Impact of rainstorm-triggered Impact of rainstorm-triggered landslides on high turbidity in a landslides on high turbidity in a

mountain reservoirmountain 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.

報告人 : 黃璿宇 俞佳成報告日期 :101/11/23

Outline

1

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)

2

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)

3

Study area – GeographicalGeographical

http://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. 6

Study area - GeologicalGeological

Fig 3. 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

7

Typhoon TrackTyphoon Track

Fig 4. 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 MatsaYear 1985 1996 2001 2004 2005

Date8/21-

247/29-8/1

9/13-198/23-

268/3-5

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

8

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 eventsTyphoon events

9

Typhoon eventsTyphoon events

19871990

19921994

1996

Fig 5. Sediment deposition (tonne), Annual precipitation (mm) and accumulated rainfall during typhoon (mm) during 1963~2005.

10

Typhoon eventsTyphoon events

Fig 6. 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)

11

Study method

• 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.

12

Study method

Statistics of Typhoon

Suspended Sediment Discharge

Turbidity

The relationship between turbidity and landslides.

13

Term descriptions

14

Table 2-3. Statistics of each typhoon event.Typhoon Nelson Herb Nari Aere MatsaLandslide area (km2) 9.83 13.95 21.32 6.71 7.02Landslide ratio (%) 1.3 1.8 2.8 0.9 0.9New 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

15

Suspended sediment discharge

Turbidity

Nephelometer

DH-48 depth integrating suspended sediment sampler

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 analysisResults analysis

17

Results analysis

Fig 7. 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 MatsaPeak 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

18

Results analysis

Fig 8. Sediment concentration had a positive relation with the water turbidity. Dashed lines indicate the 95% confidence limits.

19

Discussion

Fig 9. The diagram displays the hyperpycnal flow in the Shihmen Reservoir. 20

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.

21

Thanks for your attention.