E3S Web of Conferences 415, 02013 (2023)
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https://doi.org/10.1051/e3sconf/202341502013

Influence of approach shape of debris flow on impact load
subjected to open Sabo dam under an overturning experimentof open Sabo dam
Yuta Miyahara1*, Toshiyuki Horiguchi2, and Yoshiharu Komatsu1
1Civil and Environmental Engineering, National Defence Academy, Kanagawa, Japan
2Associate Professor, Civil and Environmental Engineering, National Defence Academy, Kanagawa, Japan

Abstract. Recently high intensity rainfall disaster induces some structural failure of steel pipe open Sabodam. To prevent such failure of the dam, a new design concept is necessary at the severe load exceeding thepresent design load. However, influence of debris flow impact load on steel pipe open Sabo dam is not clear
to make a design load. The study experimentally approaches a vertical load distribution shape and its time
history based on comparison with a wedge approach shape or a front boulder concentrated shape. The load
and moment action of a front boulder concentrated shape debris flow is greater than that of a wedge approach
shape. Mechanism making the difference is made clear by observation of vertical detail measurement ofimpact load.

Gate

1 Introduction
The magnitude of debris flow caused by torrential rains
and typhoons is increasing in Japan. Debris flow caused
damage over a wide region in a short period of time.
Debris flows are highly destructive to houses and public
property. Sabo dams are required as a countermeasure
structure. In recent years, open Sabo dam have been
constructed
mainlyduetoenvironmentalconsiderations. The structural stability calculations arebased on the design criteria for closed Sabo dams [1-3].
However, the design load originally acting on an open
Sabo dam is different load. It is known that theapproaching shape of a debris flow depends on the slopeand flow rate. The debris flow loads acting on open Sabodams differ due to the effect of the approach shape.
Mizuyama et al. [4] evaluated debris flow loads
based on two patterns of debris flow shapes from theflow rates with dimensionless channel gradients.
However, the loads acting on open Sabo dam withremain unexamined. Miyoshi et al. [5] showed from therelationship between maximum load and flow velocity
that the shape of dynamic load waveform can beclassified according to flow velocity. In particular, it
was shown that approaching waves with high velocities
produce loads with clear peaks in a short period of time.
However, the velocity of the debris flow changes theshape of the approach, and the action load changes withthe velocity. On the other hand, the effect of theapproach shape of the debris flow together with thevelocity of the flow on the applied load remainsunexplored. The distribution with respect to the height
direction used in design should be considered.
This study examines the effect of different debris

500

Sabo dam modelMoving bed

GravelFlow channel
600

θ = 15º, 12ºLine pump

Depth300

Lift㸦Unit : mm㸧

Fig. 1. Experimental channel flume

flow approach shapes on debris flow impact loads. The
dynamic difference of the impact load of the debris flow
is studied under the condition of constant flow velocity.

2 Outline of experiment
2.1 Channel flume
Figure 1 shows the channel used in the experiment. Thechannel is 4.5 m length, 30 cm width, and 50 cm depth.
The slope of the channel is adjustable from 0° to 20°.
2.2 Sabo dam model
Figure 2 shows an overview of Sabo dam model. The
Sabo dam model was designed with a height of 26 cm
due to limitation of the experimental channel. This isapproximately 1/40 of the real height of 10 m by theFroude similarity rule.

* Corresponding author:em61040@nda.ac.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 02013 (2023)
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Figure 3 shows the load measurement device. The
Sabo dam model consists of a main unit and a load
measurement section. The mass of the weight at the topof the main unit can be varied. The moment of resistancefor overturning can be adjusted as follows.
Mr = WB㺃 lB + WW㺃 lW = mB㺃g㺃lB + mW㺃g㺃lW

WeightMain unit Measure part(1)

