study on debris-flow deposition and erosion processes upstream of
TRANSCRIPT
STUDY ON DEBRIS-FLOW DEPOSITION AND EROSION
PROCESSES UPSTREAM OF A CHECK DAM
Badri Bhakta SHRESTHA1, Hajime NAKAGAWA
2, Kenji KAWAIKE
2 and Yasuyuki BABA
2
1Graduate Student, Department of Civil and Earth Resources Engineering, Kyoto University (615-8540, Kyoto)
2Disaster Prevention Research Institute, Kyoto University (612-8235, Kyoto)
Debris flows are common in mountainous areas throughout the world. It destroys houses, bridges and
claim people's lives. Sometimes it flows over even high ridge of mountain. A check dam is commonly
used for preventing the sediment disaster due to debris flow by storing the harmful sediment discharge.
Experimental works have been carried out to investigate the mechanism of debris flow deposition process
upstream of a check dam and transported of deposited sediment due to erosion process by a normal flow
discharge. The experiments are carried out on closed type and grid type check dams. From the results, it
is shown that the grid type check dam can keep their debris flow trapping capacity more effectively than
the closed type check dam.
Key Words : Debris flow, check dams, deposition, erosion, laboratory experiment
1.0 INTRODUCTION
Debris flows are common in mountainous areas throughout the world. Often triggered as mudflows by
torrential rains, debris flows contain varying amounts of mud, sand, gravel, boulders, and water. In addition
to causing significant morphological changes along riverbeds and mountain slopes, these flows are
frequently reported to have brought about extensive property damage and loss of life (Takahashi, 1991;
Huang and Garcia, 1997; Hunt, 1994). Fig. 1 shows the annual loss of life and property from the debris flow,
landslide and floods in Nepal.
0
200
400
600
800
1000
1200
1400
1600
19
83
19
84
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
20
01
20
02
20
03
20
04
20
05
Year
No
. o
f D
ea
ths
0
1000
2000
3000
4000
5000
6000
Lo
ss in m
illion
NR
s
Death
Loss of Properties
Fig.1 Annual loss of life and property from the debris flow, landslide and floods in Nepal.
Check dams are one of the effective structural measures for debris flow control. It can be classified as a
closed type and an open type check dam. In the context of Nepal, a closed type check dam is mostly used.
Debris flow stops by the closed-type check dam until the retention space behind the dam is full of sediment
and consequently could not have controlled debris flow successfully if they lose their storage capacity. It
becomes impossible for fish to move upstream and downstream freely or that not enough sediment is
supplied downstream. Accordingly, degradation of the riverbed and the retrogression of shoreline occur. In
recent years, a open type check dam has become popular. Their dams have the merits that the debris flow is
captured and the sediment discharge is small and medium floods pass through (Miyazawa et al., 2003).
The objective of this paper is to experimental study and to investigate the debris flow deposition and
erosion of deposited debris flow upstream of closed type and grid type check dams. The effectiveness of
check dam to control debris flow is presented.
2.0 LABORATORY EXPERIMENTS
The experimental works are carried out on a 5m long, 10cm wide and 13cm deep flume. The slope of
flume is set at 18 degrees. The details of experiment setup are shown in Fig. 2. Silica sand and gravel
mixtures sediment with 1.9m long and 7cm deep is positioned 2.8m upstream from the outlet of the flume by
installing a partition of 7cm in height to retain the sediment. Sediment materials with mean
diameter =md 2.53mm, maximum diameter =maxd 15mm, maximum sediment concentration at bed =*C 0.65,
angle of repose =φtan 0.7 and sediment density =σ 2.65 g/cm3 are used. The particle size distribution of
sediment mixture is shown in Fig. 3. Check dams are set at the 20cm upstream from end of the flume. The
experiment works are carried out using one closed dam of 8cm in high and two open type grid dams with
various spacing of grid. The details of the check dam types are shown in Fig. 4. Debris flow is produced by
supplying a constant water supply 260cm3/sec for 10sec from upstream end of the flume. Debris flow
deposition pattern upstream of check dams and erosion of deposited debris flow are captured by two standard
video cameras located at side and above the flume end.
Q0
Check dam
5.0m
7cm thick
0.2m
2.6m
1.9m
Saturated bed sediment
Video camera
Video camera
0
20
40
60
80
100
0.01 0.1 1 10 100
Diameter ( mm)
Per
cen
t F
iner
3.0 RESULTS AND DISCUSSIONS
(1) Deposition process upstream of a check dam Fig. 5 shows the experimental results of the debris flow deposition upstream of a closed type or a grid
type check dam. From the results, it is found that the closed dam traps all the sediments until the dam is filled
up with sediment, and their sediment storage capacities are decreased soon. In the case of grid dams harmless
sediment discharge is transported to downstream before the blockade of grid opening and keeps the sediment
storage capacities. The opening of a grid dam is blockaded by large sediment particles in debris flow. This
blockade phenomenon is influenced by the width of dam opening, the maximum particle diameter of
sediment, and the sediment concentration of debris flow. After blockade of grid dam opening, it is found that
the debris flow deposition pattern upstream of a grid dam similar as that type of a closed dam. However, the
Fig.2 Experimental laboratory flume setup Fig.3 Particle size distribution of bed sediment
Fig.4 Check dam types
100mm
80m
m
26mm
26m
m
30mm
30
mm
Closed dam Grid dam type-2 (GDT-2)
(2.5mm column/beam diameter)Grid dam type-1 (GDT-1)
(4mm column/beam diameter)
grid type check dam can store debris flow more
effectively than the closed dam.
