DYNAMIC LOAD CHARACTERISTICS OF DBRIS FLOW MODEL USING DIFFERENT GRAVEL SIZE DISTRIBUTION

Nobutaka Ishikawa1, Ryuta Inoue2, Masuhiro Beppu3, Yuji Hasegawa4, Takahisa Mizuyama5

ABSTRACT

This paper presents an experimental approach on the examination of surge formation and the dynamic load characteristics of debris flow models in the hydrodynamic channel test. First, the debris flow models with six different gravel size distributions are provided by using the natural sediment materials. Second, the dynamic load-time relations of these debris flow models are measured by the load cell which is synchronized with the front flow motion visualized by a high speed video camera. Third, the surge formation is judged by the test results of the flow depth ratio between the front and the following flow depths and of the load ratio between the peak and the constant (stabilized) loads. Finally, the cause of surge formation is discussed.

Key Words: dynamic load, debris flow model, surge formation, gravel size distribution

INTRODUCTION

Recently many debris flow disasters have occurred in the world, especially, in Taiwan, Indonesia and Japan. One of the reasons for these disasters may be caused by torrential rain fall due to global warming. Many people have been killed and many properties have been destroyed by these debris flow disasters. It is generally considered that the impulsive fluid load referred as surge in the debris flow gives the extensive damage to the village and houses.The object of this paper is to reproduce the impulsive fluid load due to the surge formation by using natural sediment materials in the hydrodynamic channel model test.Many studies have been done on the fluid load of debris flow model based on the dynamic fluid theory (Hirao,et al. 1970, Daido,1988, Miyamoto and Daido,1983, Mizuyama,et al. 1985, Miyoshi and Suzuki,1990,Horii,et al.2002). Some attempts have also been made on the impulsive fluid load by using only water, sediment with water, gravel with sediments including water and beads with water (Ishikawa,et al.2008). However, it was very difficult to make the surge formation in the debris flow model tests and no surge has been formed so far by using natural sediment materials. Surge formation was only found by the test using pumice stones (Ishikawa,et al.2008,2009).

1 Professor Emeritus of National Defense Academy, Research Advisor of Research Association for Steel SaboStructures, 2-7-5 Hirakawa-cho, Chiyoda-ku, Tokyo,102-0093, Japan(Tel:81-03-5215-8228,Fax:81-03-5215-8229, E-mail:cgishikawa@m4.dion.ne.jp)2 Civil Engineer, Kyose-Kiko,1-23-1 Shinjyuku-ku, Tokyo160-0022,Japan3 Associate Professor, Department of Civil and Environmental Engineering, National Defense Academy,1-10-20 Hashirimizu, Yokosuka 239-8686, Japan4 Researcher, Civil Engineering Research Laboratory, 904-1 Tohhigashi, Ibaraki,300-2633,Japan5 Professor, Department of Forest, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, JapanTo this end, this paper presents an experimental approach on the examination of surge formation and the dynamic load characteristics of debris flow models by using natural sediment materials. First, the specimens of debris flow model are provided as the six different gravel size distributions using natural sediment materials. Second, the surge formation is examined by observing the front flow motion visualized by a high speed video camera. Third, the dynamic load-time relations of debris flow models are measured by using load cell (force component meter) which is synchronized with the high speed video camera. Finally, the surge formation is judged from both points of view of the flow depth ratio between the front and the following flow depths in the flow motion pictures and of the load ratio between the peak and the constant (stabilized) flow loads in the load-time relations and, then the cause of surge formation is discussed.

PREVIOUS TEST USING PUMICE STONES

Figure 1 shows the front fluid motion using the pumice stones visualized by a high speed video camera which is synchronized with the load-time relation in the previous hydrodynamic channel test (Ishikawa, et al.2008, 2009). This phenomenon is known as surge (Takahashi, 2004) and is referred to as impulsive fluid load in which the feature has the steep rise time to the peak load in the load-time relation and the large load ratio between the peak and the constant (stabilized) loads. However, it has been difficult to make such a surge formation byusing natural sediment materials in the hydrodynamic channel test so far. Surge formation was only foundby using pumice stones as shown in Fig.1. Therefore, it is required to examine the surge formation by using the debris flow models with 10080loadN6040200-20

10080loadN6040200-20

10080loadN6040200-20

01000 2000 timemsec

01000 2000 timemsec

01000 2000 timemsecnatural sediment materials Fig.1 Surge shape synchronized with load-time relation using pumice stonesHYDRODYNAMIC CHANNEL TEST USING DEBRIS FLOW MODELS WITH DIFFERENT GRAVEL SIZE DISTRIBUTION

Outline of Test

Figure 2 shows the hydrodynamic channel test set-up with a slope of 18 degrees. The flow load is measured by the load cell (force component meter) which is synchronized with the high speed video camera to take the front flow motion.By pouring water of 2 into the debris flow models which is piling up to the washout height beforehand, the debris flow models are flowing at the instance of taking off the stopping panel as shown in Fig.3. The rib roughness is set up in the flow floor at the flow distance of 5.0m in order to reproduce the occurrence field of debris flow.That is to say, the debris flow models with six different gravel size distributions are flowed two times for each type and the load-time relations are measured by the load cell with the front flow motion taken by the high speed video camera.

