京都大学防災研究所年報 第 58 号 B 平成 27 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 58 B, 2015

Flume Experiment on Bedload Measurement with a Plate Microphone
Takahiro KOSHIBA (1), Tetsuya SUMI, Yasuhiro TAKEMON, and Daizo TSUTSUMI(1) Graduate School of Engineering, Kyoto university
Synopsis
Measuring the bedload transport rate in rivers or hydraulic structures is of utmost interest
to understand and quantify all sediment transport related phenomena such as sedimentation,
degradation, or bed abrasion of bedrock or concrete. Two bedload measuring systems exist:
the Japanese hydrophone and the Swiss geophone. Both systems are installed on the river
bed and estimate transport rate based on sediment impact, but both have respective
disadvantages. To improve weak points while leaving their merits, a plate microphone wasdeveloped. In this study, the calibration experiment of such plate microphone in a laboratory
flume is described. In the experiments, impact data is perceived as both sound pressure andsound pulse. As a result, the nonlinear correlation between bedload transport rate and soundpressure and sound pulses are found. However, it is also clarified that fine sediments lower
than 10 mm are not detectable. A detection rate is introduced showing that transport isunderestimated in case of continuous sediment supply with high amount of transported
sediment. Finally, it is investigated that this underestimation can be linked to the sedimentbulk density and the gravel jump length.
Keywords: bedload measurement, plate microphone, sound pressure, grain size, flume
experiment1.

(Hagmann et al. 2015). Especially invert abrasion is a

Introduction

severe problem facing most SBT which is directly
connected to increasing maintenance cost (Auel and

1.1 Background
There are more than three thousands dams in Japanfor different purposes. Some of these reservoirs face

Boes 2011, Baumer and Radogna 2015, Jacobs andHagmann, 2015, Nakajima et al. 2015).

severe sedimentation problems and require urgent

It is essential to clarify the hydraulic characteristics

sedimentation management. One advanced technique inand sediment transport mechanisms in a SBT. The invert

Japan is the sediment bypass tunnel (SBT) to reduceabrasion is caused by a combination of high flowsuspended and bed load depositions in reservoirs by

velocities and a high sediment transport rates (Auel androuting the incoming sediments around the dam (Sumi

Boes 2011, Auel 2014). The sediment which is routed

et al. 2004, Auel and Boes 2011). Japan and Switzerland

downstream of the dam during floods is also important

are leading countries in SBT operation. However,regardingmeasuring and quantifying the sediment movement in atechniques have been developed to monitor suspended

SBT is challenging and a topical research subject

sediment, for example by using turbidity current meter.

― 458 ―

river

environmental

aspects.

VariousHowever, there are limited techniques for field

1.2 Existing Measurement Techniques for Bedload

observation of bedload sediment transport to understandTransport Rates

the mechanisms and to quantify transport rates.

The pipe hydrophone system is already installed in

In

a

study

at

the

HodakaSedimentationsome rivers in Japan (Fig. 1). It consists of a rounded

Observatory, DPRI, Kyoto University, bedload transport

metal pipe installed directly in the river bed. Whenrates were measured by pipe hydrophones (Tsutsumi etgravels and sand pass over the pipe, the number ofal. 2008). A hydrophone steel pipe detects the sedimenthitting particles is measured by the hydrophone

rate by the number of pulses and sound pressure caused

microphone based on sound pressure. The plateby gravel hits. Another technique developed ingeophone (Fig. 2) was developed and used in

Switzerland is the geophone containing both a steel

Switzerland, but is also used in Italy, Austria and otherplate and vibration sensor (Rickenmann et al. 2012).

European countries. The geophone system consists of a

Switzerland also has rivers in the mountains similar to

50 cm by 36 cm steel plate with a geophone sensor

Japan. A geophone system was installed the first time inmounted below, embedded in a steel frame and installed

an SBT at the Solis dam and started operation from 2013

directly in river bed. A geophone estimates sediment(Hagmann et al. 2015). Both indirect monitoringtransport rates based on the plate vibration expressed intechniques, the pipe hydrophone and the plate geophone,voltage caused by the passing sediment. The geophone

have particular weak points. For instance, thesystem was already developed in the early 1990ies

hydrophone often underestimates bedload when the

before the microphone was developed in Japan. Eachpulses are successive and overlap. Also the hydrophonemeasuring technique has the similar sediment rateeasily deforms when hit by large stones. In contrary, ameasuring principle. However, both systems havegeophone is shock-resistant but cannot detect finedifferent features caused by the difference of sensors

sediments with a diameter smaller than 2 to 4 cm

and design (Table 1).

