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

Study on Characteristic Analysis of Closed-type Sabo Dam with a Flap due toDynamic Force of Debris Flow
Yeonjoong Kim(1), Hajime NAKAGAWA, Kenji KAWAIKE, and Hao ZHANG(1) Graduate School of Engineering, Kyoto University
Synopsis
In recent years, many research studies have examined efficient function of sabodams, which have a great impact on ecology and landscape. In that research, attention to
the aspect of analysis of impact force associated with debris flow is still lacking. The
front part of the flow is very important and complex in the case of debris flow where
there is an accumulation of large boulders. It is important to control or dampen theenergy of the frontal part of a debris flow for the safety of the downstream area because
the impact pressure of debris flow is much greater than that of clear fluid. Therefore analternative design and its resistance against debris flow impact force deserve
investigation. The objectives of this study are to analyze, firstly, the function of the
proposed closed-type dam with a flap, then to compare the vertical pressure distribution
and total pressure with other types of available dams and finally, to determine theempirical coefficients of the hydrodynamic and solid collision models. This comparison
demonstrates the future importance of the proposed sabo dam. The results from theexperimental data clearly show that the proposed dam type has the ability to capture
more sediment sustaining less force than the without flap dams under the same debris
flow. Furthermore, the empirical coefficients of hydrodynamic and solid collisionmodels were proposed and compared with available coefficients.
Keywords: debris flow, closed-type flap dam, impact force, total pressure

1.

Introduction

Debris flows are common in mountainous areas
throughout the world, which contain various amountsof mud, sand, gravel, boulders, and water. They occur
when water mobilizes large volumes of loose mud,rock, and other debris. It is generally accepted thatdebris flow disaster occurs in high mountainous areas
far away from modern cities. Recently, this disaster
simultaneously occurs in multiple locations between
the high mountainous areas and the low mountainous
areas in urban areas (Kim et al. 2013).
The debris flow disasters that occurred at Mt.
Umeyon in downtown Seoul, Korea 2011 are shownin Photo 1 (a), photo (b) shows extensive damage tothat the apartment in Caraballeda due to a passage ofa debris flow front of at least 3.5m in height (Larsen
et al. 2001). These disasters illustrate the destructive
power of the high-velocity fluid as well as debris
flow and flash floods on alluvial fans inundating
coastal communities, causing severe property
destruction, and resulting in a death toll estimated at
19,000 people. The debris flow disaster leads tomassive property damage and casualty of life.
Therefore, to estimate the disastrous effects, it isnecessary to accurately understand, evaluate, quickly
respond to the risks, and mitigate damage by
establishing an effective measure.

― 503 ―

(a) Mt. Umeyon (photo courtesy: PRESSian)
Photo 1 Debris flow disasterIn recent years, many researchers haveexperimented with the efficiency of sabo dams
having a great impact on ecology and landscape.
Such dams are also surcharged with the impulsive
forces of the debris flow. The front part of the flow isimportant and complex in debris flow where big
boulders accumulate. It is important to control ordampen the energy of the front part of a debris flow
for the safety of the downstream area because theimpact pressure of debris flow is greater than that ofclear fluid. Fig. 1 (a) shows typical features of adebris flow longitudinal section (Pierson, 1986) and
Fig. 1 (b) shows the various functions of a sabo dam(MLIT, Japan. 2011). Therefore, we have to consider
both characteristics of debris flow and sabo dam with
an effectively counterplan for mitigate the debris
flow disaster.
Much research is being carried out to improve thefunction of sabo dams as well as to clarify the impact
force of debris flow based on field observations,(b) Caraballeda

laboratory experiments (i.e. large and small scale),and numerical simulations. A few field experiments
have been conducted such as Okuda et al. (1978),Suwa and Okuda (1983), and Hu et al. (2011). In
addition to these Hu et al. (2011) reported an in-situ
test of debris flow impact at Jiangjia Ravine, andintroduced a simple approach to separate twocomponents of the impact force into fluid pressureand grain-impact loading. In those studies,
preliminary analyses were made to determine therelationships of mean velocity versus hydrodynamic
pressure, and hydrodynamic pressure versus grain
impact loading from the measured data due to thesubtracting the fluid pressure from the total impact
loading.
Many small-scale laboratory experiments havebeen performed in order to develop theoreticalmodels for the calculation of impact force. Mizuyama(1979) separated the impact force of debris flow into
the fluid force and impact force of boulder.

(a) Debris flow surge(b) Various functions of a sabo dam
Fig. 1 Sketch of the characteristics of debris flow and functions of a sabo dam― 504 ―

(a) Without flap (i.e. typical type)(b) With flap-R
Fig. 2 Sketch of closed-type dams
According to the paper, the impact force on a sabodam (i.e. without flap) is described with hydraulictheories and the theory of complete sphere elasticity
was assumed so that the impulsive force of theboulders could be derived. Scheidl et al. (2012)
analyzes the impact forces of granular and viscousdebris flow and discusses the observations of single,
short time impacts of large particles, significantly
exceeding the peak pressure values. However, there
have been very few studies that discuss how debris
flow is influenced by the shape of a sabo dam. Shiehet al. (2008) designed a new form of sabo dam bychanging the geometric shape of the upstream dam
surface to reduce the impact force of the debris flow,
with enhanced stability and reduced concrete mass
being the anticipated outcomes. Their study showedthat the curved dam experiences less impact force
than other dams under the same debris flow condition,
demonstrating the importance of curved geometry fora well-designed sabo dam. Huang et al. (2007)
applied the theory of elastic collision to devise aboulder collision impact model with four types ofdams. Recently, Shibuya et al. (2012) presented theload of debris flow with woody debris for an open
type steel frame check dam structure. Likewise,
understanding the behavior and mechanism of debris
flow and the study of preventive measures are very
important in order to manage the sediment disaster inthe river basin and prevent downstream hazards.
Preventive measures require the consideration ofvarious plans and involve the evaluation ofhydrological, hydraulic, grain size distribution,
topographical and other parameters.
The objectives of this study are to analyse theworking principle of proposed closed type dams (i.e.
in Fig. 2 (b) and (c)) with a flap over typical typedam (i.e. dam without flap as shown in Fig. 2 (a)), to
propose the most suitable flap shape (i.e. rectangular(c) With flap-T

