# IJEGE-11_BS-Suzuki-et-alii

*Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza*

*DOI: 10.4408/IJEGE.2011-03.B-067*

**MONITORING NEAR-RIVERBED SEDIMENT BEHAVIOR**

**OF DEBRIS FLOWS USING HYDROPHONES**

tutive equations have been proposed (e.g., t

*et alii*, 1982; t

*et alii*, 1982; d

*et alii*, 1989). However, in these equa-

taken into consideration. s

*et alii*(2003) and s

with larger riverbed particles. Thus, the coefficient of

resistance is large when the sediment concentration is

large or the relative flow depth is small. Therefore, it is

necessary to construct a flow model that can evaluate

the influence of riverbed roughness. To do so, the near-

riverbed sediment behavior of debris flows must be

clarified. Unfortunately, there is no established method

to measure this behavior because it is generally very

difficult to measure sediment concentration and sedi-

ment load. There is no sensor that can measure sedi-

ment concentration or sediment load directly. Moreo-

ver, a direct sampling measurement method requires

a huge amount of effort and is tremendously costly,

and long-term continuous measurement is impossible.

Therefore, sediment concentration and load are esti-

mated indirectly by measuring other physical quanti-

ties. One class of methods for the indirect estimation

involve the use of hydrophones (microphones within

steel pipes), and their effectiveness in the measurement

of bedload transport intensity has been verified (b

*et alii*, 2002). An exam-

**ABSTRACT**

port intensity. Bedload discharge and average grain

diameter can be calculated analytically using sound

pressure data. In this study, hydrophones were used to

identify debris flows. The proportional relationship be-

tween the output voltage corresponding to a grain col-

lision and its momentum was used to analyze electric

pressure distribution, which was then used to calculate

the mean diameter of colliding grains. Flume experi-

ments were conducted to verify the effectiveness of this

method in recognizing the time change of the near-river-

bed sediment discharge from debris flows and low con-

centrated flows, including their transition ranges. Total

sediment discharge can also be calculated if the collision

rate upon the hydrophones is evaluated by setting the in-

terface. In addition, the time change of the average grain

diameter can be calculated. Large grains were detected

in the debris flow surge, and the analytic values were in

rough agreement with the experimental values.

**K**

**ey**

**words***: debris flow monitoring, hydrophone, sound pres-*

*sure*

**INTRODUCTION**

shear stresses. Research on the constitutive equations

of debris flows has been carried out to construct mod-

*t. SUZUkI, y. HASEGAwA, h. MIZUNO & N. OSANAI*

*5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011*

*Q*, was calculated by dividing the sample volume,

*Vlm*, by the sampling time. The sediment concentra-

tion,

*c*

*V*

*s*

*V*

*w*

frames per second) installed 1 m above the lower end.

*CONDITIONS*

*θ*was set to 13°. The supplied sediments in

the upstream part of the channel had different grain

distributions. In two cases, a uniform grain size was

used, with

*d*= 0.294 cm and

*σ*= 2.65 (

*σ*is the specific

gravity of the sediment) (Case-Uni.). In the other two

cases, together with a sediment of

*d*= 0.294 cm, larger

grains were used with

*d*= 1.76 cm and located at 15

cm intervals on the surface (Case-Large). The total

number of larger sediment grains was 10, one grain per

15 cm interval. The supplied water discharges were 1

and 3 L/sec under each grain distribution condition.

**NUMERICAL SIMULATION**

alone. Therefore, a one-dimensional numerical simula-

the sound of sand colliding with the steel pipe contain-

ing the microphone is analyzed and transformed into

a pulse; this pulse represents the number of times the

sound level crosses a certain threshold. Thus, the sedi-

ment transport intensity is estimated on the basis of the

premise that it is positively correlated with the pulse.

However, this method has some problems. For exam-

ple, when the sediment rate is high, the sound level

is continuously high. Thus, the number of pulses de-

creases or becomes zero (m

*et alii*, 2008).

*et*

*alii*(2010) attempted to use an analytical method in-

volving the use of sound pressure data. According to

these researchers, bedload discharge and average di-

ameter can be obtained by considering the reduction

in sound pressure caused by the interference of sound

waves. While hydrophones can be used to evaluate de-

bris flow, the method of s

*et alii*(2010) has meas-

collisions. Thus, this method needs to be modified be-

fore it can be used for debris flow measurements. The

aim of this paper is to develop a method for calculating

the mean grain diameter by analyzing the electric pres-

sure distribution and to apply it to debris flows. The

method was verified by performing flume experiments.

