triggers in Cishan River** 

Cishan River watershed lies on the Western Foothills of Taiwan and has an area of 842 km<sup>2</sup> and a mean gradient of 39.3%. Its maximum elevation of 3950 m is near the Southwestern Foothills of Jade Mountain, gradually decreasing from north to south to 31 m, with mean elevation of 473 m. Obviously, there is a significant difference between the maximum and minimum elevations in this long, narrow watershed (**Figure 1**). The geologic setting is underlain by sandstone, shale, slate, and phyllite, and drains geological ages between Eocene to Pleistocene distributed from the upper to lower regions. This is clear that the geological formation is young and weak (between 5.3 and 0.01 million years ago), leading to poorly lithologic resistance to erosion. The Cishan River is 118 km in length, flowing through the watershed. The geomorphic characteristics (i.e., elevation and slope) were obtained by digitizing a 5 × 5 m digital elevation model (provided by Water Resources Agency, Taiwan, WRA) using Geographic Information System (GIS).

**Figure 2** shows a geological map of CRW (provided by the Geological Survey Center, MOEA, Taiwan). There are nine lithologic complexes and rock units in the Cishan River watershed:

**Figure 1.** Locations and topography of Cishan River watershed (CRW).

river-borne suspended-sediment discharge is only smaller than that supplied from the Beinan River (~88 Mt./y) in Taiwan [6]. The Water Resource Agency (WRA) of Taiwan continuously made stream gauging and a fortnight suspended-sediment sampling at the Shanlin Bridge station near the downstream from 1986 to 2005, restarting from 2010 till now. Based on the data recording at this gauging station, the Cishan River has a mean suspended-sediment con-

Landslides Triggered by Typhoon Morakot in Taiwan http://dx.doi.org/10.5772/intechopen.76930 17

Hsiaolin village is a village on the foothill of Hsiendu Mountain in the Cishan River watershed. At 6:16 AM (local time) on 9 August in 2009, the catastrophic landslide was triggered on the hillslope of Hsiendu Mountain when rainfall reached 1676.5 mm equal to 72-hour accumulative rainfall obtained from the rainfall record of the Jiashan station, with the peak hourly rainfall intensity of 95 mm/h (**Figure 3**). This landslide is the largest landslide occurred during Typhoon Morakot, damming the near Cishan River channel and was break at about 7:40 AM on 9 August. This dam-break flood led to abrupt change in the water level of the Cishan River downstream area, and instrumental records on the downstream of the Hsiaolin Village indicated that 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. Note that this evident water level changes in the Cishan River channel were observed at 27.8 km downstream of the Hsiaolin Village [44]. Total causality in Hsiaolin Village were more than 400 people dead and missing. The village itself

**Figure 4** shows the Formosat-II image and aerial photos of the Hsiaolin landslide. According to the digital terrain models (DTMs) with a resolution of 5 m established by the aerial photos

**Figure 3.** Hourly and accumulative rainfall for typhoon Morakot recorded by the Jiashan rainfall stations (11.4 km SSW

of the Hsiaolin landslide) during the period from 6 Aug. to 11 Aug.

centration of about 696 ppm, annual sediment yield of about 1.06 Mt./y.

**2.2. Catastrophic landslide in Hsiaolin Village**

no longer exists.

**Figure 2.** Map of lithologic complexes for the Cishan River watershed.

(a) Cholan Formation, Pliocene in age, (b) Hsitsun Formation, Eocene to Oligocene, (c) Juifang Group, Middle Miocene, (d) Lushan Formation, Miocene in age, (e) Sanhsia Formation, Late Miocene to Pliocene, (f) Shihpachungchi Formation, Eocene, (g) Tachien Sandstone, Eocene, (h) Colluvium, Pleistocene, and (i) Alluvium, Holocene. Each complex comprises different types of sedimentary rocks varying in strength. Interbedded sandstone, argillite, phyllite and slate dominate the rocks of Lushan Formation. Juifang Group, Cholan and Sanhsia Formations mainly comprise sandstone and shale. Hsitsun Formation includes the rocks of Slate, sandstone and phyllite. Shihpachungchi and Tachien Sandstone comprise sandstone, slate and shale. Alluvium and Colluvium contain weak sand, gravel and clay.

