Final Report
1994-1998 IDNDR Project

C-3: Mechanism of Viscous Debris Flows and
Countermeasures against Them

Purpose
Peoples suffer from the disasters by various types of debris flows in the world.The studies on debris flows of viscous type had not been sufficient while the boulderrich ones had been well studied.The mechanism of viscous debris flow and countermeasures against them have been the objectives of this research with the Chengdu lnstitute of Mountain Disasters and Environment,Chinese Academy of Science.
The viscous type of debris flow is often found in the regions of the geologic settings composed of highly fractured slate,mudstone,limestone,or metamorphic rock in tectonically active zone.They frequently bring about significant disasters in rainy season.

Photo 1. Debris flow runhing down Jiangjia Gully. The left is the photo on 13 August 1991 and the right on 4 August 1998.
Photo 2. Drastic change in the channel topography by debris-flow repetition. The left id the photo on 20 August 1993, and the right on 3 August 1998.


Results
The cooperative studies based on the field observation of debris flows and the topographic surveys at Jiangjia Gully,Yunnan Province; a small tributary of the upstream of Chang Jiang,the studies by flume experiments,and numerical simulation have brought about the following results.
1)The rainfall condition which is specialized by a time lag for debris-flow occurrence from an onset of rainstorm suggested an accumulation process of debris materials to the source slopes.
2)The mechanism of liquefaction of slope-failure material to debris flow,being saturated with water.
3)One event of debris flow consists of many intermittent surges(See Photo 2)whose number usually tends to several tens or a few hundreds with a long duration from a few hours to several hours.
However four causes might be possible for such intermittent surges,the process of intermittent formation of landsliding dam and its breach should be most plausible among the four causes(Cause 1: Driving force overcomes the resisting yield strength of Bingham fluids or the strength of other viscoplastic nuids; Cause 2: Successive formation of landsliding dam and its breach; Cause3: Instability of the flow itself; Cause 4: Sensitive resPonse to an impulsive succession of strong intensity of heavy rainfall).
4)The rheology of viscous debris flow can be explained by Newtonian Fluid Model,not by Bingham Fluid Model.Though the latter model have been accepted broadly by researchers to explain the viscous debris flow,it was proved that the latter was not suitable to the viscous debris flows,because observation clarified the absence of plug in the flow which must be present if the flow should be Bingham Fluid.
5)Repetition of a large number of debris-flow surges caused a remarkable migration of stream channel(See Photo 2)and generated a vast inundation of debris materials on the fan.The mechanism of such changes in the channel and fan morphology due to repetition of debris flows were clarified.
6)The previous countermeasure works against debris flows were evaluated to have been effective in many cases being based on the long term field assessments,and new methods of systematic countermeasures were proposed through the field surveys and some experimental studies.

Details: Research report of group C3 of Special Project associated with lDNDR sponsored by Ministry of Education,Science,Sports and Culture,Japan "Japan-China Joint Research on the Mechanism and the CountemleaSsres for the Viscous Debris Flow",1999,DPRI,Kyoto University,206 p.

Contents of this report are:
1. Debris-flow Hazards in Japan and China since 1994
1.1 Situation in Japan
1.2 Situation in China
2. Observational Studies on Debris Flows
2.1 Rainfall condition for the occurrence of debris flowinthe Jiangjia Gully
2.2 Stochastic analysis of rainfall for debris-flow occurrence inthe Jiangjia Guny Basin
2.3 Characteristics of the debris flow in the Jiangjia Gully
2.4 Generation,movement and sedimentation of viscous debris flows
2.5 Intermittent surges of debris flows
3. Model Experiments and Development of the Method for Surface Velocity Measurement
3.1 Field and laboratory experiments
3.2 Image analysis for the measurement of debrisflow velocity
4. Mechanics of Viscous Debris Flow
4.1 Newtonian fluid model of viscous debris flow
4.2 Two-dimensional simulation of the deposition process
4.3 Prediction of the scale of the viscous debris flow
4.4 The two-phase double-fluid mechanical model for viscous debris flow
4.5 Analysis on characteristics of fluid and motion of debris flow at the Jiangjia Gully
5. Effect of Debris Flows on the Stream-Channel Morphology
5.1 Topographic change in Jiangjia Gully by debris flow
5.2 Scour and fill of viscous debris flows
6. Mitigation of Debris-Flow Hazards
6.1 Evaluation of the Countermeasures against Debris Flow in Dongchuan City
6.2 Counter measures against debris flow in Dongchuan City
6.3 Environmental change in the area of viscous debris flow
- A case study in the Xiaojiang River Basin,Dongchuan District,Yunnan Province
6.4 Countermeasures against viscous debris noW

Authors ofthis report are:
TAKAHASHI Tamotsu,SAWADA Toyoaki,SUWA Hiroshi,MIZUYAMA Takahisa,and ARAI Muneyuki(Japanese side); KANG zhicheng,WU Jishan,OU Guoqiang,and XION Gang(Chinese side)