260 mmWhere, Mr : moment of resistance, WB, mB : weight andmass of the main unit, WW, mw : weight and mass ofdeadweight, lB, lW : horizontal distance between WB, WWand the hinge, respectively, g : acceleration of gravity.
The main unit has an effective height of 260 mm anddepth of 290 mm. The weight of the main unit is 48 N.
It can be tipped over by connecting the rear end of thebottom plate to the bottom surface with a hinge. The
distance between the hinge and the upstream face of themain unit is 245 mm. A total of 12 load cells (LMB-A500 N-P) are installed on the upstream face of the load
measuring section. The height of the first, second, third,fourth, fifth, and sixth stages are 5 cm, 8 cm, 13 cm, 17cm, 21 cm, and 25 cm, respectively. It has an opening in
the crosswise direction. Without the pressure receiving
beams, both water flow and gravel can pass freely.
Slider receiving plates (plastic plates) are attached toboth sides of the body to catch the force of the slider.
Figure 4 shows the movement of the slider and thecontact plate. Each stage of sliders can moveindependently in the horizontal direction. There is aroller at the downstream end, and when this roller
contacts the slider receiving plate, the slider is pushedfrom the upstream side through the pressure receiving
beam. When an action moment greater is generated than
the resistance moment, the main unit rotates around thehinge, the slider support plate rises upward, and theslider moves downstream. The rollers are used to reduce
friction between the slider and the receiving plate.

hinge

245 mm
335 mmFig. 2. Experimental dam modellW

WeightWW

Load measure

Main unitSlider guaiddisplacement meter
Rotation

Main unit
lBSliderLoad cell
WB

Hinge

Fig. 3. Load measure unit

2.3 Debris flow model
Figure 5 shows the initial conditions for generating afront boulder concentrated shape debris flow. The
classifying phenomenon of the debris flow has occurred,and surge has been formed. The bed slope was set to θ =15°, which is the section where the debris flows
downstream. Referring to the Matsumura et al. [6],bottom roughness of 5.0 mm height and 10 mm width
were placed on the bottom at intervals of 30 mm for alength of 50 cm upstream from the Sabo dam model. Aportion of the gravel mass was spread over 2.0 m
upstream from the top of the bottom roughness to athickness of 3.0 cm to form a moving bed. About 40 kgof the gravel mass was placed in a trapezoidal shape 15
cm thick for 1.0 m upstream, and water flow was applied
from the rear.
Figure 6 shows the initial conditions for generating
a wedge approach shape debris flow. The bed slope wasset to θ = 13°. The acrylic bottom was placed 50 cm
upstream from the Sabo dam model, and the bottomroughness was placed 50 cm upstream of the acrylicbottom. The initial placement of the gravel mass wasmade by spreading 45 kg of gravel from the top of thebottom roughness to a thickness of 3.0 cm over 1.5 m

Fig. 4. Mechanism of slider and contact plate

Fig. 5. Channel condition of front boulder concentrated
shapeFig. 6. Channel condition of wedge approach shape

upstream of the top of the bottom roughness. A
trapezoidal-shaped gravel mass was placed upstream ofit, and water flow was applied from behind it. The water2E3S Web of Conferences 415, 02013 (2023)
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fifth stage, and 0.98 s for the sixth stage. Comparing thetime difference between the onset of the maximum loadat the first stage and the onset of the maximum load at

㸸front boulder concentrated shape㸦W = 8kg㸧㸸wedge approach shape㸦W = 5kg㸧120
100120㸸1st step㸸2nd step㸸3rd step㸸4th step㸸5th step㸸6th stepab cd e f

80

60

100
Load P(N)

240
220
200
180
160
140
120
100
80
60
40
200Load P(N)

Total load Pt (N)flow was provided by a dam-break system in which 20
L of water was stored at a front depth of 30 cm and thenreleased all at once.

