International Symposium on “Appropriate technology to ensure proper Development, Operation and
Maintenance of Dams in Developing Countries”, Johannesburg, South Africa, 18 May 2016
© SANCOLD, ISBN 978-0-620-71042-8

DEVELOPMENT OF A BEDLOAD TRANSPORT MEASURING
SYSTEM FOR SEDIMENT BYPASS TUNNELS IN JAPAN
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T Koshiba , T Sumi , D Tsutsumi , SA Kantoush and C Auel
1. Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University,
Kyoto, JapanPRESENTER: T KOSHIBAABSTRACT
Sediment Bypass Tunnels are operated to divert sediment around reservoirs reducing reservoir
sedimentation. A major drawback of these tunnels is severe invert abrasion due to high velocity andsediment flows. There is an urgent need to establish innovative measurement systems of sedimenttransport rates in SBTs. In this paper, three bedload measuring systems, namely hydrophones,
geophones, and newly developed plate microphones are introduced and compared. The Koshibu SBT
is planned to operate from 2016. Plate microphones combined with geophones and other plannedsystems are installed in the tunnel. Results of preliminary tests and installation plans of bedload
measurement are presented.
1.

INTRODUCTION

There are around three thousands dams in Japan for different purposes. Some of these reservoirs arefacing severe sedimentation problems and urgent sedimentation management is required.
Sedimentation in reservoir causes several problems, namely: (1) Blockage of bottom outlet and intake
structures; (2) Losing water storage volume; and (3) Increasing flood risks by aggradation of upstream
river bed; (4) Increasing suspended sediment concentration. Moreover, sedimentation also affects thedownstream reaches as the lack of sediment downstream leads to river incision, deterioration of theriver ecosystem and coastal erosion. Strategies to counteract sedimentation in reservoirs are typicallysediment yield reduction, sediment routing, sediment transfer and sediment removal (Morris & Fan
1998, Kondolf et al. 2014). One advanced routing technique is the sediment bypass tunnel (SBT) to
reduce suspended and bed load deposition in reservoirs (Sumi et al. 2004, Auel and Boes 2011). A
SBT mitigates sedimentation by routing the incoming sediments around the dam into the downstream
river directly during flood events. Japan and Switzerland are the leading countries in SBT construction
and have the oldest tunnels in operation. However, more SBT exist worldwide, and recently a numberof tunnels are planned in Taiwan (Table 1). In the figure, data about section shape includes thesection width bt and height ht or tunnel diameter Dt. The tunnel length L, tunnel slope S, the reservoir
volume V and the catchment are A are also shown respectively. It is confirmed that a SBT works
successfully to divert sediments to the downstream reach (Auel et al. 2016). However, measuring andquantifying the sediment transport rate is still one of challenging research subjects (Hagmann et al.
2015). Especially invert abrasion is a severe problem occurring in most SBTs which is directly
connected to increasing maintenance cost (Auel and Boes 2011). Figure 1 shows the present
abrasion situation of Asahi dam SBT in Nara prefecture, Japan where the abrasion situation is severe(Nakajima et al. 2015). The tunnel invert is abraded largely, and rock and steel reinforcements are
exposed.
In order to improve the invert resistance, it is essential to clarify the hydraulic characteristics andsediment transport mechanisms in SBTs. The invert abrasion is caused by a combination of high flow
velocities and a high sediment transport rates (Auel and Boes 2011, Auel 2014). The sediment which
is routed downstream of the dam during floods is also important regarding river environmental
aspects. Various techniques have been developed to monitor suspended sediment, for example byusing a turbidity meter or sample bottles. However, there are limited techniques for field observation of
bedload sediment transport to understand the mechanisms and to quantify transport rates.

