Post-event reconstruction of the short-lived Siaolin landslide dam

1. Post-event reconstruction of the short-lived Siaolin landslide dam 莫拉克颱風引致之堰塞湖地形與水文參數分析－以小林村堰塞湖為例 Advisor：J.J. Dong Presenter：Y.S. Li Date：2010/10/14

2. Outline • Introduction • Methods • Results • The characteristics and failure mechanisms of the Siaolin landslide dam • Conclusions • Future works

3. Introduction(1/3) • Formation of landslide dams： (Casagli and Ermini,2003) • Longevity of landslide dams： Stability (Costa and Schuster,1986 )

4. Introduction(2/3) • Stability of landslide dams : DBI : ：catchment area ：dam height ：dam volume (Ermini and Casagli,2003) Discriminant analysis： (Dong et al.,2009) (Dong et al.,2009) Ds＞0：Stable ：peak flow ：dam width ( along the valley) Ds ＜ 0：Unstable ：dam height ：dam length( across the valley) (Dong et al.,2009)

5. Introduction(3/3) The database of landslide dams is incompleteness. • Why ? → Lacked for systematic data gathering on process, characteristics, control measures, and effects of landslide dams. (Costa and Schuster, 1986) → The forming and breaching of rainfall triggered landslide dam is beneficial to understand the impact of extreme weather on surface process relates to landslide dam. (Kroup, 2004) → Detailed documentation of processes of dam failure remainedscare. (Kroup, 2002) • So … Morakot event. The Siaolin landslide dam. → Reconstruction of the shape of landslide dam. →

6. Methods DTM. Field investigation & Laboratory works. Reconstruction of dam geometry Kriging. • Topography • Debris budget of • landslide • Inflow rate and overtopping time 1. Aerial photographs 1. Laboratory works 1. Water level. →Water volume. (based on DTM) ASTM→ Grain size distributions of dam materials . 2. Field investigation 2. Debris budget 2. Inflow rate is 2974cms. → overtopping time. Initial volume of a landslide. Volume of the entrained materials. (Hungr and Evans,2004) volume expansion due to fragmention. Volume of debris deposit on the slope. Volume of debris entered the channel.

7. Results(1/5) • Topographic Analysis EL. 440 ● EL. 420 ● • Laboratory works Fine grain particles Lower Hydraulic conductivity Overflow

8. Results(2/5) • Volume and debris budget of landslide 25.21 million m3 2.23 million m3 13.33% 15.41 million m3 15.31 million m3 Reference value

9. Results(3/5) • Dam height EL. 410 ● EL. 440 ● Q=2974 cms EL. 420 ● No.11 Bridge

10. Results(4/5) • Reconstruction of the shape of landslide dam

11. Results(5/5) The total dam volume is 15.34 million m3 . The breached dam volume is 7.36 million m3. 15.31 million m3 The maximum thickness of deposition is 60 m. The maximum thickness of erosion is 41 m.

12. The characteristics and failure mechanisms of the Siaolin landslide dam • Characteristics • Failure mechanisms DBI=3.2；Ds= -0.37 Unstable X Slope FS=3.46 FS=3.46 X Piping Matrix-supported. Overtopping

13. Conclusions • The total volume of landslide dam is estimated to be 15.34 million m3, the dam height of breaching point is 44m, the maximum dam height is 60m, the dam length(along valley) and dam width(across valley) is 1554m and 760m. • According to dam materials and estimated geometry and hydrologic parameters： → The Siaolin landslide dam is classified as unstable and failure mechanism is dominated by overtopping.

14. Future works • 3D landslide dam. • Collecting the data of landslide dams during Morakot typhoon.

15. Thanks for your attention！