(Rickenmann et al. 2012). To improve the weak pointsof these two techniques with leaving their advantages,
the plate microphone was developed. In this study, a
new method to measure bedload transport with steel
plate microphone is proposed and experimentallyinvestigated for various hydraulic and sedimentconditions. At present, research representatives fromJapan and Switzerland are exchanging their studies witheach other. Moreover, now the construction of Koshibu
SBT is progressing and start operation in 2016.
Therefore,

developinganinnovative

monitoring

technique to quantify bedload sediment transport is a

Fig. 1 Hydrophone installed on the Yodagiri river,
Nagano, Japan (courtesy of T.Koshiba).

topical issue.

― 459 ―

Fig. 2 Geophone installed on the Ashiarai-dani, Gifu,

Fig. 3 Plate microphone in the test flume. Design similar

Japan (courtesy of T.Koshiba).

to geophone system (courtesy of T.Koshiba).

Table 1 Comparison of Geophone and Hydrophone

1.4 Objectives
There is a gap in research regarding theexperimental investigations of plate microphones, thedesign and its configurations prior implementation in
the field. Therefore, this study attempts to optimize theplate microphone configurations for field investigations
in Koshibu SBT. One of the main objectives is tounderstand the functioning of the plate microphone forvarious sediment transport and flow conditions. The
experimental study in the laboratory flume has thefollowing objectives:

1.3 Plate Microphone

rate and acoustic pulses of the microphone.

By considering the demerits shown in Table 1, theplate microphone (Fig.3) was developed in Japan as a

To clarify the linkage between sediment transport
rate and integrated sound pressure.

method to measure the bedload transport rate. The
Microphone system consists of a steel plate like theTo find a correlation between sediment transportTo examine the minimum detectable grain sizethat can be measured by the plate microphone.

geophone and measures bedload transport rates on thebase of sound pressure like the hydrophone. Moreover,plate microphone also measures sound pressures in

2. Experimental Setup and Conditions

addition to the number of pulses. The sound pressure is
expected to help measuring sediment rate accurately.

2.1 Design of Plate Microphone Instrument

With these characteristics, the plate microphone is2.1.1 Laboratory Flume test

expected to record hitting sounds of fine sediment andalso to be robust against hitting by coarse sediment.

The experiments of this study were carried out in a
flume facility at the VAW hydraulic laboratory of ETH
Zurich, Switzerland. A schematic view of theexperimental setup and parameters are shown in Fig. 4.
The experimental setup consists of an elevated water― 460 ―

supply tank that discharges the water to a rectangular

2.7 g/cm3. The tested sediment has various shapes

flume with inner dimensions of 0.50 m width, 0.60 m

including rounded, irregular and angular particles. Fig.

height, and 8.00 m length. The plate microphone is

6 gives an example of Ds = 100 mm gravel, with Ds =placed just before the flume outlet at 7.56 m from thedecisive grain diameter.

inlet. The discharge is controlled by a gate valve. The
flow velocity is controlled by a jetbox, which has gate,

Table 2 Dimensions of tested sedimentand the flow velocity is obtained by measuring thedischarge with a magnetic flow meter and the flow depthwith point gauge measurements. At the flume end thewater plunges into a stilling basin.

Fig. 4 Schematic view of the laboratory test flume.

