Impact of rainstormtriggered landslides on high turbidity in

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. 報告人: 黃璿宇 俞佳成 報告日期: 101/11/23

Outline Introduction Study area Typhoon track and events Study method Rseult Discussion Conclusion 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 – Geographical Shihmen Reservoir --------------------------------------------------------------------------- Finishes the month Position effective storage capacity average annual precipitation slope gradient Flow direction July 1964 24. 81°N, 121. 24°E 309× 106 m 3 2556 mm 83% 30° to 50° southeast to northwest Fig 1. Geographical. http: //www. wranb. gov. tw/ct. asp? x. Item=2605&ct. Node=815&mp=5 6

Study area - Geological Table 1. Formation PERIOD EPOCH Miocene Tertiary Oligocene Formation Aoti Formation (At) Tatungshan Formation (Tt) Gangou Formation (Gg) Szeleng Sandstone Formation (Ss) Fig 3. Distribution of the rock formations in the Shihmen Reservoir catchment. 7

Typhoon Track Table 2 -1. Statistics of each typhoon event. Typhoon Nelson Herb Nari Aere Year 1985 1996 2001 2004 Date 8/21 -24 7/29 -8/1 9/13 -19 8/23 -26 Duration of measurements (hour) 96 120 168 96 Accumulated rainfall (mm) 456 700 872 996 Maximum daily rainfall (mm) 264 536 368 559 Average water discharge (m 3 s-1) 547 707 710 1398 Matsa 2005 8/3 -5 72 830 448 694 Fig 4. The location of Shihmen Reservoir catchment within Taiwan and the tracks of typhoons. 8

Typhoon events Table 2 -2. Statistics of each typhoon event. Average water discharge Peak water discharge Reservoir sediment discharge Typhoon (m 3 s-1) (106 m 3) Nelson 547 4906 3. 7 Herb 707 6363 8. 7 Nari 710 4123 0. 4 Aere 1398 8594 27. 8 Matsa 694 5322 10 9

Typhoon events 1987 1990 1994 1992 1996 Fig 5. Sediment deposition (tonne), Annual precipitation (mm) and accumulated rainfall during typhoon (mm) during 1963~2005. 10

Sediment deposition (tonne) Typhoon events Fig 6. Sediment deposition (tonne), Annual precipitation (mm) and accumulated rainfall during typhoon (mm) during 1963~2005. 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 Matsa Landslide area (km 2) 9. 83 13. 95 21. 32 6. 71 7. 02 Landslide ratio (%) 1. 3 1. 8 2. 8 0. 9 New generation ratio (%) 86. 5 88. 8 78. 5 49. 7 Reactivated ratio (%) 19. 2 17. 1 6. 8 52. 6 13. 95 21. 32 6. 71 7. 02 Landslide volume (106 m 3) 9. 83 15

Suspended sediment discharge DH-48 depth integrating suspended sediment sampler Turbidity Nephelometer 16

Results analysis Table 2 -4. Statistics of each typhoon event. Typhoon Nelson Herb Accumulated rainfall 456 700 (mm) Maximum daily rainfall 264 536 (mm) Peak water discharge 4906 6363 (m 3 s-1) Total sediment discharge 1. 67 1. 93 (106 tonne) Nari Aere Matsa 872 996 830 368 559 448 4123 8594 5322 1. 41 2. 95 1. 49 17

Results analysis Table 2 -5. Statistics of each typhoon event. Typhoon Nelson Herb Matsa Peak water discharge(m 3 s-1) 4906 6363 5322 Landslide volume (106 m 3) 9. 83 13. 95 7. 02 Total sediment discharge (106 tonne) 1. 67 1. 93 1. 49 Fig 7. Higher water discharge could drive more landslide debris. Vertical bars indicate the standard error. 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.