or triangular shape) based on apparent characteristics,
and to determine the empirical coefficients of thehydrodynamic and solid collision model.
Experiments were conducted to investigate totalpressure (combination of impact due to the collision,
static and dynamic pressures) of both flows under theconditions of closed-type dam without flap and thatwith a flap. In the experiments, total pressure
associated with major debris flows was recorded inreal time by a system consisting of four dynamic
pressure sensors (i.e. strain gages) installed at thedam. As stated above, the applied force of debris
flow is usually determined by field observations,laboratory experiments and numerical simulations.
But it is very difficult to estimate the applied force
due to the impact collision because the debris flow iscomposed of many different sizes and fractions of
sediment, which makes it difficult to estimate theactual contact area. So, the average value of the totalpressure by the maximum value of the impact
collision is determined experimentally by conducting
several experiments under the same conditions.
In all the experiments, the parameters such as theflow pattern, the surface velocity between the debris
flow and clear water, the total pressure, mass ratio ofdebris, median grain diameter of debris flow, and theuplift pressure were measured to compare thefunction of each check dam under the two different
bed sediments. The empirical coefficients of thehydrodynamic and solid collision models were also
determined and compared with the available valuesof those coefficients. Furthermore, observations of
the load behavior of debris flow, the velocity, theaverage of maximum total pressures, the upliftpressure due to the impact collision, the vertical
distribution of total pressure on the dams and theability of the proposed dam to sustain such forces, arealso discussed.

― 505 ―

2.

non-empirical relations can be found for the
calculation of the dynamic thrust.

Debris flow impact model

Several models have been developed to estimate
the impact force of debris flow against barriers.
However, it is difficult to decide the impact force dueto the diversity of substances composing the debris
flow (i.e. water, mixtures of granular and fineparticles in water and boulders) and the conditions ofdam (e.g. flexibility and properties). The impact force
of debris flow has been mainly described byhydraulic and solid collision models. The hydraulicmodels are further separated into hydrostatic andhydrodynamic models. Based on observations andtheoretical consideration, different models have beendeveloped for estimation of the debris flow impact
force.
2.1 Hydrostatic model
The hydrostatic models by Lichtenhahn (1973)
and Armanini (1997) are useful because they require
only the debris flow height, and the height of thestructures is often taken as debris flow height forchecking dam design purposes. In general, thehydrostatic formula can be written as:

Pmax  k p  d gh

(1)where Pmax = the maximum debris flow impact
pressure, kp = the empirical factor, ρd = the density ofdebris flow, g = the acceleration of gravity, and h =the depth of debris flow. The maximum impact
pressure is not related to statistical considerations, but
to the maximum pressure value in the load
distribution on the structure (Hübl et al. 2009).
Lichtenhahn (1973) proposed kp values between 2.8
and 4.4. Armanini (1997) found a maximum static
debris flow impact pressure exceeding roughly 5
times the hydrostatic pressure. Scotton and Deganutti(1997) measured the impact on an obstacle andproposed kp values between 2.5 and 7.5 from thelaboratory experiment.
2.2 Hydrodynamic model
The hydrodynamic formulas are based on theimpulse theorem. The phenomenon of debris flow
impact against an obstacle has been analysed inscientific
literature;many
empirical
andPmax  k p  d v 2(2)

Fmax  k f  d Av 2

(3)

where Pmax = the maximum debris flow impact
pressure, kp and kf = the empirical factors, ρd = thedensity of debris flow, v = the velocity of debris flow,
Fmax = the modulus of the impacting force, and A =the area of the section involved in the phenomenon
and whose height should be considered as the heightof the debris flow front. The empirical factor value
depends on the flow type. For laminar flow and finegrained material, Zhang (1993) recommends thepressure of empirical values between 3.0 and 5.0
based on field measurements at the Jiangjia Gully
station. Bugnion et al. (2011) proposed that thepressure of empirical coefficient kp approximately inthe range between 0.4 and 0.8 appear to beappropriate for objects with size of the same order ofmagnitude as the flow heights. Canelli et al. (2012)
estimated the force of empirical coefficient between
1.5 and 5.0 by a laboratory experiment with a small
scale channel. Watanabe and Ikeya (1981) proposedthat the force of empirical value changes with flow
material; for clear water, kf has been found to be
between 1 and 2, and for bentonite, kf = 2.0 as well as
value of 1.5 by Hungr et al. (1984). Besides, when
designing a sabo dam in Japan, it is usually doneusing Eq. (3) (Yamamoto et al. 1998).
2.3 Solid collision model
Conventional contact mechanics is mainly
concerned with static contact although it has been
extended to approximate solutions when impact isinvolved. For spheroidal surfaces, Hertz theory isused to obtain the force deformation relation needed
to calculate the duration of impact and the maximum
indentation. According to Jackson and Do (1969), forthe case of impact between two spheres of mass m1,
and m2, investigations show that the duration of
impact, i.e., the time during which the spheres remain
in contact, is very long in comparison with the periodof lowest mode of vibration of the spheres. Vibrations
can therefore be neglected, and it can be assumed that

― 506 ―

the force-displacement relation established for static
conditions holds during impact. The compressive
force is basically a power law written as:

16 R1 R2

k1 

9  k1  k2   R1  R2 
2

2

2
2

 5(v1  v2 ) 2  4  

 

2/5

,  

3.

Laboratory experiment

(4)

1
1
, k2 
 E1
 E2
2
1

(5)

where kf = the control empirical factor, v2 = theapproaching velocity of debris flow, and Rboulder = theradius of the boulder.