2.1.2 System of Plate microphone
The dimensions of the plate microphone used inthis experiment are shown in Fig. 5. The system consistsof three parts: a steel plate, a microphone and a datalogger. The geometry of the steel plate are: 49.2 cm
width, 35.8 cm length, and 1.5cm thickness. Water andsediment flows are perpendicular to the plate width. The

Fig. 6 Image of tested sediment stones of Ds = 100 mm.

microphone which detects the sound wave is installed(courtesy of T.Koshiba)

under the plate and connected to the data-logger.
2.1.4 Recording Systems
Several parameters were measured during eachexperiment, namely the (a) sound waveform, (b) pulses
with 6 amplitudes, and (c) sound pressure. The
mechanisms of these three parameters are as follows:
Fig. 5 Schematic view of plate microphone(a) Sound waveform
The sound waveform is raw data collected by the

2.1.3 Condition of Sediment Characteristics

microphone. A value of voltage was recorded every

In this experiment, five types of gravels are used.
Grain sizes and average weight of used gravel are shown

20 µs. Fig. 7 shows one example of a raw waveform
output.

in Table 2. The specific gravity of sediment σ is

― 461 ―

Fig. 7 Example of waveform data(b) Pulses
The pulse is determined by processes shown in
Fig. 8. First, the sound waveform is 20-times amplified
by a preamplifier and transformed into an absolute value.

Fig. 10 The process of transferring counting pulses into

Then, the specific frequency is extracted by a bandpass

pulses.

filter and an envelope curve of this wave is recorded as
the waveform and amplified 10-times again (Fig. 9).

(c) Sound pressure

The extracted wave is exported to 6 different channels,

When sediment transport rate is high, the number

in which the wave was amplified 2, 4, 16, 64, 256, andof pulses becomes low or zero due to overlapping of1024 times. The amplification was made to find the besteach separated pulse. Fig. 11 shows a case where thevalue to detect the highest number of particle impacts.

number of pulses are underestimated. In this case, theFinally, the output pulses are logged by the logger if thenumber of pulses which are higher than the threshold is

amplitude is larger than the threshold value of 2 V. Fig.

only one, nevertheless the actual pulse number is eight.

10 explains this process.

To counter this problem, the sound pressure Sp is
also measured. Fig. 11 also explains the principle ofcalculating Sp. The sound pressure is the sum of thevalues which are measured with a constant samplingtime by the logger divided by a constant interval dT (inFig. 8 Flowchart of converting wave data.

Fig. 11 The example case that the number of pulses
Fig. 9 Process of conversing raw waveform data.

are underestimated and the principle of recording
sound pressure― 462 ―

this case, this interval dT is 10 seconds). Hence, Sp is the

Table 3 Experimental Conditions

average value of the output voltage.

2.2 Experimental Procedure
The experimental procedure is identical for all
tested configurations. Prior to each experiment, thesediment and plate conditions were adapted. The pumpstarts working, then the electromagnetic flow meter anda valve are used to setup the design discharge as a target
value. Then the data-logger switched on. The flow depth
was kept constant, and sediment grains are supplied
manually within 5 seconds directly downstream of thejetbox. Finally, the plate microphone sensor measured
the impacts and outputs were recorded in the PC.

3. Experimental Results and Discussions

2.3 Experimental Conditions

3.1 Relationship between Sediment Transport Rate

The experimental conditions are presented inand Sound Pressure

Table 3. The flow depth was kept constant with 0.08 m,

Fig. 12 shows the relationship between thewhereas the discharge Q and velocity V were varied two

sediment transport rate and the sound pressure fortimes, represented as low and high flow conditions. Qs is

continuous-supply

the amount of sediment. Normally, the flow velocity inconditions. Only few of the Ds = 2 mm grains were

SBT is even higher and often reaches more than 10 m/s

detected. The figure clearly depicts that the sediment of(Auel and Boes 2011).

sizes Ds = 2 mm, 5 mm and 100 mm vary widely

experiments

using

low

flow

A constant sediment weight of 50 g for both grain

between zero and about 100 mV of sound pressure

sizes, Ds = 2 mm and 5 mm, are supplied. For Ds =indicating a positive correlation of increasing Sp with

10 mm, 50 mm, and 100 mm, a fixed number of 20

increasing diameter. However, the 10 mm, 50mm, andstones was added manually to the flow.

100 mm are classified in three groups all ranging

Moreover the experiments can be divided into two
types:

continuous-supply,

andsingle-supply.

between about 60 to 200 mV impeding a clear
correlation of Sp to the grain size.

