12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
PREDICTION OF DEBRIS FLOW DISASTERS AND THE EVALUATION OF CONTROL STRUCTURES WITH HYDRAULIC
MODEL TESTS AND NUMERICAL SIMULATION
Yuji Hasegawa1, Nobuo Sugiura2, Takahisa Mizuyama3, Kana Nakatani4 and Kazuki Matsushita5
INTRODUCTION
It is known that landslides and debris flows phenomena differ depending on the nature of the soil. Japan has areas with special soils including a type of soil called masado, which consists of disintegrated granite. This type of soil is fragile, and can easily cause landslides and debris flows. When a debris flow consist of fine sediment (ex.
masado), it behaves like a mud flow and flows a great distance compared to a gravel flow. Normally, a sabo dam is effective in reducing the damage caused by a debris flow. However, the flow conditions and erosion /deposition processes are different from those of mud
flows. When setting a closed sabo dam, debris hits the dam directly, and then upward splashing occurs (See Photo1). When setting a slit type steel sabo dam, blockage cannot be expected because flow is consist of fine sediment. In order to investigate effective countermeasures for this type of soil, we conducted two types of experiment: a channel experiment and a model experiment (model scale 1/30).
We also carried out a verification using the GUI-equipped debris flow simulator "Kanako" (Nakatani et al., 20
Dam model
Photo 1 The situation that a mud flow collides with sabo dam
08).
OUTLINE OF CHANNEL EXPERIMENT
1. The experimental channel is a rectangular straight open channel 10m long and 30cm wide with a variable slope.
0 10 20 30 40 50 60
0 5 10 15 20 25 30
Distance(m)
Velocity(m/s) Type A
Type B Type C Type D
2. The sabo dam model had a height ranging from 9m and upstream slope from 1:0.2.
3. Fig.1 shows relationship between the distance covered by the debris splashes and the velocity of flow into the dam.
4. The solid line in the diagram shows the result obtained when there was no proposed countermeasure. The plot shows the result when the proposed countermeasure was used. If the solid line is exceeded, there is no control of the splashing phenomenon.
Fig.1 Relationship between the distance covered by the debris splashes and the velocity of flow
5. A debris flow does not jump up and down if its velocity is below about 13m/s.
The splashing does not occur when (Type A) a wave
1 Researcher, Yuji Hasegawa. Civil Engineering Research Laboratory, 904-1 Tohigashi, Tsukuba, Ibaraki,300-2633,Japan (e- mail:hasegawa@crl.or.jp)
2 Vice President, Nobuo Sugiura. Civil Engineering Research Laboratory, Japan
3 Prof., Takahisa Mizuyama. Department of Forest, Graduate School of Agriculture, Kyoto University, Japan
4 PhD, Kana Nakatani. Department of Forest, Graduate School of Agriculture, Kyoto University, Japan
5 Chief Engineer, Kazuki Matsushita. Otagawa River Office, Chugoku Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Japan
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reverser is installed at the sabo dam, (Type B) the gradient upstream of the dam is steep, (Type C) a steel slit is installed upstream of the sabo dam, (Type D) small steps are made at a position upstream of the sabo dam (See Fig.2).
OUTLINE OF MODEL EXPERIMENT
1. We used a model of basin in the western part of Hiroshima Prefecture, Japan (Catchment area 0.109km2).
2. The structures which the splashing did not occur in the channel experiment were of Type A and Type D.
3. The conditions for the dam model were the same as those for the channel experiment.
4. Because the ratio of water depth (flow depth/ particle size) of the debris flow was about 100, it was classified as a mud flow.
5. This sabo dam can catch 2,477m3 of sediment, when sedimentation occurs flatly.
6. When this volume is supplied, the front flow sediment splashes to the sabo dam, and about 400m3 of sediment overflow and flow downstream.
7. If the proposed countermeasures are applied, the splashing does not occur and the flooded area decreases (See Photo2).
This is because Type A cuts the splashing, and Type D decreases the velocity of flow into the sabo dam.
OUTLINE OF NUMERICAL SIMULATION
1. We used the GUI-equipped debris flow simulator "Kanako".
The model used in Kanako is based on Takahashi et al. 's governing equation related to the flow and sedimentation of the debris flow (Takahashi et al., 1991). Using Kanako, we
can simulate from the one-dimensional steep mountainous area to the two-dimension alluvial fan area.
2. Kanako cannot reproduce the phenomenon of debris splashing because of model application limit.
However, the tendency towards flooding and sedimentation downstream from the sabo dam varied slightly.
3. If a slit type steel sabo dam is used, fine sediment does not cause a blockage even if the slit width is small. However, if there are big boulders and woody debris, a blockage occurs and fine sediment can also be trapped.
REFERENCES
Nakatani K., Wada T., Satofuka Y., Mizuyama T. (2008). Development of “Kanako”, a wide use 1-D and 2-D debris flow simulator equipped with GUI, Monitoring, Simulation, Prevention, and Remediation of Dense and Debris Flow II, WIT PRESS, pp.49-60.
Takahashi, T., Nakagawa, H. (1991). Prediction of stony debris flow induced by severe rainfall, Journal of the Japan Society of Erosion Control Engineering, Vol. 44, No. 3, pp. 12–19.
Keywords: debris flow, mud flow, hydraulic model tests, numerical simulation Fig.2 The structures that controlled the splashing
TypeA TypeB TypeC TypeD
1:0.0 9m 1:0.2
9m 0.5m
1:0.2 9m
50m
5, 10m 1:0.2
30, 50, 100m
5m 9m
Without structure
Type A
Type D
Photo2 situation of overflow in the downstream basin of the sabo dam
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