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

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

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)

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

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

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

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

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

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

Landslides in 2006, Total area 743 ha

Riverbed variation in the Wushihkeng watershed 2001-07-04 2003-10-14 2004-08-18 Riverbed variation in the Wushihkeng watershed 2004-09-17 2005-09-08 2009-07-28

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

Results of Debris Flow Monitoring C.L. Before the Chi-Chi Earthquake decrease recover 2008 05-06-07 2008-07 -16 Typhoon KALMEGI

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

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

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

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

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 ?

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.

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

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

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

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

Thanks for Your Attention