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
1 Professor Emeritus of National Defense Academy, Research Advisor of Research Association for Steel Sabo Structures, 2-7-5 Hiralawa-cho, Chiyoda-ku, Tokyo,102-0093, Japan (*Corresponding Author;
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,Japan
3 Associate Professor, Department of Civil and Environmental Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239-8686, Japan
4 Researcher, Civil Engineering Research Laboratory, 904-1 Tohhigashi, Ibaraki,300-2633,Japan 5 Professor, Department of Forest, Graduate School of Agriculture, Kyoto University, Kitashirakawa,
Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
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).
To 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.
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0 1000 2000
time（msec）
load（N）
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0 1000 2000
time（msec）
load（N）
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0 1000 2000
time（msec）
load（N）
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’ by using natural sediment materials in the hydrodynamic channel test so far. Surge formation was only found by using pumice stones as shown in Fig.1.
Therefore, it is required to examine the surge formation by using the debris flow models with
natural sediment materials Fig.1 Surge shape synchronized with load-time relation using pumice stones
HYDRODYNAMIC 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 18degrees. 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 ℓ 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.
Fig.2 Experimental apparatus set-up
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
Sabo dam model
Load cell
High speed video camera (a) Hydrodynamic channel (b) Measurement system
a
Flow distance 5.0m
Rib roughness
Flow water
18°
ell
Stopping panel Load c
Sabo dam model
h
b
Debris flow model
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.
0 20 40 60 80 100
0.01 0.1 1 10 100
Gravel size（mm）
Percentage passing by mass（%）
TypeA TypeB TypeC TypeD TypeE
TypeF D
A B
F E
C
Fig.4 Different gravel size distribution of debris flow models
TEST RESULTS AND CONSIDERATIONS Surge formation by front flow motion
The 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/ hb
Load ratio P*=Pmax/P0
Final judgement of surge formation Type Content hf（cm） hb（cm）
3 A standard 7.0 4.0 1.8 1.11 △
4 6.5 4.0 1.6 1.00 △
18 B Gravel 100%
sand 0%
5.5 4.0 1.4 0.79 ×
19 5.5 5.0 1.1 0.86 ×
14 C Gravel 70%
sand 30%
8.5 3.5 2.4 1.90 ○
15 7.5 3.5 2.1 1.57 ○
22 D Gravel 50%
sand 50%
4.0 3.5 1.1 0.86 ×
23 4.0 3.0 1.3 0.75 ×
16 E Gravel 30%
sand 70%
2.5 2.5 1.0 0.90 ×
17 4.5 3.5 1.3 0.62 ×
20 F Small gravel 70%
sand 30%
5.5 3.5 1.6 1.06 △
21 6.0 4.0 1.5 1.07 △
Note: 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 <P*< 1.5 ; quasi surge formation (2b) P*< 1.0 ; no surge formation (2c)
where,
P* = peak load / constant (stabilized) load.
-20 -10 0 10 20 30 40 50 60
0 1 2 3 4 5 6 7 8 9 1
Time (sec)
Load (N)
Type
0 A
(b) Front motion (a) Load-time relation
Fig.6 Type A ( standard type )
(The white circle in (b) is the light for the high speed video camera.)
Fig.7 Type B
(gravel 100%, sand 0% )
( The white circle in (b) is the light for the high speed video camera. )
Fig.8 Type C (gravel 70%, sand 30%)
( The white circle in (b) is the light for the high speed video camera. ) (a) Load-time relation
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0 1 2 3 4 5 6 7 8 9 1
Time (sec)
Load (N)
Type B
(b) Front motion
0
(a) Load-time relation
(b) Front motion
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0 1 2 3 4 5 6 7 8 9 1
Time (sec)
Load (N)
Type C
0
Fig.9 Type D ( gravel 50%, sand 50%)
( The white circle in (b) is the light for the high speed video camera. )
Fig.10 Type E ( gravel 30%, sand 70%)
( The white circle in (b) is the light for the high speed video camera. )
Fig.11 Type F ( small gravel 70%, sand 30%)
( The white circle in (b) is the light for the high speed video camera. ) (a) Load-time relation
-20 -10 0 10 20 30 40 50 60
0 1 2 3 4 5 6 7 8 9 1
Time (sec)
Load (N)
Type D
0 (b) Front motion
-20 -10 0 10 20 30 40 50 60
0 1 2 3 4 5 6 7 8 9 1
(a) Load-time relation (b) Front motion
Time (sec) 0
Load (N)
Type E
(b) Front motion (a) Load-time relation
-20 -10 0 10 20 30 40 50 60
0 1 2 3 4 5 6 7 8 9 10
Time (sec)
Type F
N)ad (Lo
Therefore, the load ratio are obtained as shown in Table 1 and it is confirmed that the complete surge formation is the same as Type C judged by the front flow motion.