40
200102030
40Time t(s)50

Fig..7. Relationship between totalload and time60

a b c def

0.5

1Time t(s)8060

㸸1st step㸸2nd step㸸3rd step㸸4th step㸸5th step㸸6th step

40
20000.5

1Time t(s)1.5

2Fig. 8. Load in each stage of front boulder
concentrated shape (W=8kg)

001.5

2Fig. 9. Load in each stage of wedge
approach shape (W=5kg)

the sixth stage, the time delay in the height direction isremarkable for the wedge approach shape, 0.74 scompared to 0.39 s for the front boulder concentrated
shape. The maximum load for each stage is 95 N for thefirst stage, 34 N for the second stage, 33 N for the thirdstage, 27 N for the fourth stage, 31 N for the fifth stage,and 22 N for the sixth stage. The first stage issignificantly larger than the sand and gravel load, but theimpact load above the second stage is not as large as thatof the front boulder concentrated shape.

3 Experimental results
3.1 Total load comparison
Figure 7 shows the relationship between total load andtime for each experiment near the limit of overturningfor each approach shape. The total load is the sum of themeasured loads at each stage. The maximum total load
(Pt = 227 N) for the front boulder concentrated shapereached 0.26 s after the tip of the debris flow impacted.
The wedge approach shape total load (W = 5 kg) reaches
its maximum total load (Pt = 155 N) when the time is0.65 s . The maximum total load and sediment load are
46 % and 54 % higher, respectively, for the front boulder
concentrated type. Therefore, the impact effect on themaximum total load is greater when the gravel isconcentrated at the head.

3.3 Total moment comparison
Figure 10 shows the relationship between total momentof action and time. The total moment of action is the sumof the moments of action at each stage, obtained bymultiplying the load measured at each stage by thedistance from the hinge. The maximum total actingmoment (Mac = 30.9 N 㺃 m) for the front boulder
concentrated (W = 8 kg) is reached 0.40 s after theimpact of the debris flow. In the wedge approach shape(W = 5 kg) of the relationship between total action
moment and time, the maximum total action moment(Mac = 22.2 N 㺃 m) is reached 0.72 s after impact.
Comparing between the total reaction moment of thefront boulder concentrated shape and the wedge
approach shape, the maximum total reaction moment is39 % greater for the front boulder concentrated shape.
In other words, the front boulder concentrated shape hasa greater impact effect on the moment of action as wellas the impact load.

3.2 Comparison of load to time relationship foreach stage
Figure 8 shows the relationship between load in eachstage and time of the front boulder concentrated shape.
The maximum load on each stage increases rapidly upon
impact of the flow. Its impact occurs with a time lag ofapproximately 0.04 s at each stage. On the other hand,
the maximum load generation time is 0.14 s for the firststep, 0.17 s for the second step, 0.30 s for the third step,
0.31 s for the fourth step, 0.43 s for the fifth step, and0.53 s for the sixth step.
The maximum load on each step is 113 N for the firststep, 93 N for the second step, 61 N for the third step, 57
N for the fourth step, 54 N for the fifth step, and 39 N
for the sixth step. The values decrease with the height of
the impact point. This is thought to be due to theinfluence of the preceding stop gravel, which causes thefollowing gravel to lose velocity due to repeated minor
collisions with the preceding stop gravel [7-8].
Figure 9 shows the relationship between load in eachstage and time of the wedge approach shape. Its impacts
occurred with a time lag of approximately 0.06 s foreach stage. The maximum load generation time is 0.24 s
for the first stage, 0.35 s for the second stage, 0.49 s forthe third stage, 0.61 s for the fourth stage, 0.68 s for the3.4 Moment to time relationship for each stage
Figure 11 shows the relationship between moment ofaction in each stage and time of the front boulder
concentrated shape (W= 8 kg). The maximum total
moment of action is almost the same as the time of themaximum moment under the fifth stage shown in Figure
9, indicating that the dominant factor is the upper impact
load. The maximum moment for each stage is 5.7 N㺃mfor the first stage, 7.7 N㺃m for the second stage, 7.6 N㺃
m for the third stage, 9.7 N㺃m for the fourth stage, 11.63