**EXPERIMENTS**

*EXPERIMENTAL DEVICES*

The channel is 9 m long and 10 cm wide, with glazed

sides (Fig. 1). The sides in the downstream part of the

channel (4.5 m) are as high as 10 cm. A sand rough-

ness was positioned at the downstream part (4.5 m);

the upstream part (1.8 m) was filled with sediment.

Water was regularly supplied from the upper end and

a debris flow was generated.

*MEASUREMENT METHOD*

stalled 1 m above the lower end. A hydrophone was

installed at the lower end of the channel, as shown

in Fig. 2. The output voltage of the microphone was

amplified to the range of ±10 V.

*Fig. 1 - Experimental setup*

*Fig. 2 - Schematic design of the hydrophone setup*

**MONITORING NEAR-RIVERBED SEDIMENT BEHAVIOR OF DEBRIS FLOWS USING HYDROPHONES**

*Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza*

*e*is the coefficient of restitution,

and

*k*

*f*

*c*=

*c*

*e*

*h*=

*h*

*e*

*u*=

*M*/

*h*

*e*

*h*

*e*

sults in terms of time-series variation of

*h*and sedi-

ment concentration around the surge front.

*c*-Exp and

*h*-Sensor refer to the

measured

*c*and

*h*, and

*c*-Cal. and

*h*-Cal. refer to the

simulation results of

*c*and

*h*using calibrated param-

eters. From 72 to 85 sec of the 1

*l*-case and from 31 to

34 sec of the 3

*l*-case, the

*h*-Sensor decreased drasti-

cally. These changes were due to the transition from

debris flow to low concentrated flow after most of the

grains were eroded. Thus, the simulation results agree

well with the experimental results. Therefore, these

simulation results are useful as comparison values for

hydrophone measurements.

**ANALYTICAL METHOD**

tial frequency components were extracted with a band-

pass filter (Fig. 6). Sound pressure data correspond to

the line connecting the local maximum points of the

extracted data (Fig. 7), and

*Sp*is the average value.

*et alii*(2010) confirmed the relationship

*Sp*and bedload discharge,

*Qs*, as follows:

*α*is the proportionality coefficient,

*R*is the detec-

tion rate, and

*N*is the number of collisions per second.

Equation (14) indicates that

*R*is a function of

*N*. The re-

lationship between

*R*and

*N*can be obtained from exper-

imental results under a wide range of conditions. How-

ever, it is unrealistic because a tremendous amount of

data is necessary for experimental accuracy. Therefore,

s

*et alii*(2010) proposed a method for estimating

*R*and

*N*using superposition

simulations. Their method is described in the following.

*rd*(

*t*), is given

*t*is the elapsed

*h*is the flow depth,

*u*is the flow

*M*=

*uh*,

*g*is the acceleration due to gravity,

*ρ*

*m*

*H*=

*h*+

*z*

*b*

*z*

*b*

*E*is the erosion velocity, τ

*c*is the average sediment concentra-

tion,

*c*

*t*

*c*

***

*τ*

*c*

*t*

*et alii*(2009) was applied for

*E*:

*T*is the relaxation time of erosion or

*e*is the equilibrium sediment concen-

tration, expressed as follows:

*h*

*e*

*τ*

moto & ito (2002),

*τ*

stress,

*τ*

*y*

*τ*

*D*

*f*

*b*

*k*

*g*

(9)

*t. SUZUkI, y. HASEGAwA, h. MIZUNO & N. OSANAI*

*5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011*

*m*is an arbitrary counting number.

*T*

*T*

*T*

*T*

*m*parts,

*p*th part is designated as

*V*

*p*,

*t*), where

*p*is the

*m*and

*t*is the elapsed time

from the starting point of every part. Transformed data

are designated as

*V*

*t*) and calculated as Eq. (19),

*T*

*T*

*Sp*

set of simultaneous equations to calculate

*Qs*and

*d*.

However, this method has measurement limitations

with respect to the number of grain collisions. Thus,

the method needs to be modified before it can be used

for debris flows. In the present study, a new method

was developed to calculate the grain diameter distri-

bution by analyzing the electric pressure distribution.

This new method is detailed in the following.

*Th*, is set at

*Th*=

*N*/100000 for arbitrary

*N*. When

*rd*(

*t*) is lower than

*Th*,

an individual collision wave datum, which is obtained

by preliminary experiment, is added to the wave data

being produced.

*R*is calculated using Eq. (13) from

the data computed in this way. The relationship be-

tween

*R*and

*N*is obtained when

*N*is changed over a

wide range. Thus,

*R*decreases as

*N*increases due to

the effects of sound wave interference (Fig. 8).