F  n 3/ 2

2
Fmax  k f 48.2v1.2
2 Rboulder

m 1  m2
m1m2

where F = the compressive force (i.e. the force thatacts during the period of impact between the spheres),υ1 and υ2 = Poisson’s ratio, E1 and E2 = Young’s
moduli, v1 and v2 are the velocities of the colliding
particles, R1 and R2 = the radii of spherical surfacesof the two bodies at the point of contact, and m1 andm2 = the mass of sphere. Mizuyama (1979) calculated
the impact load between sabo structure (relates
parameters with subscript 1 in above equation) anddebris flow (relates parameters with subscript 2 inabove equation) based on different assumed values ofparameters used in equation 4. Mizuyama assumed
the Young’s modulus of material forming the sabo
structure E1 = 5×108 kg/m2 (as of concrete) and thematerials forming the debris flow E2 = 2×109 kg/m2(as of stone), Poisson’s ratios of sabo structurematerial υ1 =1/6 (as of concrete) and material ofdebris flow υ2 =1/5 (as of stone), and v1 is equal tozero (since sabo structure is stationary) so that v1 + v2
is taken as equal to v2 (the approaching velocity ofdebris flow). The impact load is rewritten using theproperties of concrete (i.e. sabo dam) and large
boulder as:

To clarify the characteristics of debris flow incomparison with clear water as well as to compare
the functions of the proposed dams with typical dam(i.e. without flap), experiments were conducted in a
flume located at the Ujigawa Open Laboratory(UOL) of the Disaster Prevention Research Institute(DPRI), Kyoto University, Kyoto, Japan.
3.1 Working principle of sensor
The dynamic pressure measurement system is
produced by a Japanese company, Kyowa. The
measuring system was composed of four dynamic
pressure sensors (diaphragm type), sensor Interface(PCD-300B), and four channels of Adaptor andNote-PC by the dynamic data acquisition software(DCS-100A) as shown in Fig. 3. These sensor
transducers have a bridge of strain gages inside,achieving ultra-thin compact structure. Kyowa strain
gages are available for the measurement of various
types of strain, from static to dynamic strain andimpact-initiated strain. Strain gages are used not only
for stress measurement but also as sensing elementsfor various transducers owing to their excellent
repeatability and linearity. Dynamic strain is a strain
whose magnitude changes as time passes or which is
initiated by vibration or impact. Since ever-changing
strain cannot be read out on analog and digitalindicators, a data recorder was used to obtain theFig. 3 Dynamic pressure sensor and interface with four channels (unit: mm)― 507 ―

detected data. Table 1 shows the specification of thesensors. Pressure sensors are measuring devices that
produce an output signal proportional to the applied
dynamic pressure and the total pressure is calculatedby the following relation as proposed by the company,
Kyowa.

 Strain ampifier ' s output ,   10 
6Pressure(Pa)  Rated output on labe, mV/V  (6)Capacity (Pa) 2000  10 / mV/V 

Fig. 4 Experimental set-up for dam-break upstreamof the proposed dam

Compression pressure values are used to measure
the positive force along a single axis. The
information from the sensor monitor is thentransferred to a recorded or other computerized datacollection system. The sampling frequency (fscan)
adopted in the experiments is 500 Hz based on
Nyquist-Shannon sampling theorem as shown in Eq.
(7), and is able to show the dynamic variation withinevery 0.002 s.

where vavg = the average velocity of debris flow anddmax = the maximum grain diameter.

debris flow were tested. To clarify the characteristicof debris flow, hydraulic model tests with clear water
were performed for comparing the flow profile andtotal pressure in a rectangular flume that was 100 cm
long, 15 cm wide, and 40 cm high. At a distance of
60 cm from its downstream end, a vertical gate wasinstalled as shown in Fig. 4. The walls and bottom are
made of transparent smooth acrylic plank, whichallows for lateral observation. At the dam section, theflume is equipped with a smooth gate that can open
by hand. The flow was filmed with a digital video
camera (Sony: HDR-CX560) placed on the side of
the channel. The efficiency of the two types of dam
with plan-1 (without and with flap-R structure) is
discussed comparing the total pressure and flow
profile near the dam.

3.2 Experimental setup
3.2.1 Experiment for hydraulic characteristic
The experiments confirmed the characteristic ofdebris flow and observed the energy dissipation
phenomenon near the dam. Only two types of closed
dams (i.e. without and with flap-R) under the twoflow conditions i.e., the clear water flow and the

3.2.2 Experiment for debris flow
The debris flow experimental facility consists of a
5.0 m long horizontal smooth flume with arectangular section 10cm wide and 14cm high. One
side of the walls is made of transparent glass and theother side is opaque made of PVC. The slope of the
flume is set at 18°. The sabo dams were built using

f scan  2 f max , ( f max 

6

vavg
d max

)

(7)

Table 1 Specification of the sensor
Sensor-A

Sensor-B

Sensor-C

Capacity

200 kPa (2.039 kgf/cm )Rated output

1mV/V (=2000×106 strain) ± 20 %Safe excitation

3V

Sensitivity

Sensor-D

2

0.718 mV/V ± 1 %Input & output
Resistance

0.748 mV/V ± 1 %350 Ω ± 10 %*Note: 1mV/V corresponds to 2000×10-6 equivalent strain― 508 ―

Table 2 Median grain diameter of sediments (mm)
Type
D50

G1
10.0

S1
4.26

S2
2.56

S3
1.85

S4
0.94

S5
0.67

S6
0.29

Table 3 Properties of bed sediment material
Sediment

D50 (mm)

Dmax (mm)Sediment-A

1.783

10.871

Sediment-A

2.304

11.142

Fig. 5 Particle size distribution curves

acrylic planks 10 mm thick. To generate the debris
flow, a section of the flume (2.8 m upstream from theoutlet) having dimensions 1.9 m long and 7 cm deepis filled with the sediments supported at thedownstream by a 7 cm high weir. The reason behind
the installation of weir at the downstream of thedebris flow generation section is to make thesediment pre-saturated before the debris flow
generation. Then the debris flow is generated
overflowing the installed weir. As for the sedimentused in the experiments, silica sands (S1, S2, S3, S4,
S5, S6) and gravel (G1) were mixed in equal
proportion by weight to prepare the bed sediment-A.
Silica sand (S1, S2, S3, S4, S5, S6) in proportion (1.6,
1.5, 1, 1, 1, 0.7) and gravel (G1) in (1.7) by weight
were mixed to prepare the bed sediment-B. Table 2
shows sediments with median grain diameters andproperties of sediment material are shown in Table 3.
Fig. 5 shows particle size distributions of the
prepared material for bed sediment A and bed
sediment B. The bed sediments have an angle ofrepose, tanϕ = 0.7 and sediment density σ = 2.65
g/cm3 and are saturated with water. Debris flow is
produced by supplying a constant water discharge of
300 cm3/sec for 10 sec from the upstream end of the
flume. Details of the experimental setup are shown in
Fig. 6.
To measure the approaching velocity of debris
flow in this study, a high-speed camera was installedat the front of dam. The flow was filmed with ahigh-speed camera (Casio: EX-ZR300) placed on theside and top of the channel (as shown in Fig. 6). A
powerful imaging technique is exploited to measure
the tracer velocities and flow patterns. At the chosen
video rate of 480 frames per second, the digital
images have a resolution of 224×160 pixels. Fig. 7

