Impact of rainstorm-triggered landslides on high

turbidity in a mountain reservoir

Lin, G. W., Chen, H., Petley, D. N., Horng, M. J., Wu, S. J., Chuang, B., 2010. Impact of rainstorm-triggered landslides on high turbidity in a

mountain reservoir. Engineering Geology.

報告者：林雅詩指導教授：蔡龍珆 老師

2010/12/23

1

Outline

• Introduction• Objectives• Study method• Results• Discussion• Conclusions

2

Introduction

• Landslide is the key influence on sediment delivery in upland river catchments, which controls both amount and characteristics of sediment released. (Al-Sheriadeh et al., 2000; Korup et al., 2004; Johnson et al., 2008)

• Landslide is also increasingly considered as a primary factor dominating the turbidity of rivers and reservoirs. (Jordan, 2006; Sobieszczyk et al., 2007)

3

Introduction

• Several studies indicate that much of the sediment produced in upper basins often does not immediately migrate downstream but is instead deposited in the riverbed, resulting in channel aggradation. (Kasai et al., 2004; Koi et al., 2008)

4

Study area – Geographical

5http://www.wranb.gov.tw/ct.asp?xItem=2605&ctNode=815&mp=5

Shihmen Reservoir------------------------------------------------------------------------------------------------------------------------------------------- ------------

Finishes the month July 1964Position 24.81°N, 121.24°Eeffective storage capacity 309×106 m3 average annual precipitation 2556 mmslope gradient 83% 30° to 50°Flow direction southeast to northwest

Fig 1. Geographical.

Study area - Geological

6Fig 2. Distribution of the rock formations in the Shihmen Reservoir catchment.

PERIOD

EPOCH Formation

Tertiary

Miocene Aoti Formation (At)

Oligocene

Tatungshan Formation (Tt)

Gangou Formation (Gg)

Szeleng Sandstone Formation (Ss)

Table 1. Formation

Typhoon Track

7Fig 3. The location of Shihmen Reservoir catchment within Taiwan and the tracks of typhoons.

Table 2-1. Statistics of each typhoon event.

Typhoon Nelson Herb Nari Aere Matsa

Year 1985 1996 2001 2004 2005

Date8/21-

247/29-8/1 9/13-19

8/23-26 8/3-5

Duration of measurements (hour) 96 120 168 96 72

Accumulated rainfall (mm) 456 700 872 996 830

Maximum daily rainfall (mm) 264 536 368 559 448

Average water discharge (m3s-1) 547 707 710 1398 694

Table 2-2. Statistics of each typhoon event.Typhoo

nAverage water

discharge (m3s-1)Peak water

discharge (m3s-1) Reservoir sediment discharge (106m3)

Nelson 547 4906 3.7Herb 707 6363 8.7Nari 710 4123 0.4Aere 1398 8594 27.8

Matsa 694 5322 10

Typhoon events

8

Typhoon events

9

19871990

19921994

1996

Fig 4. Sediment deposition (tonne), Annual precipitation (mm) and accumulated rainfall during typhoon (mm) during 1963~2005.

Typhoon events

10Fig 4. Sediment deposition (tonne), Annual precipitation (mm) and accumulated rainfall during typhoon (mm) during 1963~2005.

S

ed

imen

t d

ep

osi

tion

(to

nn

e)

Objectives

• To study the relationship between water turbidity and the landslide debris of the Shihmen Reservoir.

• To reconstruct the process and impact of forming high turbidity water in the reservoir area.

11

Study method

12

Statistics of

Typhoon

Suspended

Sediment Discharg

e

Turbidity

The relationship between turbidity and landslides.

Term descriptions

13

14

Table 2-3. Statistics of each typhoon event.

Typhoon Nelson Herb Nari Aere Matsa

Landslide area (km2) 9.83 13.95 21.32 6.71 7.02

Landslide ratio (%) 1.3 1.8 2.8 0.9 0.9

New generation ratio (%) 86.5 88.8 78.5 49.7

Reactivated ratio (%) 19.2 17.1 6.8 52.6

Landslide volume (106m3) 9.83 13.95 21.32 6.71 7.02

Sample? NTU

Suspended sediment discharge

Turbidity

Nephelometer400NTU

DH-48 depth integrating suspended sediment sampler

15

16

Table 2-4. Statistics of each typhoon event.

Typhoon Nelson Herb Nari Aere MatsaAccumulated rainfall (mm) 456 700 872 996 830

Maximum daily rainfall (mm) 264 536 368 559 448

Peak water discharge(m3s-1)

4906 6363 4123 8594 5322

Total sediment discharge (106 tonne)

1.67 1.93 1.41 2.95 1.49

Results analysis

Results analysis

17Fig 5. Higher water discharge could drive more landslide debris. Vertical bars indicate the standard error.

Table 2-5. Statistics of each typhoon event.

Typhoon Nelson Herb Matsa

Peak water discharge(m3s-1) 4906 6363 5322

Landslide volume (106m3) 9.83 13.95 7.02

Total sediment discharge (106 tonne)

1.67 1.93 1.49

Results analysis

18Fig 6. Sediment concentration had a positive relation with the water turbidity. Dashed lines indicate the 95% confidence limits.

Discussion

19Fig 7. The diagram displays the hyperpycnal flow in the Shihmen Reservoir.

Conclusion• High landslide ratios do not correspond to

high sediment discharge because sediment discharge is still dominated by water discharge and landslide debris possibly still stay on slopes.

• Factors causing high turbidity in the reservoir water were (1) landslides and surface weathering in the upstream catchment; (2) the high density hyperpycnal flow between upstream channel and the reservoir bottom.

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Thanks for your attention.

21

• 水庫原來的取水口是195 公尺，但遇到颱風，壩頂淤泥會被攪亂，導致底層原水混濁，自來水廠難以淨化，就會影響供水

• 在石門水庫大壩左岸處岩壁新建水位在236 、 228 、 220 公尺高度的高、中、低 3個取水口，以因應原水濁度太高時取水之用

22http://www.tycg.gov.tw/ehousekeep/chat_detail.aspx?sn=17415

Table 4. Statistics of each typhoon event.

Typhoon Nelson Herb Nari Aere Matsa

Peak water discharge(m3s-1) 4906 6363 4123 8594 5322

Landslide number 977 516 715 2062 3712

Landslide area (km2) 9.83 13.95 21.32 6.71 7.02

New landslide area (km2) 12.07 18.93 5.26 3.49

Reactivated landslide area (km2) 1.88 2.39 1.45 3.53

Landslide ratio (%) 1.3 1.8 2.8 0.9 0.9

New generation ratio (%) 86.5 88.8 78.5 49.7

Reactivated ratio (%) 19.2 17.1 6.8 52.6

Landslide volume (106m3) 9.83 13.95 21.32 6.71 7.02

Landslide driven into the channel (106m3) 2.16 3.77 7.68 1.81 1.83

Suspended discharge (106 tonne) 1.4 1.62 1.18 2.47 1.25

Bedload discharge (106 tonne) 0.27 0.31 0.23 0.48 0.24

Total sediment discharge (106 tonne) 1.67 1.93 1.41 2.95 1.49

Reservoir sediment discharge (106m3) 3.7 8.7 0.4 27.8 10

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