*Qs*is expressed as

*Qs*and

*d*using just sound data obtained with a

hydrophone; this was attempted because it is difficult to

measure the time-series variation of grain size distribu-

tion. Thus, Eq. (16) has two unknown values, d and

*N*,

while

*Sp*is measured variable. Therefore, it is impossible

to obtain

*Qs*from

*Sp*only through Eq. (16). s

*et*

*alii*(2010) proposed a method for calculating the trans-

formed data that satisfies Eq. (17) by dividing the origi-

nal data equally and summing them linearly as follows:

*Qs*

*Fig. 3 - Experimental and simulation results of flow height, h, and sediment concentration, c (Case-Uni.-1l)*

*Fig. 4 - Experimental and simulation results of flow height, h, and sediment concentration, c (Case-Uni.-3l)*

**MONITORING NEAR-RIVERBED SEDIMENT BEHAVIOR OF DEBRIS FLOWS USING HYDROPHONES**

*Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza*

*r*

*i*

*d*is calculated as the volume

*d*in

*Nf*(

*N*) to be calculated.

The relationship between

*N*and

*Nf*(

*N*) (Fig. 10) was

computed easily from the

*Nf*(

*N*) relationship (Fig. 9).

*Nf*(

*N*) was substituted into the

*Nf*(

*N*) relationship, and

*N*was calculated. Finally, substituting

*d*and

*N*into

Eq. (15) allowed

*Qs*to be obtained.

*i*th section,

*V*

*i*

*et alii*(2010) confirmed that the maximum

*V*

relationship between

*V*

*β*is the proportionality coefficient and

*v*is the

velocity of a grain. Assuming that

*v*is constant in cer-

tain durations, the diameter corresponding to the

*i*th

section,

*d*

*i*

*γ*is the proportionality coefficient and is found

to have a value of 0.6

*β*from the results of the present

preliminary investigations.

*Nm*

*i*

*i*th section, the volume of

grains in the ith section, Vol

*i*

*k*is the proportionality coefficient. Therefore, the

ratio of Vol

*i*

*r*

*i*

*Fig. 5 - Raw wave data*

*Fig. 9 - Schematic diagram of the data transformation*

*Fig. 6 - wave data extracted with a band-pass filter*

*Fig. 7 - Sound pressure data*

*Fig. 8 - Relationship between the number of collisions, N,*

*and the detection rate, R*

*Fig. 10 - Relationship between the number of collisions, N,*

*and Nf(N)*

*t. SUZUkI, y. HASEGAwA, h. MIZUNO & N. OSANAI*

*5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011*

*Qs*and

*d*were calculated every 0.01

0.02 sec. For

*Qs*, a 0.1 sec moving average was computed.

*Qs*-Cal(Interface = 1.7)] are shown, and they

are in agreement with the analyzed results.

*c*was high, the analytic results of

*d*were

*c*was low (low

concentrated), the analytic results of

*d*were lower.

This was due to the noise of the water, and the prob-

lem should be rectified in future studies.

*CASE-LARGE*

*Qs*-Hp,

*d*-HP,

*Qs*-Cal., and

*d*-Cal. around the surge

of large grains determined from high-speed video also

are shown.

*Qs*and

*d*increased instantaneously when

large grains were considered to have collided with the

hydrophone. About five grains in the 1

*l*-case and two

grains in the 3

*l*-case were discriminated. It is presumed

that other grains did not collide with or graze the hy-

drophone. The maximum analysis value of

*d*was about

0.8 cm, which is lower than the actual value of 1.76

cm. This is because the analytic value of

*d*was the vol-

ume mean diameter. The volume mean diameter,

*d*

**RESULTS**

*CASE-UNI*

*Qs*-Hp and

*d*-HP referring to the analytic results of

*Qs*and

*d*

(see the previous paragraph), and

*Qs*-Cal. and

*d*-Cal.

referring to the simulation results of

*Qs*and

*d*(see

the section titled “Numerical Simulation”). In the

1

*l*-case,

*Qs*-HP and

*Qs*-Cal. agreed quantitatively.