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Table 1. List of SBT in Japan and Switzerland (Sumi et al. 2004, Auel & Boes 2011)bt

ht or Dt

L

Run Time

V

A

[-]

[days/a]

3

[m ]

[km2]

0.03

~200

0.15

390

0.014

4

0.48

50

268

0.043

~100

0.06

25

360

0.026

10

0.4

108

6.20

1760

0.02

2-5

4.26

140

3.45

3.42

450

0.04

1-5

0.5

25

4.40

4.68

968

0.019

1-5

4.1

900

Archway

2.90

2.90

258

0.013

0.76

10

1998

Archway

3.80

3.80

2384

0.029

15.47

39

2004

Horseshoe

7.80

7.00

4300

0.01

29.95

311

Reservoir
nameCountry

Commissioning year

Section
shape

[m]

[m]

[m]

Pfaffensprung

Swiss

1922

Horseshoe

4.70

5.23

282

Runcahez

Swiss

1962

Archway

3.80

4.27

572

Ual da Mulin

Swiss

1963

Circular*

2.50

3.70

Egschi

Swiss

1976

Circular*

1.20

2.80

Palagnedra

Swiss

1978

Circular*

3.70

Rempen

Swiss

1986

Circular*

Solis

Swiss

2012

Archway

Nunobiki

Japan

1908

Asahi

Japan

Miwa

Japan

S

~100

Circular*: Circular shape with plain invert
SBTs in the process of construction: Koshibu (Japan), Matsukawa (Japan), Nanhua (Taiwan), Shimen (Taiwan),
Zengwen (Taiwan), Rizzanese (France), Nagle (South Africa)

Figure 1. Accumulated abrasion depth (1998 to Nov.2011) at Asahi dam SBT㸦㸦Nakajima 2015㸧
2.

BEDLOAD MEASURING SYSTEMS

Measuring the bedload transport rate is important to support managing appropriate SBT maintenanceworks and evaluate the efficiency of the tunnel. Moreover, it is known that the volume of sedimentyield in mountainous area does not only depend on flow discharge of the river but also on the scale ofsediment supply origin and the seasonal variation. Therefore, it is essential to monitor the amount of
sediment yield from mountains for comprehensive sediment management in the basin and mitigationof sediment or river related disasters.
2.1

Existing Measuring Methods

In a study at the Hodaka Sedimentation Observatory, DPRI, Kyoto University, bedload transport rates
were measured by pipe hydrophones (Tsutsumi et al. 2014). As shown in Figure 2a, a hydrophoneconsists of a steel pipe and a microphone installed in it. The sediment transport rate is detected by the
number of pulses and resulting sound pressure levels caused by gravel hits. Another techniquedeveloped in Switzerland is the geophone (Figure 2b) containing of a 36 by 50 cm steel plate and avibration sensor below (Rickenmann et al. 2012). A geophone system was installed the first time in an
SBT at the Solis dam which started to operate from 2012 on (Hagmann et al. 2015). Principles of bothindirect monitoring techniques, the pipe hydrophone and the plate geophone, are similar but they haveparticular weak points. Their cons and pros are shown in Table 2. The most remarkable difference is
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that the hydrophone often underestimates bedload when the pulses are successive and overlap
because the hydrophone pipe wide in the streamwise direction compared to 36 cm for the geophone
plate. Furthermore, the hydrophone may easily deform when hit by large stones. In contrary, ageophone is shock-resistant but the minimum detectable grain size is 10mm to 40mm (Rickenmann etal. 2012). Figure 3 shows the temporal changes of hydrophone impulses (1 min averaged) observed
at Erlenbach watershed during a rainfall event on July 29, 2013. In the figure, (A) is Hydrophone’s
result, and (B) is Geophone’s one (Tsutsumi et al. 2014). It is evident that reactions of both systems
are similar and worked well. A major difference is the number of pulses detected. The hydrophone
detects about 600 times more than the geophone. This is due to two effects, the hydrophone candetect fine sediments, and the acoustic wave has a larger frequency compared to the vibration wave.

Figure 2. a) Hydrophones installed at Yodagiri River, b) Geophones installed at Ashiarai dani,
Hodaka Observatory, DPRI, Kyoto University in Japan

Figure 3. The difference of observed impulses between (A) Hydrophone and (B) Geophone(Tsutsumi et al. 2014)
Table 2. Pros and Cons of Hydrophone and Geophone
Hydrophone

Geophone

Made inJapan

Switzerland

Shape

Steel Pipe

Steel Plate

Principle

Counting number of pulses based on
sound pressure

Measuring voltage of vibration caused
by impacting sediment

Merit

z

Inexpensive system setupzStrong for large stonesz

the minimum detectable grain sizeis 10mm to 40mmz

Thicker plate can deal with larger
stones up to decimetresz

Deforms easily when large stones
hitzInitial installation cost is highzzOverestimate bedload rate duringhigh sound or large amount of
sediment