Continuous-supply is repeated 50 times for every

Fig. 13 shows the plot of both single-supply andcondition, whereas single-supply experiments arecontinuous-supply experiments of Ds = 100 mm at low

carried out one time throwing 10 stones consecutively

flow conditions. The straight line connects the origin(conducted for Ds = 2 mm, 5 mm, 10 mm, and 50 mm).

and single-supply uniform grainsize experiment (Ds =For Ds = 100 mm, the experiment is repeated two times

100 mm, average of throwing two times three

throwing only three consecutive stones. The purpose ofconsecutive particles) assuming that the sound pressure

single-supply type is to examine the output of a single

is proportional to the supplied sediment weight.

particle.

However, analyzing the continuous-supply experiments― 463 ―

reveals that with increasing impact number, hits of

3.2 Relationship between the amplitude and thedifferent particles will overlap, which damps the soundnumber of pulses

pressure lower than the proportional relationship. The

Fig. 14 shows the relationship between thedata of continuous-particles experiments appear to be

amplitude and the number of pulses for high flow

lower than what is expected from the relationshipconditions of Ds = 10 mm. This graph reveals that theassumed for the single-supply experiments. In order tonumber of pulses increases with increasing amplitude.

confirm this theory, it’s necessary to have more

The reason is that more particles exceed the amplitude

experiment with a wider range of sediment transport

threshold, thus more particles are detected.

rates.
1000

120

10mm

100
80
60

100

40
20
0

2

4

16

64

256

1024

Amplitude [-]102mm

Fig. 14 Relationship between amplitude and the number

5mm
10mm

of pulses in the continuous-supply (High flow, Ds = 10

50mm

mm)

100mm

1
1

10

100

1000

10000

100000Bedload transport rate [g/sec]

3.3 Analysis of Plate Microphone Governing

Fig. 12 Relationship between sediment transport rate

Parametersand sound pressure for continuous-supply (Low flow)

3.3.1 Pulses Detection Rate
To know how much sediment is detected by the500

plate microphone compared to the total sediment rate,
the detection rate Rd was defined. Rd is defined as the

400

number of detected pulsed divided by total supplied
number of gravels. This means that for Rd > 1.0 a

300

particle hits the plate more than one time.
Fig. 15 and Fig. 16 show the relationship between

200

Rd and the amplitude in order to identify the best
amplitude to detect all particle impacts. Fig. 15 shows

100

the low flow and Fig. 16 the high flow conditions,

100mm single
100mm continuous

respectively.

0
0

2000

4000

6000

8000

10000

12000

14000

These figures reveal that Rd of Ds = 2 mm and 5

Bedload transport rate [g/sec]

Fig. 13 Correlation between sound pressure and bedload

mm are low making it hard to estimate an accuratetransport of continuous-supply and single-supply (Lowsediment transport rate. However, the detection rate for

flow, Ds = 100 mm).

particles with Ds > 10 mm is good in general for both

― 464 ―

low and high flow conditions, if a minimum amplitude
2.5

of 16, better 64 or 256 is considered.
However, there are differences between the low

50mm

2

flow and high flow conditions for Ds = 50 mm and 100

100mm

mm. In both cases Rd increases with amplitude, but for

1.5

the low flow case Rd even exceeds 1. That can be

10mm

explained by the bedload transport mode, where1

particles are rolling and hitting the plate more times. For
the high flow experiment, Rd does not exceed 1 for

0.5

5mm

amplitudes 16 to 256. It is assumed that the gravelsaltates due to high flow velocity and does not hit the

2mm

0
1

10

100

1000

10000

plate often. However, surprisingly the amplitude 1024

Amplitude [-]largely exceeds Rd = 1. The reason is that the water

Fig. 16 Relationship between detection rate andinduced noise causes values Rd > 1 at the highest

amplitude of continuous-supply (High flow)

amplitude. According to both figures Rd for Ds = 10 mm
is around 0.5 for amplitudes 64 and 256 which can be

3.3.2 Saturation Rate
One governing parameter which influences theseen as a satisfying result to detect this particle size.
Finally, it can be concluded that the optimal

detection rate is the number of particles transported overthe plate at the same time representing a bulk density.

amplitude is 64 or 256.

