Recent rainfall-induced landslides and debris flow in northern Taiwan 專題討論 Recent rainfall-induced landslides and debris flow in northern Taiwan 49842018 蔡怡臻 Oct. 30 2009

Route of Xangsane

土石流潛勢溪流 研究範圍

Location of the study area Huang mountain Datun mountain

Research method 1. Photographs comparing 2. Field surveys on geology 3. Field measurement of channel cross-sections 4. Laboratory assessment of slope material properties 5. Slope stability analysis

Environmental geology map showing rock formations in the study area

Photographs of the debris flow source area

Distribution of hourly and accumulated precipitation between Oct Distribution of hourly and accumulated precipitation between Oct. 30 and Nov. 2,2000

(A) Aerial photograph taken in 1979. (B) Aerial photograph taken in 1994

Aerial photograph taken in 2001 showing devastation caused by Typhoon Xangsane.

Geomorphological map

Locations of open pits and bedrocks where samples for laboratory tests were taken.

Particle size distributions of river sediment from different locations.

Physical properties of river sediment Pit no #10 passing #200 D50 Cu Wn γint γsat e n Sat LL PL PI Cf (%) (mm) (t/m3) Up01 27.54 2.32 55 450 22.45 1.44 1.62 0.78 43.91 77.87 – NP GP Up02 23.26 1.6 90 478 21.96 1.38 1.57 0.77 43.36 78.68 Mp01 20.82 2.95 43 412 13.91 1.75 1.93 0.66 39.69 56.87 38.14 35.86 0.28 Mp02 17.87 1.32 51 192 17.1 1.76 0.76 43.16 59.39 40.57 35.62 0.95 Mp03 19.07 1.67 83 367 10.44 1.81 2.02 0.61 37.86 46.34 Mp04 13.28 1.51 60 123 9.26 1.94 0.51 33.64 50.59 36.43 29.67 Mp05 14.91 61 126 8.74 1.85 2.03 0.48 32.47 49.56 Dp01 17.4 1.47 38 375 11.49 1.83 0.63 38.52 49.83 41.12 32.72 0.4 Dp02 19.93 1.9 80 440 16.55 1.92 0.71 41.57 62.2 33.88 27.12 Dp03 15.6 68 148 18.79 1.78 43.19 67.02 37.91 32.27 0.25 Dp04 11.65 1.03 58 73 10.68 2.08 0.68 40.59 42.78 35.69 31.97 0.27 Dp05 16.78 1.46 50 115 11.5 1.87 2.1 0.73 42.36 35.46 30.15 0.31

Physical properties of rock samples from the study area Rock type Wn γint γsat e n Sat Dry condition Saturated location (%) (t/m3) condition c f (kg/cm2) (  ) R01 Muddy sandstone 3.86 2.33 2.47 0.16 14.11 54.8 0.1 34.2 – R02 4.15 2.41 2.55 0.15 13.31 65.12 34.3 R03 3.32 2.54 13.32 52.09 35.3 R04 12.5 1.92 2.21 0.41 29.17 57.64 0.2 30.2 R05 11.79 1.95 2.32 0.57 36.44 39.82 32.6 R06 5.24 2.29 2.57 0.38 27.61 31.5 36.4 R07 Tuffaceous conglomerate 1.15 2.62 2.68 0.07 6.18 45.57 35.5 R08 2.16 0.09 8.67 52.7 28.2 R09 2.74 2.83 9.41 67.42 36.5 0.5 33.0 R10 1.53 2.71 9.15 39.75 35.0 30.5 R11 1.09 2.78 0.05 4.45 64.44 39.0 0.7 32.5 R12 1.1 2.73 4.75 60.18 38.0 0.3 31.8 R13 2.61 2.77 8.41 76.25 38.3 31.0 R14 Lava 0.36 0.04 3.94 24.39 48.2 37.1 R15 2.75 3.67 26.26

Volume of erosion and deposition in different sections of the study area

Examples of safety factor analysis for slopes in the study area (t/m3) C (kg/cm2)  () Geometric model Safety factor Condition Remark Ocp01(1) 2.42 0.1 34.2 Plane 1.07 Dry Critical stable 0.96 Saturated Fail (3.3 ma) Ocp01(2) Wedge 1.06 0.74 Fail (3.1 ma) Ocp02 2.51 34.3 Topple 1.17 Stable 0.98 Ocp04 2.16 0.2 30.2 1.1 0.94 Fail (2.5 ma) Ocp09 2.81 36.5 1.19 Ocp11 2.77 39 1.2 0.95 Fail (2.8 ma) Ocp13 2.75 38.3 1.18 Fail (8.3 ma) Ocp14 2.74 48.2 2.18 1.47 Ocp15 2.76 37.1 1.41 0.89 Fail (14.7 ma)

Conclusion 1.Intense rainfall caused groundwater to infiltrate through the volcanic bedrock material along the fractures in the exfoliated zones. 2. The tuffaceous conglomerate and the weathered lava became saturated with water became unstable. 3.The largest landslide in the upper reach mobilized debris as a landslide mass and it flowed down with water along the stream channel as a debris flow. 4.The rapidly enlarging debris flow eroded the stream sidewalls but at the same time left some sediment on the riverbed, until it stopped in the downstream area. 5.The accumulated deposits, provided a source for future debris flows triggered by rainfall.

The End