Fig. 6 Sketch of experiment flume for debris flow

― 509 ―

(a) plan-1(a) Measuring system(b) plan-2
Fig. 8 Schematic diagram of three closed dams(b) Generation of debris flow at the upstream
Fig. 7 Experimental set-up for debris flow
shows the measurement systems and generation ofdebris flow at the upstream. The pressures data were
sampled at frequency rates of 500 Hz, converted via
an interface board and then recorded on a hard diskfile.
The schematic diagrams of the three dams withdifferent plans are shown in Fig. 8. The total pressure
associated with major debris flows was recorded inreal time by a system consisting of four sensors
installed in the two different plans. The sensors were
installed vertically along the flow depths (plan-1) andhorizontally at the same flow depth (plan-2). The
uplift pressure and vertical distribution of totalpressures were measured by plan-1 and the totalpressure due to the impact collision was measured byplan-2. Furthermore, Table 4 and Table 5 show theexperimental condition with both plans.
Debris flow velocity, the radius of the boulder
and approaching height are very important factors to
discuss the impact force. So far, to discuss the impact
force of debris flow, there have been few
experimental studies. Each experiment used threedifferent generation methods. Itoh et al. (2011)
explained the generation methods of debris flow asfollows:a) Type- NL: Natural landslide dam break.
b) Type-U: Sediment and water is supplied steadily
in upstream end of channel.

Table 4 Experimental conditions for horizontal total pressure
Bed sedimentSensor installation

Measurement

RepetitionA

Plan-2

Total pressure, velocity, depth

10 times

B

Plan-2

Total pressure, velocity, depth

10 times

Table 5 Experimental conditions for vertical total pressure
Type

Bed sedimentSensor installation

Measurement

Repetition

Without flap
Flap-R dam

A and B
A and B

Plan-1
Plan-1

Total pressure
Total pressure

10 times
10 times

Flap-T dam

A and B

Plan-1

Total pressure

10 times― 510 ―

c)

Type-QS: Bed sediment is set on the bed in
upstream reach of channel, saturating upstream
sediment with water, and supplying to input
discharge from upstream.
In most studies, type-U has been used for debris flow
generation. In this study, the phenomenon of thedebris flow is approximated by the type-QS
generation method. As the generation type is QS, thesediment composition and degree of saturation might
not be uniform throughout the sediment layer. Also,
the standard size of the boulder cannot be measured
when a debris flow hits on the obstacle. Therefore theexperiments were repeated several times underidentical conditions. Debris flow produced in theexperiments is the stony debris flow type and thelargest particles are accumulated in the forefront. To
measure the thickness of deposition (i.e. the flow
depth plus the deposition thickness in the process andfinal stage) accurately, the graduations are marked on
the side of the flume.
4.

Fig. 9 Results of the total pressure due to clear waterand debris flow

Results and discussions

Fundamental experiments were conducted toinvestigate dam-break and debris-flow for
improvement of functions of the sabo structure.
Considering quantitative and qualitative results fromthe experiments, the characteristics of the proposed
closed dams were discussed and summarized asfollows:
4.1 Comparison of flow profile near the dam
In order to confirm the characteristics (flowprofile and total pressure), firstly only two types ofclosed dams (without and with flap-R) under the twoflow conditions i.e. the clear water flow and thedebris flow were tested with plan-1. The experiments
were conducted for both flow conditions in order toobserve the energy dissipation phenomenon near thedam. In the case of dam without flap, after suddenly
opening of the gate, the flow hit the dam body andwhole flow move vertically upward and then some
Table 6 Results of surface velocity in both flows
Flow typeSurface velocity (m/sec)

Clear water
Debris flow-Sediment.A

1.176
1.218

Debris flow-Sediment.B

1.244

flows overtopped the dam while rest falls down
upstream. But in the case of dam with flap, after
hitting the dam surface by the flow, the flow moves
towards the below part of the flap vertically. Then,the flap reflect back the flow upstream which looks
like a bore traveling toward the upstream direction.
The flow pattern with debris flow was observed
without considering the flap structure. In such case,the flow pattern is similar to clear water case but due
to the mixture of different size of sediments in thedebris flow, we observed that some portion of thedebris flow overflows carrying large sediments while
rest of the flow remains upstream depositing
remaining particles at the bottom part of the dam. In
contrast, if we consider the flap structure, almost all
the particles were captured and deposited upstream of
dam due to the reflection from the flap.
Table 6 shows the average values of the surface
velocities measured for different flow conditions (i.e.
clear water and debris flows) in front of dam by ahigh-speed camera. Although the flow pattern and theapproaching surface velocity are very similar, thetotal pressure is entirely different in both flow cases
as shown in Fig. 9. This is due to the fact that thedebris flow has a huge energy in the front part of the― 511 ―

