1. Chjeng-Lun SHIEH, Yu-Shiu CHEN, and Yuan-Jung TSAI
Threshold variation of debris flow occurrence after the Chi-Chi earthquake
Chjeng-Lun SHIEH, Yu-Shiu CHEN, and Yuan-Jung TSAI

2. Introduction Chi-Chi Earthquake on 21st Sep. 1999
ML 7.3
Central Taiwan was suffered from a severe damage
Plenty of landslides occurred (14347 hectares)
Debris flows occurred frequently and easily after the Chi-Chi earthquake

3. Debris Flow Warning Criteria in Taiwan
Critical line (C.L.) is installed as the criteria of debris flow warning in every debris flow potential area in Taiwan
Debris Flow occurred
Debris Flow didn’t occurred
(mm/hr)
(mm)

4. Purpose of this paper
After the Chi-Chi earthquake, the threshold of debris flow occurrence varied severely. Debris flolw warning became difficult
This paper study on the threshold variation of debris flow occurrence

5. Study Sites
Black Stone watershed in central Taiwan, near the Chelungpu fault
There was few debris-flow histories before the earthquake
Debris flow occurred easily and frequently in this watershed after Chi-Chi earthquake
Black Stone watershed

6. Study Sites – Black Stone watershed
Monitoring Station of Debris Flow
Sediment Sampling

7. Landslides in 1999, Total area 58 ha (before Chi-Chi earthquake)

8. Landslides in 2001, total area 488 ha (after Chi-Chi earthquake)

9. Landslides in 2004 , Total area 703 ha (after Aere Typhoon)

10. Landslides in 2006, Total area 743 ha

11. Riverbed variation in the Wushihkeng watershed
Riverbed variation in the Wushihkeng watershed

12. Monitoring & Survey Debris Flow Monitoring System River Survey
Rainfall gage
Ultrasonic water level meter
Real-time camera
Data log & network system
River Survey
Longitudinal section
Cross section
Sediment sampling & grain size analysis
Suspended load survey
Landslide Survey
Landslide volume survey

13. Results of Debris Flow Monitoring
C.L. Before the Chi-Chi Earthquake
decrease
recover
2008
Typhoon KALMEGI

14. Results of Landslide & River Survey
The Chi-Chi
Earthquake
Typhoon
Toraji
Typhoon
Mindulle & Alle
Typhoon
Matsa
Landslide area ratio = Landslide area / watershed area

15. Result of the Grain Size Investigation
Landslide
Upstream
Midstream
Downstream
Percent Passing (%)
#200
Sieve
Fine
Particles
Size (mm)

16. The Record of Fine Particle Concentraion at Yi-Li Staion (Near the river mouth of black stone stream)
Suspended Load (p.p.m.)
Discharge (cms)
Data from Water Resource Angency

17. The Concentration of Suspended Load in Same Discharge
Qw= 100 cms

18. Why does the C.L. Vary ?
Because of the landslide volume variation on slope ?
Because of the sediment volume variation in river ?
Because of the sediment grain size variation ?
Because of fine partial concentration variation ?

19. View Point of C.L. Variation
Debris Flow is a two-phase flow which include fluid phase and solid phase. When fluid becomes hyper concentration due to fine particle supply by riverbed or landslide, then coarser particle become easily to move.
The amount of fine particles which supply from landslide or erod from river material could be the dominant factor of the C.L. variation.

20. The higher concentration, the smaller hc
Influence of Fine Particle Concentration on Occurrence Condition of Debris Flow
The condition of debris flow occurrence (Takahashi,1977)
hc : critical water depth of debris flow occurrence
rw = [cfσ + (1-cf) ρ]g
σ：particle density
ρ：water density
cf：concentration of fine particle in fluid
The higher concentration, the smaller hc

21. Influence of Fine Particle Concentration on Occurrence Condition of Debris Flow
The higher fluid concentration, the lower occurrence
condition of debris flow
Study site C* = 0.71 & Φ = 34o
hc / D

22. Influence of Fine Particle Concentration on Occurrence Condition of Debris Flow
For example, the river gradient is about 8 degrees. Compare the critical water depth hc in the case of hyper concentration (rw=1.5) and clear water (rw=1.0) as follow
It shows that when the fluid specific weight become 1.5, then hc of debris flow occurrence become 1/3 of the clear water case.
hc (rw =1.5)
hc (rw =1.0) = 1 3

23. Conclusion
Debris-Flow occurrence varied severely before and after the Chi-Chi earthquake
Occurrence condition of debris flow become lower after the Chi-Chi earthquake
However, recovered gradually in the sequential years
Fine particle concentration maybe the dominant factor of the variation of debris flow occurrence

24. Thanks for Your Attention