京都大学防災研究所年報 第 56 号 B 平成 25 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 56 B, 2013

Influence of a Series of Sabo Dams on Debris Flow Deposition
Namgyun Kim(1), Hajime NAKAGAWA, Kenji KAWAIKE and Hao ZHANG(1) Graduate School of Engineering, Kyoto University
Synopsis
Since ancient times, debris flow has been recorded. It is a kind of mixture flow ofwater and sediment. It causes damage of human’s lives and properties. Large boulders
in the front of surge and big destructive force are common for debris flow. To mitigate
debris flow disaster, various kinds of sabo dams have been constructed. Control
function of such sabo dams have been reported through a lot of studies. However there
are still few studies about combination of sabo dam. In this study, experimental andnumerical works have been performed for influencing on capture capacity of sabo dam
by upper located sabo dam. Results indicate that depending on the conditions ofarrangement of sabo dam can affect the storage capacity.
Keywords: debris flow, sabo dam, distance, deposition, potential storage volume1.

Introduction

Debris flows are found in mountainousenvironment, which consist of fully saturated
mixtures of water, sediment, debris. Debris flows
are widely recognized as one of the geomorphic
processes in steep mountainous area. (Takahashi etal., 1992; Hunger et al., 2001; Vandine et al., 2002)
Debris flows initiated high on the slope of hill andit increased in volume by entraining material fromthe channels. It flows down a slope undergravitational force. Furthermore, the mass of debris
are transported downstream and debris flow disasterspreads in residential area(Fig. 1). The effects ofdebris flows can cause morphological change,
serious casualties and damage to properties due tolarge boulders are in front of wave. Debris flow
disasters have been occurred in East Asia. Fig. 2
shows the number of occurrence of debris flow inJapan. The debris flows occur annually. Therefore,structural countermeasures have been designed topreserve humans and their properties areFig. 1 Stages of debris flow (Takahashi, 1992)
constructed in order to prevent debris flows.
Countermeasures can be classified as structuraland non-structural measures. In the structural
measures, sabo dams are one of the effective
structural countermeasures to control the debris
flow. Sabo dams, which are often constructed inseries, have been used in East Asia and Europe tomitigate the debris flow hazard. Fig. 3 shows aclosed-type sabo dam constructed to mitigate― 531 ―

Fig. 2 The
numberof debris
flow
occurrence inPuta figurein thisspaceJapan

numerical works have been performed on the size,
shape and structure of torrential Sabo dams
(Mizuyama et al., 1988; Johnson et al., 1989).
Takahashi et al. proposed a method to take into
account the effect of a closed-type sabo dam. Thesestudies have contributed to technical guidelines
related to debris flows. However, further studies areneeded to develop general guidelines of sabo dam.
Understanding the processes that debris flow
deposition upstream of a sabo dam is important forthe design Sabo dams.
This study is focused on the optimal distance ofeach closed-type sabo dam. It is conducted
experiments and simulation to propose optimal
distance of each closed-type sabo dam. The
fundamental one dimensional depth averaged modelis used. The deposition velocity equations proposedby Shrestha et al. is chosen for deposition process
upstream of a sabo dam.
2.

Laboratory experiments

The laboratory experiments were carried out
using rectangular flume of 4.7m long, 20cm high,Fig. 3 Closed-type sabo dam

Put a figure in this spacedebris flow in downstream area. Sabo dams aredesigned to control sediment volume, sediment sizeand hydrograph of debris flow. Such functions ofsabo dams are described in many experimental andcomputational studies(Honda et al., 1997; Imran, J.
et al., 2001). Also, considerable theoretical and10cm wide. The slopes of flume is set. The
details of the experimental setup are shown in fig.
4. A sediment bed of silica sand and gravel
mixtures sediment with 1.5m long and 10cm deep
was positioned 3m to 4.5m upstream measured
from the outlet of the flume. This laid sediment bed
is saturated by water as seepage flow. Sedimentmaterials with mean diameter,
maximum diameterand density
were used. Fig. 5 shows particle

Fig. 4 Experimental flume set up― 532 ―3.