The CRW drains tropical monsoon climate zones. The mean annual rainfall is about 3267 mm and the mean annual temperature is 25.1°C. The relative atmospheric moisture averages 75.6%. In this region, the majority of rainfall occurs between June and October because periodic typhoons and trade wind can provide abundant moisture sources during wet season. Conversely, the Center Mountain usually blocks moisture brought by northwestern trade wind, leading to less rainfalls in other months during dry season.

Cishan River watershed drains the Western Foothills of Taiwan, which has a decadal erosion rate of ~30 mm/y. The Cishan River is a main tributary of the Kao-Pin River (the largest river of Taiwan for total river basin) that supplied 49 Mt./y during 1990 and 1999 and this value of river-borne suspended-sediment discharge is only smaller than that supplied from the Beinan River (~88 Mt./y) in Taiwan [6]. The Water Resource Agency (WRA) of Taiwan continuously made stream gauging and a fortnight suspended-sediment sampling at the Shanlin Bridge station near the downstream from 1986 to 2005, restarting from 2010 till now. Based on the data recording at this gauging station, the Cishan River has a mean suspended-sediment concentration of about 696 ppm, annual sediment yield of about 1.06 Mt./y.

#### **2.2. Catastrophic landslide in Hsiaolin Village**

(a) Cholan Formation, Pliocene in age, (b) Hsitsun Formation, Eocene to Oligocene, (c) Juifang Group, Middle Miocene, (d) Lushan Formation, Miocene in age, (e) Sanhsia Formation, Late Miocene to Pliocene, (f) Shihpachungchi Formation, Eocene, (g) Tachien Sandstone, Eocene, (h) Colluvium, Pleistocene, and (i) Alluvium, Holocene. Each complex comprises different types of sedimentary rocks varying in strength. Interbedded sandstone, argillite, phyllite and slate dominate the rocks of Lushan Formation. Juifang Group, Cholan and Sanhsia Formations mainly comprise sandstone and shale. Hsitsun Formation includes the rocks of Slate, sandstone and phyllite. Shihpachungchi and Tachien Sandstone comprise sandstone,

The CRW drains tropical monsoon climate zones. The mean annual rainfall is about 3267 mm and the mean annual temperature is 25.1°C. The relative atmospheric moisture averages 75.6%. In this region, the majority of rainfall occurs between June and October because periodic typhoons and trade wind can provide abundant moisture sources during wet season. Conversely, the Center Mountain usually blocks moisture brought by northwestern trade

Cishan River watershed drains the Western Foothills of Taiwan, which has a decadal erosion rate of ~30 mm/y. The Cishan River is a main tributary of the Kao-Pin River (the largest river of Taiwan for total river basin) that supplied 49 Mt./y during 1990 and 1999 and this value of

slate and shale. Alluvium and Colluvium contain weak sand, gravel and clay.

wind, leading to less rainfalls in other months during dry season.

**Figure 2.** Map of lithologic complexes for the Cishan River watershed.

16 Environmental Risks

Hsiaolin village is a village on the foothill of Hsiendu Mountain in the Cishan River watershed. At 6:16 AM (local time) on 9 August in 2009, the catastrophic landslide was triggered on the hillslope of Hsiendu Mountain when rainfall reached 1676.5 mm equal to 72-hour accumulative rainfall obtained from the rainfall record of the Jiashan station, with the peak hourly rainfall intensity of 95 mm/h (**Figure 3**). This landslide is the largest landslide occurred during Typhoon Morakot, damming the near Cishan River channel and was break at about 7:40 AM on 9 August. This dam-break flood led to abrupt change in the water level of the Cishan River downstream area, and instrumental records on the downstream of the Hsiaolin Village indicated that 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. Note that this evident water level changes in the Cishan River channel were observed at 27.8 km downstream of the Hsiaolin Village [44]. Total causality in Hsiaolin Village were more than 400 people dead and missing. The village itself no longer exists.