E3S Web of Conferences 415, 02013 (2023)
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3) The impact load of a debris flow decreases graduallywith height. This is because the velocity reduction
effect due to fine collision friction with the gravelN㺃m for the fifth stage, and 9.9 N㺃m for the sixth stage.
The following is an example of the use of a single unitof the same size.
35
㸸front boulder concentrated shape㸦W = 8kg㸧㸸wedge approach shape㸦W = 5kg㸧Total moment Mac(N࣭m)

30
2520
15
10500

10203040Time t(s)50Fig. 10. Relationship between
total moment of action and time

60

Fig. 11. Moment of action in each stage offront boulder concentrated shpe (W=8kg)

Fig. 12. Moment of action in each stageof wedge approach shape (W=5kg)

mass deposited after the preceding collision has agreater effect on the wedge approach shape type.
4) The front boulder concentrated shape has a higher
impact load on the Sabo dam than the wedge
approach shape. However, quantitative differencesin their effects need to be investigated throughstructural analysis.

Figure 12 shows the relationship between momentof action in each stage and time of the wedge approach
shape (W = 5 kg). The maximum total moment is almost
coincident with the time in the moment of the sixth stage
shown in Figure 11. The maximum moment of action
for each stage is 4.8 N㺃m for the first stage, 3.1 N㺃m forthe second stage, 4.4 N㺃m for the third stage, 4.6 N㺃mfor the fourth stage, 6.9 N㺃m for the fifth stage, and 5.7N㺃m for the sixth stage. In the wedge approach shapetype, the load in Figure 9 was maximum at the firststage, but the moment of action in Figure 12 wasmaximum at the fifth stage, and it increased inproportion to the height direction. Therefore, the heightof the impact point has a significant effect on the total
moment of action for both the front boulder
concentrated shape and the wedge approach shape.

References
1.

2.

3.

4 Conclusion
This study examines the influences of the shape of theapproaching debris flow on the impact load acting on
open Sabo dam. The results obtained can be summarized
as follows.
1) The maximum total load of the wedge approach shape
is about 70 % of that of the front boulderconcentrated shape. the maximum moment of action
is relatively small, about 80 % of that of the front
boulder concentrated shape. After the maximumload, the load converges to a static gravel load, which
is about 45 % of the maximum load.
2) The maximum total load of the wedge approach shape
is about 70 % of that of the front boulder
concentrated shape, and the maximum moment ofaction is about 80 % of that of the front boulder
concentrated shape, which is relatively small. Afterthe maximum load, the load converges to a static
gravel load, which is about 45 % of the maximumload.

4.

5.

6.
7.

8.

4

JSCE Structural Engineering Committee, Toward
the Preparation of Guidelines for Civil Engineering
Structural Loads - Framework and Guidelines(2000)
J. Daido, Impact Force of Debris Flow onStructures, Proceedings of the 1985 Erosion
Control Society of Japan, pp. 104-107 (1985)
SABO & LANDSLIDE TECHNICAL CENTER,
2009 Edition Steel Erosion Control Structures
Design Handbook (2010)
T. Mizuyama, H. Shimoto, H. Nakanish, K.
Matsumura, Journal of Japan Society for Erosion
Control, 37, 30 -34 (1985)
I. Miyoshi, M. Suzuki, Journal of Japan Society forErosion Control (New Sediment Control), 43, 11 19 (1990)
K. Matsumura, T. Horiguchi, S. Katsuki, Journal ofStructural Engineering, 66, 928 -940 (2020)
K. Komatsu, T. Horiguchi, T. Kozuki, N. Ishikawa,and T. Mizuyama, Journal of StructuralEngineering, 64, pp.779 -788 (2008)
O. Shimakawa, T. Horiguchi, Y. Komatsu, S.
Katsuki, Journal of Japan Society of Civil
Engineers, Ser. A2 (Applied Mechanics(AM)), 75,2, I_175-I_185