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., Impact of rainstorm-triggered landslides on high turbidity in a mountain reservoir. Engineering Geology. 2010/12/23 1

Outline Introduction Introduction Objectives Objectives Study method Study method Results Results Discussion Discussion Conclusions 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 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) 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) 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 5 Shihmen Reservoir Finishes the month July 1964 Position 24.81°N, °E effective storage capacity 309×10 6 m 3 average annual precipitation 2556 mm slope gradient 83% 30° to 50° Flow direction southeast to northwest Fig 1. Geographical.

Study area - Geological 6 Fig 2. Distribution of the rock formations in the Shihmen Reservoir catchment. PERIODEPOCHFormation Tertiary MioceneAoti Formation (At) Oligocene Tatungshan Formation (Tt) Gangou Formation (Gg) Szeleng Sandstone Formation (Ss) Table 1. Formation

Typhoon Track 7 Fig 3. The location of Shihmen Reservoir catchment within Taiwan and the tracks of typhoons. Table 2-1. Statistics of each typhoon event. TyphoonNelsonHerbNariAereMatsa Year Date8/21-247/29-8/19/13-198/23-268/3-5 Duration of measurements (hour) Accumulated rainfall (mm) Maximum daily rainfall (mm) Average water discharge (m 3 s -1 )

Table 2-2. Statistics of each typhoon event. Typhoo n Average water discharge (m 3 s -1 ) Peak water discharge (m 3 s -1 ) Reservoir sediment discharge (10 6 m 3 ) Nelson Herb Nari Aere Matsa Typhoon events 8

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

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

Objectives To study the relationship between water turbidity and the landslide debris of the Shihmen Reservoir. 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. To reconstruct the process and impact of forming high turbidity water in the reservoir area. 11

Study method 12 Statistics of Typhoon Suspended Sediment Discharge Turbidity The relationship between turbidity and landslides.

Term descriptions 13

14 Table 2-3. Statistics of each typhoon event. TyphoonNelsonHerbNariAereMatsa Landslide area (km 2 ) Landslide ratio (%) New generation ratio (%) Reactivated ratio (%) Landslide volume (10 6 m 3 )

Sample ? NTU Suspended sediment discharge Turbidity Nephelometer 400NTU DH-48 depth integrating suspended sediment sampler 15

16 Table 2-4. Statistics of each typhoon event. TyphoonNelsonHerbNariAereMatsa Accumulated rainfall (mm) Maximum daily rainfall (mm) Peak water discharge (m 3 s -1 ) Total sediment discharge (10 6 tonne) Results analysis

17 Fig 5. Higher water discharge could drive more landslide debris. Vertical bars indicate the standard error. Table 2-5. Statistics of each typhoon event. TyphoonNelsonHerbMatsa Peak water discharge(m 3 s -1 ) Landslide volume (10 6 m 3 ) Total sediment discharge (10 6 tonne)

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

Discussion 19 Fig 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. 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. 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

Table 4. Statistics of each typhoon event. TyphoonNelsonHerbNariAereMatsa Peak water discharge(m 3 s -1 ) Landslide number Landslide area (km 2 ) New landslide area (km 2 ) Reactivated landslide area (km 2 ) Landslide ratio (%) New generation ratio (%) Reactivated ratio (%) Landslide volume (10 6 m 3 ) Landslide driven into the channel (10 6 m 3 ) Suspended discharge (10 6 tonne) Bedload discharge (10 6 tonne) Total sediment discharge (10 6 tonne) Reservoir sediment discharge (10 6 m 3 )