(a) Clear water case without flap(b) Clear water case with flap-R(c) Debris flow case without flap(d) Debris flow case with flap-R
Fig. 10 Results of the flow pattern for two different types of dam due to clear water and debris flow
flow that is much greater than clear fluid flow
because of the accumulation of large boulders at thefront.
Fig. 10 shows the test results of the flow pattern
for two different types of dam in the case of clear
water and debris flow. The flow patterns in the caseof clear water flow are shown in Fig. 10 (a) for thedam without flap and the Fig. 10 (b) for the dam withflap-R dam whilst Fig. 10 (c) and (d) correspond to
the debris flow condition for both types of dam. An
important point is that each flow pattern is very
different for each different type of dam but thepattern is the same in both flow cases. However, it
shown that flap-R dam generated a larger volume ofspray than the dam without flap downstream in bothcases. A comparison of the total pressures due to theimpact collision (i.e. the value is taken as the averageof the total measured maximum value of pressure foreach sensor in the number of experiments) reveals thefollowing important points:(1) The sensor D indicated nearly similar totalpressure in both types of flow for the dam without
flap (as shown in Fig. 9). The reason for thissimilarity is that, in both the flow cases, themovement of flow at the top of the dam (where
sensor D located) is vertical i.e. most of the forces are

uplift (due to the vertical nature of flow at the top as
shown in Fig. 10 (a) and 10 (c)) and the dam without
flap has no capability to absorb the uplift force since
it has no flap. But the total pressures in the case of
sensor A with debris flow have increased nearly 7
times in comparison to other types because at thebottom part of the dam (where sensor A is located)the debris flow has more impact force due to thesediments than the clear water since the flow here isnot completely vertical as in sensor D as shown by
Fig. 10 (b) and 10 (d).
(2) In the case of dam with flap-R, the values of
sensor D with bed sediment B indicated that the totalpressure has increased nearly 6 times in comparison
to the clear water type and sensor A shows a 5 times
increase in the value of total pressure. The reason for
the increase in value is that the flap structure fordebris flow absorbed higher uplift pressure than theclear water obviously and the bottom part has notchanged significantly from point (1).
The above result shows that the dam with flap-R
can reduce the quantity of spray transportation more
efficiently than the dam without flap due to the flow
profile so that the reduction in energy andovertopping time occurs. This experiment should also
help us better understand the energy dissipation― 512 ―

process involved in the flow weakening. Even if theflap structure was shocked by the uplift pressure, it
shows that the newly designed check dam with flap
has the advantage of changing the total pressurecompared to the dam without flap due to generationof the reflected flow. Furthermore, this result showsthat the dynamic force of debris flow due to theimpact collision is much greater than the clear fluidof dynamic force in comparison to the total pressurein the flow due to the average of maximum totalpressure.
4.2 Debris flow experiment
Debris flow experiments were conducted using
two types of bed sediment in order to check theefficiency of all three types of dam underconsideration namely without flap, with flap-R, and(a) Without flap: Initial condition, at t = 0.0sec(b) With flap-R: Initial condition, at t = 0.0sec

with flap-T. All experiments measured the totalpressure using a measuring system. As a generationof Type-QS, the sediment composition and degree ofsaturation might not be uniform throughout the
sediment layer. So the data obtained fromexperiments repeated several times under the same
hydraulic conditions were used for data analysis. In
the experiments, in order to test if the average of thetotal pressure (by the maximum value for each case)is statistically significant with different types of dam,as well as to remove the outlier data from the original
data, we used graph analysis with a boxplot method.
A boxplot is a device used to represent the median ofthe data, the upper and lower quartiles, and any data
points that possibly are outside (outlier) values, and it
is also useful for summarizing a data set.

At time = 6.67sec

At time = 7.43sec(c) With flap-T: Initial condition, at t = 0.0sec
At time = 6.30sec
Fig. 11 Movement of debris flow at different time step: sediment A― 513 ―

4.2.1 Flow patternFig. 11 shows the characteristics of debris flow
movement in case of all three types of dam (i.e. a:
without flap, b: with flap-R, and c: with flap-T) at
different time steps. The figure clearly reveals that, inthe case of a dam without flap, the large particles areseen to overflow whereas in other two types of dam
such particles are seen to be captured. This leads tothe conclusion that flap-T and flap-R types are more
efficient regarding the capturing of large debris mass.
This is because the flap structure is more efficient in
capturing the large particles. The flow patterns of the
proposed dams obtained by the experiments aredescribed briefly as follows:(1) In the case of a dam without flap: the flow
pattern is influenced by the characteristic of debris
flow (especially the approaching flow depth andvelocity). First the dam was immensely shocked by
the impact collision; secondly flow uprush occurred
vertically with large boulders due to the upliftpressure quickly and finally the debris flow separated
into two flows at the top of the dam: some debris
flow is transformed as an overflow with few large
boulders, and other debris is dropped by gravity fromthe top of the dam. A dead zone is developed at thebottom of the dam by the incoming debris flow
simultaneously with the above process. Due to those
processes, now the channel bed will rise upstream ofthe dam because of the captured debris mass.
(2) In the case of the proposed dams (i.e. flap-R
and flap-T dams), when debris flow occurs in front of
the dam, the impact collision and the vertical lift ofthe debris mass occur in similar fashion as in the caseof the dam without flap but there will be no overflowof the debris mass and all the debris mass is captured
upstream of the dam due to the generation of thereflected flow as shown in Fig. 11 (b) and Fig. 11 (c).
But later after deposition above the dam height theoverflow of debris mass may occur. Therefore, it isinferred from this process that the proposed dam typewill capture more large boulders than the other typesof dam and also the proposed type will reduce theoverflow time (which is not quick enough like thedam without flap) and reduce the impact energy on
the dam from the incoming debris flow due to thedeposited mass before the dam.
Although the figure shows that the final stage ofthe debris flow pattern is similar in all three types ofdam, it should be understood that the process to reachthe final stage is different for each type of dam asshown by the research.
4.2.2 Vertical distribution of total pressure
To estimate the external force of flap structure,
the vertical distributions of total pressure for the threedams under the two different bed sediment conditions
are investigated (Table 5 shows the experimental
conditions).
Table 7 compares the test results on the averageof maximum total pressure values by plan-1.
Proposed closed dams are the extension of a typicalvertical dam with flap installed at the top. But, thismight be vulnerable to accidental bumps due to the
uplift pressure. In order to check the uplift pressure,sensor D was installed within the flap. The results
obtained by comparing the total pressure and upliftpressure are summarized as below:(1) In the case of sediment A: the uplift pressureof the flap-R dam has increased by nearly 3 times incomparison to other types. The reason for thisincrease is that the rectangular type has to absorb allof the uplift pressure directly while the flap-T typehas more smooth control due to its shape and thetypical type has no flap at all.
(2) In the case of sediment B: even if the main
total pressures due to the impact collision are very
similar by the value of sensor A, the values of upliftpressure for the flap-R type have increased by nearly