Numerical model

3.1 Spacing between sabo dams

Fig. 5 Particle size distribution of sedimentmaterials

Table 1 Experimental condition
Supplywater
Experiment
Discharge
No.
𝑠Time(sec)Case.A-1

1030

Case.A-2

1055

1080

2030

Case.B-2

2055

Case.B-3

2080

Case.A-3
Case.B-1

300

Distance ofeach dam(cm)Fig. 6 Schematics of closed-type sabo dam

Put a figure in this space
As shown in Fig. 6, the spacing between sabodams depends on stream gradient, dam height,
angle of deposition of material behind the dam.
Chatwin et al. provides a formula for the spacing ofsabo dams:(1)

size distribution. Two closed-type sabo dams wereused for this study. One sabo dam was set at 450cm
from upstream end of flume. The other sabo dam
was set at 420cm, 395cm and 365cm from upstream
end of flume. Sabo dams of 6cm high, 1cm
thickness were used.
The experimental conditions are shown in Table
1. Debris flow was generated by supplying a
constant water dischargefor 10sec
from the upstream end of the flume. Debris flow
produced in the experiments is the fully stony typedebris flow and the largest particles wereaccumulated in the forefront. Using two standard
video cameras, debris flow deposition patterns
upstream of each sabo dam were captured. Also,
debris flow captured volume of each sabo dam wasmeasured.

where
is the height of dam,is original
channel gradient,is the equilibrium bed slope.
Minimum spacing between dams is determinedby captured volume of sabo dam. Since theevaluation of the control function of sabo dams isbased on the assumption that Ssbo dams are
initially filled with sediment up to crest level,
estimation of the sediment volume trapped by eachsabo dam is carried out based on the potential
storage volume ( ) of each dam, which can bedefined by equilibrium and initial bed deposition
slopes.
is described as follows.

{}

(2)

where,
height of dam,is the flow width,
is the initial bed slope of storage area before
occurring debris flow.

― 533 ―

Deposition velocity, if
3.2 Governing equations
Debris flow is described using one dimensional
depth averaged equations. The equations for themass conservation of water-sediment mixture (3)and only sediment (4) can be described as following
equations.

(3)

(8)

whereis erosion coefficient,is deposition
coefficient,is mean diameter of sediment,is the equilibrium sediment concentration describedas follows(Nakagawa et al.,2003):
If>0.138, a stony type debris flow
occurs, and

(9)

(4)
The equations for momentum conservation are
as follow;

If 0.03<flow occurs, and, an immature type debris{}

(10)

(5)
where(=is flow flux indirection,
ismean velocity,is flow depth, erosion
ordeposition
velocity,is the sedimentconcentration in the flow,
is maximum
sediment concentration in the bed,
is momentum
correction factor equal to 1.25 for a stony debris
flow,
is the acceleration due to gravity,isbottom shear stress,
is mixture density,
is density of thesediment particle,
is density of the water.
The equation of bed variation is described asfollows:

If, a turbulent water flow with bed
load transport occurs, and(Erosion velocity, if(7)

√ )
(11)

whereis water surface gradient,of the interstitial muddy fluid,friction angle of the sediment, and{is density
is internal}

(12)

(13)

(6)
whereis erosion or deposition thickness of thebed measured from the original bed surface
elevation,is bed erosion or deposition velocity.
The erosion and deposition velocity have been
given by Takahashi et al. are described as follows.

)(

(14)

wherestress andis the non-dimensional critical shear
is the non-dimensional shear stress.

3.3 Bottom shear stress equations
For a fully developed stony debris flow
constitutive equations that have given by Takahashi
et al. are described as follows.

― 534 ―

Fig. 7 Definition of arrangement of variables onmeshes

whereis experiment constant,is thecollisions angle of particle,
is the coordinate
perpendicular to the bed and positive upward in thenormal direction of flow,
is static pressure.
The bottom shear stress for a stony debris flow isderived by substituting the constitutive equations
into the momentum conservation equation under asteady and uniform flow conditions;(| |){}{⁄(17)}

An immature debris flow occurs whenis lessthan 0.4 and the bottom shear stress is describedas follows;

()

| |

(18)

In case of turbulent flow, the Manning’sequation is used to determine the bottom shear
stress. WhenFig. 8 Definition of the variables and flow
surface gradient at closed-type Sabo dam{⁄}

( )
(15)
(16)

is less than 0.02 as follows;| |where(19)

is Manning resistance coefficient.