(2) Erosion of deposited debris flow upstream
of a check dam. The large boulders deposited upstream of a
check dam can not be transported by a normal
scale flood flow. If we remove large boulders
deposited upstream of a grid dam or blockaded
large boulders at open spaces of grid, deposited
sediment upstream of a grid dam may be
transported by a normal scale of flood flow.
Hence, the experiments on flushing out of
deposited sediment upstream of a check dam due
Fig.5 Experimental results of debris flow deposition upstream of a check dam.
0
2
4
6
8
10
020
4060
80100
120
Distance (cm)
Dep
th (cm
)
Before flushing, GDT-1 After flushing, GDT-1
Before flushing, GDT-2 After flushing, GDT-2
Before flushing, Cosed dam After flushing, Closed dam
Fig.6 Experimental results of flushing out deposited sediment due
to erosion and variations in depth, CASE-I
02
4
68
10
010
2030
4050
Distance (cm)
Dep
th (cm
)
GDT-1 GDT-2 Closed dam
a. At 0.4sec
0
2
46
8
10
010
2030
4050
Distance (cm)
Dep
th (cm
)GDT-1 GDT-2 Closed dam
b. At 0.8sec
0
2
4
6
8
10
010
2030
4050
Distance (cm)
Dep
th (cm
)
GDT-1 GDT-2 Closed dam
c. At 1sec
0
2
4
6
8
10
010
2030
4050
60
Distance (cm)
Dep
th (cm
)
GDT-1 GDT-2 Closed dam
e. At 4sec
02
4
6
8
10
010
2030
4050
6070
Distance (cm)
Dep
th (cm
)
GDT-1 GDT-2 Closed dam
f. At 6sec
0
2
4
6
8
10
0
10
20
30
40
50
Distance (cm)
Dep
th (cm
)
d. At 2sec
GDT-1 GDT-2 Closed dam
θ=18o θ=18
o
θ=18o θ=18
o
θ=18o θ=18
o
θ=18o
to erosion process are carried out in two cases. In CASE-I: some large boulders deposited upstream of the
check dam are removed and supplying discharge at a rate of 260cm3/sec for 15sec. Fig. 6 shows the variation
in depth after flushing out of deposited sediment upstream of check dams due to erosion process by normal
flow. In CASE-II: firstly flow discharge at a rate of 260cm3/sec is supplied for 15sec, and after that some
deposited large boulders are removed, then again flow discharge at a rate of 260cm3/sec is supplied for
15sec. Fig. 7 shows the experimental results of flushing out of deposited sediment in the case II. The
deposited sediment could not be flushed out effectively by erosion of water supplying before removing large
boulders. It is found that deposited sediment upstream of grid dam can be flushed out more effectively by a
normal scale flood if we remove deposited large boulders.
4.0 CONSLUSIONS
Debris flow deposition and erosion upstream of a check dam are investigated through several laboratory
experiments. The deposited sediment upstream of grid dam can be flushed out due to erosion process by a
normal scale flood flow if deposited large boulders are removed. From the results, it is shown that the grid
type check dam can keep their debris flow trapping capacity more effectively than the closed type check
dam.
In the further study, the numerical model will be developed to investigate debris flow deposition and
erosion process upstream of a check dam.
REFERENCES Huang, X., and Garcia, M.H., A perturbation solution for Bingham-plastic mudflows, J. Hydr. Eng. ASCE, 123 (11) 986-994, 1997.
Hunt, B., Newtonian fluid mechanics treatment of debris flows and avalanches. J. Hydr., Eng. ASCE, 120(12) 1350-1363, 1994.
Miyazawa, N., Tanishima, T., Sunada, K., and Oishi, S., Debris-flow capturing effect of grid type steel-made sabo dam using 3D
distinct element method, Proc., 3rd Conf. on Debris-Flow Hazards Mit.: Mech., Pred., and Assessment, 527-538, 2000.
Takahashi, T., Debris flow, IAHR Monograph Series, Rotterdam: Balkema, 1991.
a. Experimental results of flushing out deposited
sediment before removing large boulders
b. Experimental results of flushing out deposited
sediment after removing large boulders
Fig. 7 Experimental results of flushing out deposited sediment upstream of check dam and variations in depth, CASE-II
0
2
4
6
8
10
020
4060
80100
120
Distance (cm)
Dep
th (cm
)
Before flushing, GDT-1 After flushing, GDT-1
Before flushing, GDT-2 After flushing, GDT-2
Before flushing, Cosed dam After flushing, Closed dam
θ=18o
0
2
4
6
8
10
0
20
40
60
80100
Distance (cm)
Dep
th (cm
)
Before flushing, GDT-1 After flushing, GDT-1
Before flushing, GDT-2 After flushing, GDT-2
Before flushing, Cosed dam After flushing, Closed dam
θ=18o