Table 7 Results of total pressure without outlier data (unit: kPa)
Sediment A

Sediment A

Without

Flap-R

Flap-T

Without

Flap-R

Flap-T

Sensor-A

20.01422.88016.09019.85017.61419.397

Sensor-B
Sensor-C

6.478
3.003

5.764
3.891

9.749
4.128

11.9854.704

11.317
4.055

7.728
6.596

Sensor-D

2.689

7.645

2.611

3.120

17.1990.492― 514 ―

35 times in comparison to other types. So the flap-T
type is still efficient in comparison to the flap-R typeregarding uplift pressure. Therefore, the flap-T typecan more effectively control the uplift pressure than
the flap-R type due to generation of the reflected flow.
Even if both flap types were shocked by the upliftpressure, it shows that the newly designed check dam
with flap has the advantage of changing the dynamic
pressure compared to the dam without flap due togeneration of the reflected flow.
Fig. 12 shows the trend line of vertical
distribution of total pressure using the average ofmaximum values for the three dams (as shown in thedotted line) and the trend line (as shown in the blackline) without the uplift pressure for both dams (i.e.
flap-R and flap-T dams by sensor D) are estimated by
the following relation:x  Me Ny(8)where x: total pressure, y: height of dam, and M and
N are constant coefficients. The results obtained bycomparing the trend line of vertical distributions are
summarized as below.
In the case of sediment A and B, all dam types
show a similar trend of vertical distribution of totalpressure. However, the flap-R dam shows twopatterns regarding the vertical distribution of totalpressure (i.e. the first pattern is decrease from bottom
to below the flap part; the second pattern is rapid
increase at the flap part due to the uplift pressure).
This is because the flap-R type has to absorb all of
the uplift pressure directly. So, it shows theeffectiveness of adding the rectangular flap to theconventional dam. But the flap-T shows a similar
trend to the dam without flap due to the fact that it
has a smooth control of the pressure because of its
shape. So, the pressure calculation while designing
the sabo dam with flap-T does not require any
different approach to that of the without flap dam but
the flap-R does require a different approach for theconsideration of uplift force.
4.2.3 Mass ratio of debris
To clarify the function of flap structures, thecaptured and overflowed sediment of debris flows
between the without and with flap-T dams under the

Fig. 12 Vertical distribution of total pressure for threedams
two different sediment conditions were observed. The
experiments were repeated 5 times under identicalconditions. The mass of debris (i.e., the overflow andcaptured) were measured directly at the final stage by
an electronic scale. After the generation of debris
flow, the bed sediments remained in the upstream endof the flume. In both cases of sediment A and B, thegeneration of debris flow mass was calculated forwithout and with flap-T cases by summing up the
captured and overflow mass of debris. Initially for theexperiment, the sediment (density of 2.65 g/cm3 andporosity of 0.35) of about 23 kg was fed through thechannel. Table 8 compares the test results regarding
the overflow, captured and generated, of the debris
mass for the three dams. Out of 23 kg mass forgeneration of debris flow, the actual generation ofdebris flow as measured by summing of the captured
and overflow debris flow mass was found to be
37.60 % of the total (8.648 kg) for sediment A and
39.62 % of the total (9.114 kg) for sediment B in thecase of without flap dam. The results indicate that asimilar quantity of debris flow was generated during
the experiment. For the flap-T type also, the recorded
values of actual generation of debris flow were
38.13 % and 35.95 %, respectively for sediment A― 515 ―

Table 8 Results of total pressure without outlier data (unit: kPa)
Sediment A
Without flap
With flap-T

Sediment B
Without flap
With flap-T

Overflow (kg)

2.116

2.570

1.245

1.397

I (%)

24.44

29.23

13.73

17.48

Captured (kg)

6.532

6.119

7.868

6.872

II (%)

75.56

70.77

86.27

82.52

Generated (kg)III (%)

8.648
37.60

8.769
38.13

9.114
39.62

8.269
35.95

*I: comparing the mass between generated and overflow*II: comparing the mass between generated and captured*III: comparing the mass between generated and input sediment*Input sediment mass at upstream for generation of debris flow = 23 kg
and sediment B cases. Based on this result, weconcluded that there is very good reproducibility ofthe tests, giving confidence in the results obtained byapplying the generation method of debris flow.
4.2.4 Median grain diameter
Table 9 shows the median diameter of sediments
for dams of without flap and with flap-T under two
different sediments. The results obtained bycomparing the captured and overflow debris mass
grain size distribution for three stages with two
different check dams are summarized below:(1) Bed sediment A: The median size of particles(d50) of overflow debris mass increased by nearly
11 % compared to the generated debris flow mediansize for the case of the dam without flap. But, in thecase of the dam with flap-T, nearly 4 % decrease inmedian size was observed. These results conclude
that the flap structure will capture more debris mass
than the without flap since less overflow means more
capture. So flap-T type is more efficient to capture
large debris mass than the without flap.
(2) Bed sediment B: The median size of particles(d50) of overflow debris mass increased by nearly

17 % and 5 % compared to the generated debris flow
median size for the case of the dams of without flap
and with flap-T, respectively. These results conclude
that even though the median grain size increased forboth dam types, the percentage increment for theflap-T type is smaller compared to the dam without
flap. The flap-T type is still efficient in comparison towithout flap regarding the capture of debris mass.
4.2.5 Force behavior of debris flow
The force behaviour of debris flow can beclassified into three stages. The maximum totalpressure will occur due to the impact force (by thecollision) when debris flow just reaches the sabo dam
at first. Secondly, both the dynamic and static
applying condition will occur due to moving debris
flow. Finally, the static applying condition will occur
due to the static pressure when debris flow isdeposited on the front of the sabo dam. Fig. 13 as thecase of the flap-R dam shows the movement of debris
flow due to the main forces for each stage condition.
Fig.14 shows the variation of total pressure due tothe main applying force with time. The debris flow infront of the dams became strongly turbulent due toTable 9 Results of the median grain diameter (d50)
Sediment A
Without

Sediment B
Flap-T

Without

Flap-T

Overflow (mm)

2.201

1.658

3.330

2.853

I (%)

(▲) 10.5(▼) 4.3(▲) 16.6(▲) 5.4

Capture (mm)II (%)

1.931(▼) 3.1

1.789(▲) 3.2

2.764(▼) 3.3

2.669(▼) 1.4

Generated (mm)

1.992

1.733

2.857

2.707

― 516 ―

1st stage (impact force)Without force (initial condition)

2nd stage (dynamic and static forces)
3rd staged (static force)
Fig. 13 Movement of debris flow due to main forces with bed sediment B (Flap-R dam)

Fig. 14 Variation of total pressure due to the main forces with bed sediment B
the impact (stage: 1), dynamic (stage: 2) and static(stage: 3) forces that are shown by the sharp increasein the value in the graph shown in the figure. Afterthe debris flow stopped, the static pressure was themain force since the debris mass has deposited in
front of the dams and the depth of water has
increased.