-20 -10 0 10 20 30 40 50 60
0.0 2.0 4.0 6.0 8.0 10.0
Time(sec)
Load(N)
Type C (No.15)
Load ratio P*＝Peak load/ constant load Peak load
Constant load
Fig.12 Definition of load ratio (Type C(No.15))
Final Judgement for Surge Formation
The right column in Table 1 shows the final judgement for surge formation from both points of view of the flow depth ratio by Eq.(1) and the load ratio by Eq.(2). These values are caluculated by the test results of debris flow model specimens with six different garvel size distributions of Types A-F. Herein, symbols ○,△,× express the complete surge formation, the quasi surge formation and no surge formation, respectively.
Consequently, it was recognized that the debris flow model of Type C including gravel 70%
and sand 30% can form the complete surge phenomenon which shows the impulsive fluid load which shows the very steep rise time in the load-time relation and the large load ratio as shown in Fig.12. Furthermore, it is noted that Types A and F also show the quasi surge fromation.
Cause of Surge Formation
The cause of surge formation in the flow motions of Types A,C and F may be considered as follows. One of the reasons may be caused by the gravel size sorting phenomenon in which the large gravels flow toward the front and the sand flows back behind the gravels as shown in Fig. 13.
That is to say, the sorting phenomenon associated with the inverse grading phenomenon is explained as follows.
(1) The large gravels are transported to the front flow and dropped to the floor.
Bagnold (1968) proposed that this is based on the concept of dispersion pressure due to the impact between two gravel particles. This means that the dispersion pressure is proportional to the gravel diameter to the 2nd power and, as such, large gravels are moved to the direction of the minimum shearing velocity, i.e., the direction of the upper free surface.
Middleton (1970) opposed Bagnold’s remark that small gravels are dropped down to the floor and the large gravels go up to the upper surface by the dynamic sieving effect.
On the otherhand, Takahashi(1980) expalained that the large gravels are transported to the front flow as if the the soil attached to the bulldozer were jumped to the front direction rather than the moving velocity of the bulldozer itself, because the upper flow velocity is faster than the lower one in the flow depth direction.
(2) The friction between gravel and floor makes the flow velocity slow at the bottom and gives resistance to the flow motion. Therefore, the upper surface flow velocity becomes faster than the lower floor velocity in the flow depth direction.
(3) The large gravels are pushed up to the flow surface due to the interaction between gravel and sand in the debris flow motion. That is, small gravels and sand are transported to the back, because of slow velocity in the lower layer and, therefore, large gravels are conveyed to the upper surface (Takahashi,1980).
On the contrary, Type B with the single gravel size distribution (only gravels and no sand) could not produce the pushing up phenomenon such as the condition (3) mentioned above, because there is no interaction between gravel and sand. Furthermore, Types D and E showed the dispersion of gravels and could not make the aggregation because of the small quantities of gravels as shown in Figs.9(b) and 10(b).
Consequently, Types A, C and F were suited for the surge formation because of satisfying the conditions (1)-(3) within the range in this test.
(1)
(2)
(3)
Flow velocity
Fig.13 Sorting phenomenon of gravel and sand
CONCLUSIONS
The following conclusions are drawn from this study.
1. It was found that the surge shape was well formed in Type C which has the gravel size distribution ratio of gravel (16~19mm) of 70% and sand (less than 2mm) of 30% among six kinds of gravel size distribution.
2. The quasi surge formation were also made in Types A and F, because of the relatively good gravel size distributions. The reason may be due to the size sorting phenomenon.
3. It was recognized that surge shape was formed in the case where the flow depth ratio between front and following flow depths was larger than 2.0 and the load ratio between peak and constant loads was larger than 1.5.
4. The surge formation may be caused by the gravel size sorting phenomenon in which the large gravels are conveyed toward the front flow and the small sands are transported to the back behind the large gravels.
5. The surge formation was not made in Types B, D,E, because there was no sand in Type B and the small quanties of gravel percentage in Types D and E.
6. The detailed explanation will be made by simulating the gravel size sorting phenomenon using the particle method in the near future.
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