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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.
20
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
23
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