4.2.6 Impact pressure and velocity
To clarify the impact pressure due to the impact
collision, parameters such as total pressure,approaching velocity and depth are observed duringexperiment. In the experiments, the data wereobtained from the 10 times repeated experiments
under the same hydraulic conditions with plan-2― 517 ―

Table 10 Results of the total pressure and surface velocity
Sediment A

Sediment B

Total pressure(kPa)

Surface velocity(m/sec)Total pressure(kPa)

Surface velocity(m/sec)Q1

19.176

1.151

23.9401.130MedianQ3

37.62159.136

1.231
1.272

33.01173.477

1.231
1.338

IQ range

39.9610.120

49.538

0.208

Average42.415

1.218

43.512

1.244

Max88.255

1.333

117.644

1.548*Q1: Lower quartile*Q3: Upper quartile*IQ Range: Range of Q1~Q3(Table 4 shows the experimental conditions).
Since plan-2 consists of sensor installation at thebottom part only, the results are extraneous as to dam
shape but the values are directly connected to the bed
sediments. Therefore, this experiment can obtain four
values of the total pressure per one case and averagesof the maximum values observed are used for thedata analysis. Also, the box plot method of data
analysis is performed as shown in Fig. 15.
Table 10 shows the test results for the averaged
total pressure and surface velocity of debris flow
under the two bed sediments. It shows that there arealmost similar values of the average total pressuresand velocities in the case of both bed sediments.
However, in the case of bed sediment B, themaximum total pressure value has increased bynearly 1.3 times and Q1 (lower quartile) and Q3(upper quartile) values have also increased by 1.2
times in comparison to sediment A. This should not
be ignored. The reason for this increase is that theamount of large particles in the composition of

sediment B is more than that of sediment A andmaximum pressure is the result of sudden impact by
the large particles. From this result, it is concluded
that the maximum total pressure generated by debris
flow is not proportional to the amount of debris flow,
but strongly depends on the particle size distributionof debris flow. In addition, the results reveal that thecolliding total pressure is different in all three typesof dam although all the influencing parameters as
described by Hu et al. (2011) such as generation
pattern, initial sediment size, properties and volume,
impact angle etc. are similar. The reason for this isdue to the contact area of the sediment particles withthe sensor. So the contact area of the different
particles should be one of the important parameterswhen deciding impact pressure on the dam.
4.2.7 Impact model of debris flow
Pressure is the force on an object that is spreadover a surface area. The equation for pressure is theforce divided by the area where the force is applied.

*lower quartile: split lowest 25% of data (= 25th percentile)*upper quartile: splits highest 25% of data, or lowest 75% (=75th percentile)
Fig. 15 Results of the boxplot under the two bed sediments― 518 ―

Although this measurement is straightforward when asolid is pushing on a solid, the case of a solid pushingon a liquid or gas requires that the fluid be confinedin a container. The force can also be created by theweight of an object. When we apply a force to a solid
object, the pressure is defined as the applied force
divided by the area of application. The equation forpressure is:

P

F
A

sediment concentration equation. The equilibrium
sediment concentration and the mixture density ofdebris flow are described as follows: According to
Nakagawa et al. 2003, if a stony debris flow occurs( tan  w  0.138 ), then tan  w
    tan   tan  w 

C

(10)

 d   C   (1  C )

(11)

(9)

where P = the total pressure, F = the applied force,and A = the surface area where the force is applied. In
this experiment, the total pressure from debris flow
was measured by the dynamic pressure sensor. To
calculate the applied force (i.e. impact force due tothe collision) by Eq. (9), we have to estimate theactual contact area. But it is very difficult to obtain
the actual contact area. So, in this study, the sensor
area is considered as the actual contact area (A =0.2826 cm2). Likewise, Scheidl et al. (2012) also
estimated the peak forces by multiplying measured
pressure by the sensor contact area. To calculate theforce of debris flow by Eqs. (3) and (5), various
parameters such as the density of debris flow, theapproaching surface velocity of debris flow, the flow
depth, the discharge of debris flow, and the radius of
the boulder are required. The approaching surface
velocity of debris flow is obtained from theexperiments by measurement from the high-speed
camera. However, the flow depth is taken as averageof 10 values of measurement of flow depth (= 2.0cm), the radius of the boulder is obtained from thevalue of dmax from particle size distribution curves ofgravel, and the discharge of debris flow was
calculated from the channel geometry, approachingsurface velocity, and depth of debris flow. Since theexact size of the colliding particles of debris flow
(since it consists of variety of particle sizes) changes
with time and the debris flow is accumulated with thelarge boulders at the front part of the flow, so forsimplicity the maximum size of the particle isadopted for the calculations (i.e. 0.5 cm for sediment
A and 0.6 cm for sediment B, refer Table 3). Finally,
the density of debris flow is very difficult to measurein the flow through the experiment. So, the density ofdebris flow is calculated using an equilibrium

