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

2. Route of Xangsane

3. 土石流潛勢溪流
研究範圍

4. Location of the study area
Huang
mountain
Datun
mountain

5. 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

6. Environmental geology map showing rock formations in the study area

7. Photographs of the debris flow source area

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

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

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

11. Geomorphological map

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

13. Particle size distributions of river sediment from different locations.

14. 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

15. 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

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

17. 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)

18. 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.

19. The End