Hence, the saturation rate R s is proposed as:
Rs =

2.5

(1)

Here, ∆t: impact time caused by one particle hit on the

100mm

2

∆t × PnT

plate microphone [s], Pn : the number of particle used in
one experiment, T: The time between first collision of

1.5

first particle to the last of the last particle in one

50mm

experiment. Fig. 17 explains these parameters.

1

10mm

0.5

5mm
2mm

0
1

10

100

1000

10000

Amplitude [-]Fig. 15 Relationship between detection rate andamplitude of continuous-supply (Low flow)

Fig. 17 Parameters which compose saturation rate
Fig. 18 shows the relationship between R d and R susing amplitude 64 and 256 for Ds = 10 mm, 50 mm,
100 mm, and amplitude 256 for Ds = 5 mm. According― 465 ―

to this figure, as R s increases R d also tends towhere G𝑠 : relative density, g: gravitational acceleration

decrease. When R s rises, both the waveforms whichand shear stress τb isinterfere each other and the sediment transported
τb = ρu∗ 2

without hitting on the plate microphone increase leading(5)

to a decreased value of Rd.
where ρ: density of water and u∗ : shear velocity:

2.5
Low flow, Horizontal plate
High flow, Horizontal plate2(6)u∗ = √ghib
1.5

1With ib = gradient of channel or energy line gradient.
Since ib of the testing flume in this experiment is 0°,0.5

ib is represented by the energy gradient. Thus,
probability based on jump height

0
0.5

1

1.5

2

2.5

3

3.5

4

P(Hp )

andprobability based on jump length P(Lp ) was calculated

Rs [-]

as follows.

Fig. 18 Relationship between saturation rate anddetection rate of continuous-supply (Ds = 50mm,

P(Lp ) =Amplitude: 256)

B
Lp

(7)

with B = plate length.

3.3.3 Jump Length and Jump Height
It is obvious that gravel and sand jump length andCalculation results of Eq. (7) are shown in Fig. 19,

height affect the relationship with R d . Jump length andwhere R d is given as a function of P(Lp ). Similarheight for supercritical flows can be calculated as

markers represent each diameter, and transparent

follows (Auel 2014):

symbols represent low flow and black one high flow
conditions. Fig. 19 reveals that P(Lp) increases withHP
= 5.9θ + 0.65
Ds

(2)

Lp
= 251θ
Ds(3)

decreasing fluid velocity. At high flow velocities,
particles tend to jump very long and hence do not hit theplate often, whereas at the low flow, particles tend to rollor only jump short distances and therefore show a higher
detection rate Rd. However, particles Ds ≥ 50 mm show

Where, HP : jump height, Lp : jump length. θ: Shield’ssatisfying Rd ≥ 1 even for the high flow. That means thatparameter. θ is calculated by following formulae

in average every particle hits the plate once although
P(Lp) is only about 20%. Consequently not only theθ=τb
ρG𝑠 gDs(4)jump length but also the height Hp has to be considered
to calculate the hitting probability.

― 466 ―

curve and formulization of detection rate to propose an

2

accurate measurement of bedload transport rate. An
ideal fitting curve would be calculated by carrying out
1.5

more flume experiments with wider varied conditions
and calibration of prototype data from the river andsediment bypass tunnel.

1

0.5

0
0.1

0.2

0.3

0.4

0.5

0.6

0.7

P(Lp) [-]

Fig. 19 The relationship between hitting probability
P(Lp) and detection rate of continuous-supply Rd

Fig. 20 Concept of correlation between sound pressure(Amplitude: 256). Black marker: high flow, transparent

and bedload transport rate

marker: low flow.
4. Conclusion and Outlook
3.4 New Concept of Correlation between Sound
Innovative techniques to measure the bedload

