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DEBRIS FLOW TAMOTSU TAKAHASHI Disaster Prevention Research Institute, Kyoto University Publishedfor the lnternational Associationfor Hydraulic Research by A.A.BALKEMA l ROTTERDAM l BROOKFIELD l 1991 Table of contents PREFACE IX l THE NATURE OF DEBRIS FLOW OBSERVED IN SOME MOUNTAIN TORRENTS 1.1 Observation at the gully Kamikamihorizawa of M t. Yakedake, Japan 1.1.1 Process of generation of debris fiow (a) The situation of occurrence (b) Rainfall condition for occurrence of debris ftow 1.1.2 Process of ftow (a) Behavior of flow (b) The translation ve1ocity of the forefront (c) The origin of the ftow material (d) Specific density and the pressure in the ftow 1.1.3 Process of deposition (a) Stoppage of the debris ftow and the arrivai distance (b) Characteristics of the debris lobes and their distribution on the debris fan (c) Topographical change on the debris fan 1.2 1.2.1 1.2.2 1.2.3 Observation at some other basins in Japan Observation at the Name River Observation in the gullies on the Sakurajima Volcano Observation in the experimental basi n of the Hodaka Sedimentation Observatory l 2 2 2 5 5 5 8 lO 11 Il 12 14 15 15 17 19 1.3 Observation at Jiang-Jia Ravine, China 1.3.1 Process of occurrence of the debris flow 1.3.2 Particle size 19 21 23 l .4 23 Observation at Mt.Thomas, North Canterbury, New Zealand 2 MECHANICS OF FLOW 26 2.1 Momentum conservation equations 26 2.2 Constitutive relations 27 v 2.3 Debris Oows in the incrtial range 2.3.1 Stony dcbris fl ow (a) The first approximation (b) The second approximation (c) Comparison to the field data 2.3.2 Immature debris flow (a) Definition and thickness of particle flowing layer (b) Velocity distribution (c) Resistance formula (d) Sediment discharge (e) Cross-sectional mean solids concentration 2.3.3 Turbulent mud ftow (a) Modeling of the ftow (b) Resistance law (c) Karman constant 2.3.4 A hybrid of stony and muddy debris ftows 2.3.5 Existence criteria of various type ftows 32 32 32 2.4 2.4.1 2.4.2 2.4.3 57 57 Debris flow in the macro-viscous range Mechanics of flow Existing criteria of the flow in the viscous range Solids concentration 36 40 4J 41 43 45 46 47 48 48 50 51 52 55 59 61 3 PROCESSES OF OCCURRENCE, DEVELOPMENT AND DECLINATION 63 3.1 Mechanism of occurrence 3.1.1 Debris ftow produced by appearance of/increase in surface water fiow on the gully bed 3.1.2 Liquefaction of landslide mass 3.1.3 Debris ftow generation due to natura! dam collapse (a) Erosive destruction due to overtopping (b) Abrupt sliding collapse (c) Progressive failure 63 3.2 Formation of hydrograph 3.2.1 Development of a debris flow on a varying slope bed (a) Description of the process (b) Fundarnemal equations (c) Comparison of experimental results with calculated ones 3.2.2 Debris ftow due to landslide dam failure (a) Debris fiow prediction mode! for the overtopping type failure (b) Debris flow prediction mode! for the instantaneous slip failure type (c) Debris flow prediction mode! for the progressive failure type VI 63 70 73 74 75 75 75 75 75 76 79 83 83 85 89 3. 3 Deformation of hydrograph 3.3.1 Kinematic wave theory 3.3.2 The big mud flow in the Stava River, Italy 90 90 92 4 CHARACTERISTICS OF FULLY DEVELOPED FLOW 100 4.1 Model of quasi-steady ftow and the shape of the snout 100 4 .2 Roll waves 103 4.3 Mechanics of accumulation of boulders at front of debris fiow 4.3. 1 Mechanism of particle segregation in a stony debris ftow 4.3.2 A numerica! simulation of convergence of larger particles to the front l05 105 IlO 4.4 Transportation of huge stones 113 4.5 Debris flow around bend 116 5 PROCESSES OF DEPOSITION 121 5.1 121 Stoppage of inertial stony debris flow 5.2 Analysis of deposition in the channel of constant width 5.2.1 Surface slope on the deposit of a stony debris flow 5.2.2 Longitudinal profile of the deposi t of stony debris tlow (a) In the case of ed ~ y (b) In the case of y <ed< ec 5.2.3 Numerica! simulation of the depositing process of a stony debris fiow 5.2.4 Analysis of deposition of turbulent muddy flow 124 124 125 125 127 129 132 5.3 The debris eone formation process 5.3.1 Description of the experimental results 5.3.2 Numerica! simulation (a) Fundamental equations (b) Verification of the mathematical model by experiment 5.3.3 Particle size distribution in the debris eone (a) Field and experimental data (b) Mathematical mode1 5.3.4 Analyses of the actual debris fiow/mud flow deposition (a) Horadani debris flow (b) Armero mud fiow 134 134 136 136 138 140 140 141 145 145 APPENDIX- NOTATIONS 155 REFERENCES 161 147 VII