3.4 conditions of closed-type sabo dam
The closed-type sabo dam is set at thecalculation point of flow discharge per unit width.
The effective flow depth,, at the dam point toTable 2 Debris flow captured volume(unit : cm3)
Lower located sabo dam

Upper located sabo dam

Total

ExpSimExpSimExp

Sim

Case.A-1

1357.81224.5

1978.32021.53336.1

3246

Case.A-2

1649.81352

1912.6

1997.53562.43349.5

Case.A-3

1635.21353.5

1868.82041

3504

3394.5

Case.B-1

1843.25

2010.52617.052383

4460.34393.5

Case.B-2

2744.82037

2752.12342

5496.94379

Case.B-3

2117

2123

2802.72334.54919.74457.5No.

― 535 ―(a) Case.A-1(b) Case.A-2(c) Case.A-3(d) Case.B-1(e) Case.B-2(f) Case.B-3

Fig.9 Final debris flow deposition upstream of sabo dams
calculate the outflow flux and the flow surface
gradient,, are described as follows;

described as{{(20){}(21)

The gradient,, needed to calculate theequilibrium sediment concentration,, is}(22)Fig. 7 and Fig. 8 shows the definition ofarrangement of variables with the finite difference
mesh.
4.

Results and discussions

The numerical simulations and experiments were― 536 ―captured volume at upper located sabo dam is
accounted for around 55% of the total captured
volume. Also, in simulated result, sedimentcaptured volume at upper located sabo dam is
accounted for around 56% of the total captured
volume.
In this study was evaluated the debris flow
captured volume by distance of each sabo dam. The
Fig. 10 Sediment concentration at downstream
end of flume
performed to investigate capture capacity of sabo
dam according to distance of between dam.
Fig. 9 shows the simulated results andexperimental results of debris flow deposition
upstream of a series of sabo dams. From the figure,simulation results are quite agreement with theexperimental results. Because a decrease in distanceof between dams is able to increase equilibrium bed
slope by upper located sabo dam. However, somediscrepancies are found. The equilibrium bed slopeof lower located sabo dam is changed by upper
located sabo dam. In such case(Case A-1, Case B-1,Case B-2), experimental results shows that upper
located sabo dam is almost buried by sediment. Incontrast, simulated result shows that sedimentdeposited form is oscillating. When the debris flow
overflows the sabo dam, sediment concentration isdecreasing (Fig. 10). This may change thedeposition process of lower located sabo dam.
Sediment concentration is keep going around 40%
without sabo dam. If sabo dam is installed in thechannel, sediment concentration is decreasing at
forefront of debris flow.
The sediment volume is captured by a series ofsabo dams are presented in Table 2. This table showthat distance of each dams change, the equilibrium
bed slope,, is changed. Because debris flow
deposition form upstream of lower located sabo
dam is changed by upper located saboam. Case.A-1, Case.B-1, Case.B-2 shows that thechanged equilibrium bed slope.
Total debris flow captured volume is the sum ofcaptured volume at lower located sabo dam andupper located sabo dam. All of case, upper locatedsabo dam captured more sediment than lower
located sabo dam. In experimental result, sedimentequilibrium bed slope,is changed by distanceof each sabo dam. By considering the depositing
processes on lower located sabo dam, more clearlycalculated result could be found .
5.

Conclusions

This study has shown that debris flow
capture capacity by arrangement of sabo dam.
As a result, it is shows that a decrease indistance of between dams will increase
equilibrium bed slope of lower located sabo
dam due to upper located sabo dam. Second,
Upper located sabo dam could capture more
sediment than lower located sabo dam.
Through considering the combination of sabodams, it is expected to seek furtheradvancements in technical criteria andguidelines. However, it is necessary to modify
the mathematical model about changes ofoverflowed debris flow characteristics.
Acknowledgements
This research is supported by JSPS AA Science
Platform Program(Coordinataor: H. Nakagawa) andsupport of Wakate fund program by Kyoto
University Global COE Program(GCOE-ARS).
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(Received June 10, 2013)