Pressure and Bedload Transport Rate
In order to estimate sediment transport rate in thetransport rate such as hydrophone and geophone areriver or sediment bypass tunnel, formulization of thepresented herein. However, it is known that bothcorrelation between sound pressure and bedload

systems have particular weak points about durability

transport rate is necessary as shown in Fig. 14. In thisand minim detectable grain size depending on

section, a new concept for the formulization is proposed

instrumentation technical characteristics. The present

which should be implemented based on morestudy introduces a new instrument that combines theexperimental and field data. Fig. 20 explains thisadvances of both measuring systems namely the plateconcept. In this figure, a correlation between bedload

microphone. Several hydraulic, sediment, geometricaltransport rate (BTR) and sound pressure is shown as

conditions have been tested in a laboratory flume andcurves of Rp and P(Lp ) . The lower curve alreadyessential properties and optimized parameters of plate

considers the detection and saturation rate. The “Ideal Sp

microphones are revealed. Moreover, a correlation

curve” describes the fitting curve under the condition

between sound pressure and bedload transport rate isthat all sediment hit the plate microphone once. Hence,developed showing nonlinear behavior. It can be

this fitting curve depends on the flow and plateconcluded that, the ratio of the sediment rate which

conditions and varies at every river. The presented

passed over the plate microphone can be estimated if theconcept is useful to enlarge the estimation of sedimentdetection rate is known. The optimal amplitude to detect

transport with flow conditions being beyond thealmost all stones Ds ≥ 10 mm is 64 times or 256 times

experimental ones. For example, for high flows withfor plate microphone. The detection rate is a significant

large stones. It is necessary to clarify the ideal fitting

variable affected by saturation rate and hitting― 467 ―

probability which is based on jump length and jump

„Dams under changing challenges“ (A.J. Schleiss &height. Further investigation of various ranges of

R.M. Boes, eds.), 79th Annual Meeting, Lucerne.

detection rates are necessary to accurately measure andTaylor & Francis, London, UK, pp. 403-412.

estimate the bedload transport rate. It is suggested that

Baumer, A. and Radogna, R. (2015). Rehabilitation of

future studies should include experiments with more

the Palagnedra sediment bypass tunnel (2011-2013).

varying conditions to observe the movement of particles

Proc. Int. Workshop on Sediment Bypass Tunnels,in the water flow. The following conditions are

VAW-Mitteilung 232 (R. Boes, ed.), ETH Zurich,suggested in detail: Hydraulics: Higher flow velocity of

Switzerland, pp. 235-245.

about 10 m/s to account for typical conditions in

MIZUYAMA, T., MATSUOKA, M., and NONAKA, M.

prototype SBTs. Sediment: Experiments with tracking(2008): Bedload measurement by acoustic energy withthe movement of sediment with a high speed camera.

Hydrophone for high sediment transport rate. Journalof the Japan Society of Erosion Control Engineering,61(1), 35-38.

Acknowledgements
For writing my thesis and accordingly this paper,

Hagmann, M., Albayrak, I. and Boes, R.M. (2015).Field

a lot of persons helped me. Therefore, I would like toresearch: Invert material resistance and sediment

thank every of my cooperators. First, I sincerely express

transport measurements. Proc. Int. Workshop on

the greatest thanks to my supervisors Prof. Dr. Tetsuya

Sediment Bypass Tunnels, VAW-Mitteilung 232 (R.

Sumi and Assoc. Prof. Dr. Yasuhiro Takemon. Second, I

Boes, ed.), ETH Zurich, Switzerland, pp. 123-135.

thank to Dr. Daizo Tsutsumi. He helped me a lot in

Mizuyama, T., Tomita, Y., Nonaka, M., and Fujita, M.

Hodaka sabo observatory and experiments in Zürich.

(1996): Observation of sediment discharge rate using a

Moreover, I acknowledge Assoc. Prof. Dr. Sameh

hydrophone. J. Jap. Soc. of Erosion Control Eng, 49(4),Kantoush, Dr. Sohei Kobayashi, and Dr. Christian Auel.

34-37.

They gave me really meaningful advises and kindlychecked this paper. I greatly appreciate the support of all
members of VAW in ETH Zürich. They not only let us
use the experimental flume but also helped so much.

Nakajima, H., Otsubo, Y. and Omoto, Y. (2015).
Abrasion and corrective measures of a sediment bypass
system at Asahi dam. Proc. Int. Workshop on Sediment

Finally, I deeply thank my laboratory members. Their

Bypass Tunnels, VAW-Mitteilung 232 (R. Boes, ed.),help made me have good environment to write my thesis

ETH Zurich, Switzerland, pp. 21-32.

and this paper.

Rickenmann, D., Turowski, J. M., Fritschi, B., Klaiber,A., and Ludwig, A. (2012): Bedload transport

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