Load cell

Sabo dam model

High speed video camera

(a) Hydrodynamic channel (b) Measurement system

Fig.2 Experimental apparatus set-up

Stopping panel

Load cell

Debris flow model

Sabo dam model

Fig.3 Outline of debris flow model test

Debris flow model specimens

The debris flow model specimens are provided by using the natural sediment materials with six different kinds of gravel size distributions which illustrate the percentage passing by mass gravel size relation as shown in Fig.4. That is to say,

Type A (standard type): the equal gravel size distribution curve of gravel (16-19mm) and sand(0.01-15mm).Type B: the ratio of gravel 100% and sand 0%, Type C: the ratio of gravel 70% and sand 30%, Type D: the ratio of gravel 50% and sand 50%, Type E: the ratio of gravel 30% and sand 70%,Type F: the ratio of small size gravel (4.75-9.5mm) 70% and sand 30%.

It should be noted that Type A has the standard gravel size distribution, Type B has only gravels and no sand, Type C consists of the ratio of gravel and sand which is 7:3, Type D has the ratio which is 5:5, Type E is composed of the ratio of 3:7, Type F has the ratio of small gravels and sand which is 7:3. Therefore, the test was performed twice for each Type to examine if the surge could be formed among the debris flow models with six different kinds of gravel distribution mentioned above or not.

100

Percentage passing by mass%80

60 TypeA TypeBTypeC ETypeDTypeE

TypeF D

40 F

C20A B

00.01 0.1 1 10 100Gravel sizemmFig.4 Different gravel size distribution of debris flow models

TEST RESULTS AND CONSIDERATIONS Surge formation by front flow motionThe front and following flow depths were measured by the flow motion pictures taken by the high speed video camera as shown in Fig.5. Then, the ratio (h*) beween front and following flow depths is calculated as shown in Table 1. The results from Table 1 show that if the ratio of h* is more than 2.0, then the surge was formed at the front flow motion. That is to say, the surge formation is judged from the viewponit of the flow depth ratio as follows.

h*>2.0 ; complete surge formation (1a)2.0>h*>1.5 ; quasi surge formation (1b)1.5 >h*> 1.0; no surge formation (1c)where, h*=hf / hb : flow depth ratio, hf : front flow depth,hb: following flow depth.

(a) Type C (Surge formation) (b) Type E (No surge formation)

Fig.5 Front flow motions of Type C and Type E( The white circle is the light for the high speed video camera. )Table 1 Test results for surge formation

Case

Debris flow model

Front flow depth

Following flow depth

flow depth ratio h* = hf/ hbLoad ratio

P*=Pmax/P0

Final judgement of surge formation

TypeContenthfcmhbcm

3

A

standard7.04.01.81.114

6.54.01.61.0018

B

Gravel 100%sand 0%5.54.01.40.7919

5.55.01.10.8614

C

Gravel 70%sand 30%8.53.52.41.9015

7.53.52.11.5722

D

Gravel 50%sand 50%4.03.51.10.8623

4.03.01.30.7516

E

Gravel 30%sand 70%2.52.51.00.9017

4.53.51.30.6220

F

Small gravel 70%sand 30%5.53.51.61.0621

6.04.01.51.07Note: symbols , , express the complete surge, the quasi surge and no surge formations, respectively.

Surge formation by load-deformation relation

Figures 6-11 show the load-deformation relations with front flow motions of Types A-F, respectively.Type A (standard type) shows the steep rise time in the load-time relation and the quasi-surge formation in the front flow motion as shown in Fig.6.Type B illsutrates the slow rise time to the peak load in the load-time relation and no surge formation in the front flow motion as shown in Fig.7.Type C shows the very steep rise time in the load-time relation in Fig.8(a) and the complete surge shape is forming in the front flow motions as shown in Figs.5(a) and 8(b).Type D has no clear peak load in the load-time ralation and the precise surge formation was not seen in the front motion as shown in Fig.9.Type E has very slow rise time in the load-time relation and the no surge formation was found in the front motion as shown in Fig.10.Type F shows the steep rise time phenomenon in the load-time relation and the quasi surge formation was seen in the front motion as shown in Fig.11.In order to examine the dynamic load characteristics of different gravel size distribution of Types A-F, the load ratio P* is defined as the ratio between the peak and constant loads shown in Fig.12 and the surge formation is judged from the viewpoint of the load ratio as follows.

P*> 1.5 ;

complete surge formation

(2a)1.0