**Figure 4** shows the Formosat-II image and aerial photos of the Hsiaolin landslide. According to the digital terrain models (DTMs) with a resolution of 5 m established by the aerial photos

**Figure 3.** Hourly and accumulative rainfall for typhoon Morakot recorded by the Jiashan rainfall stations (11.4 km SSW of the Hsiaolin landslide) during the period from 6 Aug. to 11 Aug.

**3. Methodology and results**

**3.1. Rainfall brought by typhoon Morakot**

On August 8, 2009, Typhoon Morakot was "born" at approximately 22.4°N and 133.8°E in the North Pacific Ocean, about 1000 km far from Northeastern Philippines, moving west at a speed of 10–30 km/h towards Taiwan. In retrospect, Typhoon Morakot had not been considered a serious threat before striking Taiwan. However, contrary to the prediction, after landing Taiwan, it caused more damage than any other typhoon because of massive rainfall, especially in Taiwan's southwestern region. **Figure 5** shows the spatial distribution of rainfall in Taiwan for the six-day rainfall during Typhoon Morakot. This "monster "brought significant rainfalls causing severe debris flows, shallow and deep landslides, and debris dam-break in the mountainous areas of the central and southern Taiwan. Consequently, 675 people were

Landslides Triggered by Typhoon Morakot in Taiwan http://dx.doi.org/10.5772/intechopen.76930 19

**Figure 6** shows the time series of hourly rainfall data and cumulative rainfall during Typhoon Morakot obtained from Alishan station in central Taiwan. On the basis of this hourly rainfall

dead; 34 people were hurt; the economic loss was up to 164 million NT dollars.

**Figure 5.** Spatial distribution of rainfall in Taiwan for Typhoon Morakot during six days.

**Figure 4.** Formosat-II image and aerial photos of the Hsiaolin landslide in the Cishan River watershed, Taiwan.

provided by the Agricultural and Forestry Aerial Survey Institute of Taiwan, Kuo et al. [7] indicated that the major body of the Hsiaolin landslide had the extent of 57 × 104 m2 and was estimated to have a volume about 24 ± 2 million m3 , distributed at an average depth of 42 ± 3 m. Moreover, it is 3.2 km long in an E–W direction and 0.8 to 1.5 km wide. The total fall height was 830 m from the top of the head scarp, at an elevation of 1280 m, to the toe of the landslide deposit at 450 m [8]. Comparing the Hsiaolin landslide with the 1:25000 geologic map (provided by CGS, MOEA) shows that this landslide crops the late Miocene to early Pliocene Yenshuikeng Formation composed of mudstone, sandstone, and shale. Strength of sandstone is much greater than mudstone and shale, and its corresponding uniaxial compressive strength is about 15 Mpa [9]. The source area of the Hsiaolin landslide was the dip slope of the east limb of the syncline that could exhibits simple traces of strata on a horizontal crosssection, however, because the strata and slopes on the east region of the Cishan River shows the similar characteristics, the bedding traces have rather complicated patterns [8].

A cascade of loose sediment produced by the Hsiaolin landslide deposited on the Cishan River and generated the barrier lake that was suddenly broken about 1 to 2 hours after its generation. The landslide dam was estimated to have a volume of 15.4 million m3 . This is clear that only half of the total sliding mass contributed to the main body of the landslide dam [10]. The maximum elevation produced by this catastrophic landslide is about 475 m, on the west bank of the Cishan River and the maximum water level that can overtop the dam crest is about an elevation of 413 m. The height of the dam was about 44 m and the deepest deposit was a thick of 60 m. Moreover, this dam drained about 354 km2 watershed area and trapped about 9.9 million m3 water before overtopping occurred [10]. On the basis of data recorded by continuous river stream gauging from Shanlin Bridge station, the water level of the Cishan river dropped to 118 m nearly at 08:00 AM after the landslide dam formation and rapidly rise up to 126 m at about 9:40 AM [11]. Hence, we can infer that the landslide dam was suddenly flushed out by river water after the dam formation during the period of only 2 hours.