where ρ = the density of water, θw = the water surface
slope, σ = the density of sediment particle, ϕ = theinternal friction angle of the sediment, ρd = themixture density of debris flow, and C = the sedimentconcentration of debris flow. The equilibrium
sediment concentration is taken as 1.866 g/cm3
calculated by Eq. (11) using the value of the densityof sediment particles and water as 2.65 and 1 g/cm3,
respectively. The water surface slope is taken as 18ºequivalent to the flume slope, and the angle ofinternal friction of the sediment is assumed as 35º.
The results of impact force calculated using equations(3), (5) and (9) are shown in Table 11. This tableshows the calculated applied force (i.e. due to theimpact collision) of debris flow with different impact
models for bed sediments A and B. The parameters of
the equations are measured from the experiments
under the same hydraulic conditions with plan-2
performing experiment 10 times. The maximum,
minimum and average values for equations (3) and(5) in the table means the impact force is calculatedusing maximum, minimum and average flow
velocities in 10 experiments while for equation (9)those values are calculated using maximum,
minimum and average measured total pressure.
Furthermore, Table 12 shows a comparison of theempirical coefficients (i.e. pressure and force) used inboth hydrodynamic and solid collision models withthe experimentally obtained value from this study.
The result shows the new range of values regarding
the use of the equations of both models (i.e.
hydrodynamic and solid collision model). The result
from the experiment reveals that the values of kp (i.e.
coefficient of impact pressure) as proposed by other― 519 ―

Table 11 Results of the impact force with three theory (unit: N)
Sediment A
Eq. (3)Eq. (5)Eq. (9)Eq. (3)

Sediment B
Eq. (5)

Max.

6.632

1.701

2.494

8.944

2.931

3.325

Min.

4.649

1.375

0.425

3.891

1.779

0.591

Avg.

5.560

1.528

1.199

5.860

2.259

1.230

Parameters

d , v, Q

v, RAd , v, Q

v, RA

Eq. (9)

Table 12 Comparison of empirical coefficients
Empirical coefficient
Hydrodynamic
model
Solid collisionmodel

kp by Eq. (2)kf by Eq. (3)kf by Eq. (5)In this study
Bugnion et al. (2001)
In this study
Yamamoto et al. (1998)
In this study
Mizuyama (1979)

Sediment A

Sediment B

0.1~0.2

0.1~0.2
0.4~0.8

5.5* (1.9~11.8)

6.5* (1.8~12.6)
1.0

1.8* (0.6~4.0)

2.5* (0.7~4.4)
1.0( ): Range value from experimental data*: Average value
researchers are relatively smaller for thehydrodynamic model, but the values of kf (i.e.
coefficient of impact force) as proposed by otherresearchers are comparatively higher for both thehydrodynamic and solid collision models.
5.

Results and discussions

In all the experiments, the effects of sabo
structures, with emphasis in the sabo dams, theparameters such as the flow pattern, the surface
velocity between the debris flow and clear water, thetotal pressure and the uplift pressure, mass ratio ofdebris, and median grain diameter were measured tocompare the function of each sabo dam under the twodifferent bed sediments. The empirical coefficients of
the hydrodynamic and solid collision models are also
determined and compared with available values of
those coefficients. Furthermore, the characteristic ofdebris flow and proposed dams are confirmed due tothe experiments. The experimental data analyses leadto the following conclusions:(1) From the experimental results, although theflow pattern and the approaching velocity are very
similar, the total pressure is entirely different in bothflow cases. This is due to the fact that the debris flow

has high energy in the front part of the flow, morethan clear fluid flow because of big boulders
accumulation at the front.
(2) The proposed closed-type sabo dams with flap
structure can reduce the quantity of spraytransportation more efficiently than the dam without
flap (i.e. typical type).
(3) Flap-T type (triangle type) can control the
uplift pressure more effectively than flap-R type(rectangular type) due to the generated reflected flow
and is more efficient to capture large debris mass thanthe dam without flap (i.e. typical type). Even if bothflap types were shocked by the uplift pressure, it was
shown that the newly designed check dam with flap
has the advantage of changing the dynamic pressurecompared to the dam without flap due to reflection.
Moreover, the triangular flap shows a similar trend
like the dam without flap because it has smooth
control of the pressure due to its shape. So, thepressure calculation while designing the sabo dam
with flap-T does not require any different approach tothat of the without flap dam but the flap-R does
require a different approach for the consideration of
uplift force. Therefore, the triangular flap structure ismore suitable than the rectangular shape for actual
application.

― 520 ―

(4) The results of mass ratio of debris indicatethat the similar quantity of debris flow was generated
during the experiment. For the dams of without flap
and with flap-T, the recorded values of the actual
generation of debris flow were 38 % for sediment A
and sediment B cases, respectively. From this result,
it is concluded that the experiments revealed very
good reproducibility of the tests, giving confidence inthe results obtained by the applied generation methodof debris flow.
(5) From the results of the median grain diameter,these results conclude that the flap structure willcapture more debris mass than the dam without flap
since less overflow means more capture. The case of
sediment A shows the advantage clearly (i.e. theflap-T type is more efficient to capture large debris
mass than the dam without flap). Even though thecase of sediment B increased the median grain sizefor both dam types, the percentage increment for theflap-T type is small compared to the ordinary type.
So, the flap-T type is still efficient in comparison tothe ordinary one regarding capture of the debris mass.
(6) The force behaviour of debris flow can beclassified into the three steps due to the main applied
force (i.e. impact force, dynamic and static force, andstatic force).
(7) The empirical coefficients were estimated by
the hydrodynamic and the solid collision models
from the experimental results. The result from theexperiment reveals that the values of kp (i.e.
coefficient of impact pressure) as proposed by otherresearchers are relatively smaller value for thehydrodynamic model but the value of kf (i.e.
coefficient of impact force) is comparatively higher
for both the hydrodynamic and solid collisionmodels.
(8) The maximum of total pressure generated bydebris flow is not proportional to the amount ofdebris flow, but depends strongly on the particle size
distribution of debris flow. Besides, the contact areaof the different particles should be one of theimportant parameters when deciding impact pressure
to the dam.
This study was carried out with the stony debris
flow, but the case of mud debris flow proves equallyapplicable. The reason is that the mud debris flow iseasy to generate and does not consist of larger
particles compared to the stony debris flow. Also, theflap structure should build monolithically with main
dam structure to correlate this research results.
Acknowledgements
This research is supported by JSPS AA Science
Platform Program (Coordinator: H. Nakagawa) andsupport of Wakate fund program by Kyoto University
Global COE Program (GCOE-ARS).
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― 522 ―

(Received 11, June, 2013)