However,

*Qs*-HP of the 3

*l*-case was lower than

*Qs*-

Cal. The underestimation of

*Qs*for the 3

*l*-case can

be explained fairly well if one considers that, during

the experimental tests, the grains in the upper layer

likely did not collide with the hydrophone. Assum-

ing that only grains under a certain interface height,

IF, collide with the hydrophone, the collision rate,

*r*

*c*

*u*(

*z*) is the velocity at

*z*. Substituting the typical

velocity distribution of a debris flow [Eq. (28)] into

Eq. (27),

*r*

*c*

*Fig. 11 - Analytic results of sediment discharge, Qs, and average grain diameter, d (Case-Uni.-1l)*

*Fig. 12 - Analytic results of sediment discharge, Qs, and average grain diameter, d (Case-Uni.-3l)*

**MONITORING NEAR-RIVERBED SEDIMENT BEHAVIOR OF DEBRIS FLOWS USING HYDROPHONES**

ated by setting the interface.

its range depends on the performance of the micro-

phones. Because of this, hydrophones are applicable

within a specific range. The method described in this

study can be used to perform highly accurate meas-

urements of the time change of sediment discharge

within experimental results of this study . However,

this method also has some limitations. For example,

when sediment concentration is low, the values of

the grain diameter obtained are lower than the actual

values. Moreover, it is presumed that the interface

varies with the scale of the debris flows, the condi-

tions under which the hydrophone is installed, and

the terrain conditions at the installation site. There-

fore, further improvement of the method is necessary

to resolve these problems.

part of basic studies of debris flows. However, it may

also be applied to field monitoring. In such cases,

the hydrophone’s steel pipe must be large enough to

endure the impact of realistically scaled debris flows.

Therefore, verification of the applicability of large-

size hydrophones is a necessary next step in the de-

velopment of the method.

*Qs*

*S*

*d*

*L*

*Qs*

*L*

*Qs*

*L*

*d*= 0.294,

*d*

*L*

*Qs*

*S*

*d*

*d*

**CONCLUSIONS**

analyze debris flows. An analytical method for calcu-

lating the grain diameter distribution from measure-

ments of the electric pressure distribution was devel-

oped by considering the fact that the output voltage

corresponding to a grain collision is proportional to

its momentum. Also introduced herein was the exist-

ing method of s

*et alii*(2010). Flume experi-

discharge and the average diameter of near-riverbed

sediment can be measured quantitatively by this

method. Total sediment discharge can also be calcu-

*Fig. 13 - Analytic results of sediment discharge, Qs, and average grain diameter, d (Case-Large.-1l)*

*Fig. 14 - Analytic results of sediment discharge, Qs, and average grain diameter, d (Case-Large.-3l)*

*t. SUZUkI, y. HASEGAwA, h. MIZUNO & N. OSANAI*

**REFERENCES**

*Acoustic sensors(hydrophones) as indicators for bed load transport in a mountain torrent*,

*Hydrology in Mountain Regions, 1*. Hydrological Measurements; the Water Cycle, Proceedings of two Lausanne Symposia,

August 1990), IAHS Publ. No.

**193.**

*Mathematical model of two-phase flow*. Ann. Rev. Fluid Mech., 261-291.

*Constitutive equations of debris flow.*Annuals, Disas. Prev. Res. Inst.,

**32B**-2: 487-501. (in Japanese)

*Short-time relations between runoff and bed load transport in a steep mountain torrent*,

**249**: 317-324

*Numerical simulation method of debris flow introducing the erosion rate equation*, Journal of the

**55**(

**2**): 24-35. (in Japanese)

*Sediment measurement with a hydrophone at Tsuno-ura karyu Sabo Dam in the*

*Joganji River*. Journal of the Japan Society of Erosion Control Engineering,

**55(3**): 56-59. (in Japanese)

*Bedload measurement by acoustic energy with Hydrophone for high sediment*

*transport rate*. Journal of the Japan Society of Erosion Control Engineering,

**61**(

**1):**35-38. (in Japanese)

*New results from sediment transport measurements in two Alpine torrents*. IAHS Publ.,

**248**: 283-289.

*Constitutive relationships for fluid-solid mixtures*, Proc. ASCE,

**108**,

**EM5**: 748-763.

*Influence of riverbed roughness on debris flows*, Journal of the Japan Society of

**2)**: 5-13 (in Japanese)

*Resistance of the debris flow on the roughness boundary, Disaster Mitigation of Debris Flows,*

*Slope Failures and Landslides*. Proceedings of the INTERPREVENT International Symposium, 129-139.

*Numerical simulation method of debris flow introducing the non-entrainment*

*erosion rate equation, at the transition point of riverbed gradient or the channel width and in the area of sabo dam*, Journal

of the Japan Society of Erosion Control Engineering,

**62**(

**3)**: 14-22. (in Japanese)

*Basic study on sediment rate measurement with a*

*hydrophone on the basis of sound pressure data*, Journal of the Japan Society of Erosion Control Engineering,

**62**(

**5)**:

18-26. (in Japanese)

*Debris flow on prismatic open channel*. Proc. ASCE,

**106**, HY3: 381-396.

*Grain Stress and Flow Properties of Debris Flow*. Proc. JSCE,

**317:**79-91.