## 2 You-Ye-Lin landslide
### Climate, Topography and Geology of the Landslide  
The average temperature in You-Ye-Lin landslide area is about 23.0 \({}^{\circ}\)C. The annual average rainfall approximates 3050 mm and the annual average number of rainy days is 166 based on the rainfall  
Figure 1: Satellite image of the location of You-Ye-Lin landslide, study area, topography and flows in the landslide. records from 2003 to 2016. Most of the precipitation falls during summer (from May to September). From October to the following March, the weather turns into a dry season and the plum rain season starts from the end of May to the beginning of June. The terrain of the landslide is wavy with a small flat ground area. The landslide area descends from north to south between the elevation of 820 m and 1110 m with an average elevation of 1000 m. The slope \(S\) (%) or slope angle \(\beta\) (degree\({}^{\circ}\)) of the landslide area varies from grade-3 (\(15\%<S\leqq 30\%\) or \(8.53^{\circ}<\beta\leqq 16.70^{\circ}\)) to grade-6 (\(55\%<S\leqq 100\%\) or \(28.81^{\circ}<\beta\leqq 45^{\circ}\)) and most of the landslide is at grade-5 (\(40\%<S\leqq 55\%\) or \(21.80^{\circ}<\beta\leqq 28.81^{\circ}\)). The landslide area can be delineated in the watershed of the Shen-Mao-Shu River which flows through the southern valley of the landslide area (refer to Figure 1). The slope of the valley is steep, with well-developed ditch shape erosion gullies which are liable to collapse. Meanwhile, because of severe longitudinal and transverse erosions of the channel bed of Shen-Mao-Shu River, the slopes within the watershed become very unstable during the typhoon season. In addition, Hai-Tu-Lun Creek which flows through the landslide area from north to south cuts the landslide area into eastern and western topographic blocks (refer to Figure 1). Due to the watershed boundaries formed by the eastern and western mountain ridges of Hai-Tu-Lun Creek, the run-off of watershed is collected into Hai-Tu-Lun Creek and drains southward into the Shen-Mao-Shu River. The removal of the slope toe of Hai-Tu-Lun Creek due to severe bank erosion may lead to a slope failure. This failure mechanism is verified by the wide spreading of scarps and tension cracks at the Creek banks. Moreover, the construction disturbance of the county road Route-No. 166, which crosses through the entire hillside of You-Ye-Lin landslide, frequently triggers slope failures in the typhoon season. The sliding soil mass of You-Ye-Lin landslide can be conjectured as ancient colluviums underlain by the bedrock. The colluviums slide downward along the interface with the underlain sandstone because the soil mass absorbs the rainwater and increases the self-weight. After the inspection, there seems no indication that the sliding plane extends into the bedrock and as a consequence the ground movement is only within the uppermost colluviums. The strike of bedding plane of soil stratum in the You-Ye-Lin landslide directs towards north by east (NE) and topographically constitutes a dip slope. The Lu-Ku fault passes through the southern edge of the landslide with a strike of north by east (NE) and turns into a reverse fault inclined to the east with an inclination angle of \(50^{\circ}\). The outcrops of the You-Ye-Lin landslide, which are situated on the western side of the Lu-Ku Fault, can be categorized as part of the Pliocene Cho-Lan Formation which consists of thick sandstone (Ss), shale (Sh), and sandstone/shale (Ss/Sh) alternating as shown in Figure 2a-c.  
Field investigations and boring explorations indicate that the surficial colluvium of You-Ye-Lin landslide has a thickness of 8.65-35.8 m and is composed of unconsolidated sandy, silty, or clayey soil mixed with weathered rock or rock fragments as shown in Figure 2d. The bedding plane is not available in the weathered rock or rock fragment, as a result, the overall mechanical behavior of the colluvium is dominated by fine particle soils.