Minimum detectable grain size is
10mm to 40mm

Demerit

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2.2

Plate microphone and vibration sensor

A plate-microphone was developed in Japan as a method to measure the bedload transport rate toovercome disadvantages of the pipe-hydrophone as shown in Table 2. The plate-microphone system
consists of a steel plate like the geophone and measures bedload transport rates based on soundpressure like the pipe-hydrophone. With these characteristics, the plate microphone is expected torecord hitting sounds of fine sediment and also to be robust against hitting by coarse sediment.
Additionally, a plate microphone contains a vibration sensor which has a different resonancefrequency from a geophone in order to conduct more accurate observation using vibration data. Figure
4 and 5 show an inside view of the geophone and plate microphone, and a schematic view of a plate
microphone, respectively.
The calibration experiments of these two devices were carried out in a laboratory flume at theLaboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich, Switzerland (Figure 6)(Koshiba et al. 2016). The flume was 7.56 m long, 0.5 m wide, and the intake was equipped with a jetbox to allow for precise flow depth adjustment. The devices were installed at the end of the flume. The
flow depth of 80 mm and five grain size fractions were used: Mean diameter D = 2 mm, 5 mm, 10 mm,
50 mm, and 100 mm. Two flow velocities were tested: U = 2.7 m/s and 4.5 m/s. Although these values
are below the ones of about 10 m/s in a prototype SBT (Auel & Boes 2011), they are higher thantypical values in rivers and considered to be high enough to gain insight into the high-speed flow
behaviour in a SBT. The impacts of hitting grains were recorded in a logger based on soundmeasurements at the plate microphone and vibration measurements at the plate sensor.

Figure 4. The inside of geophone (left) and plate microphone and vibration sensor (right)

Figure 5. Schematic view of plate microphone and vibration sensor

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Figure 6. Sketch of a model flume at ETH Zurich
It was found that both devices can detect fine sediments of 2 mm and 5 mm grain size. Especially, it isfound that the plate vibration sensor can detect the fine grain sizes. Figure 7 shows the wave form ofgrain hitting which explains the energy where x-axis and y-axis are time and voltage, respectively. The
upper graph is plate microphone and bottom one is the plate vibration sensor. Left (a) and right (b) arecases of grain size of 100mm and 2mm, respectively. Both water velocity was fixed to 4.5m/s. Figure7(b) shows that only vibration sensor detects hitting of 2mm grain clearly, and the time width of avibration sensor wave which caused by one grain’s hitting is shorter. It insists that waves by vibration
sensor is hardly to interference with other waves. The experiments also clarify that almost all of theseresults are, however, underestimated or overestimated from the actual particle numbers. It leads theunderestimation or overestimation of bedload transport rate, therefore it is essential to quantify anequation which convert the measured bedload transport rate into correct one by using flow andsediment conditions like jump length, sediment density, flow velocity and so on. Some relationships
were already found by the experiment, but it is require to carry out more flume experiments to clarify
the relationship. Moreover, it is also required to conduct in-situ measurement using real scale
sediment volume and grain sizes, because the flume experiments are conducted with a little amount of
bedload sediment load comparing with the actual condition in SBT. Regarding the problem, acalibration experiment with using real SBT will be started in concurrence with the commencement of
the Koshibu dam SBT operation in 2016. Likewise, the geophone has been already installed in the
Solis dam SBT in Switzerland and started operation. In the next section, details of these observations
and results in the Solis SBT and expected results in the Koshibu SBT are introduced.

Figure 7. Wave forms (a) 100mm, (b) 2mm, top red is acoustic wave, and bottom yellow isvibration one
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3.1

ON SITE APPLICATION OF BEDLOAD MEASURING SYSTEMS
Solis SBTThe Solis dam is located in Switzerland and impounds the Albula river in the canton of Grisons in theeastern part of the Swiss Alps. It is operated by the electric power company of Zurich (EWZ). The dam
was built in 1986 and is 68 m high with a width of 75 m. The initial capacity of the reservoir was about
4.1 million m 3, and the annual inflow is about 800 million m 3 (Auel et al. 2011). This value is a large
compared to the reservoir volume because the catchment area of the reservoir is approximately 900
23
km . Due to the large inflow, the reservoir suffered from 80,000 m of sedimentation annually on
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average. Sedimentation in the reservoir reached to the minimum operation level and active storage
capacity has started to decrease. The operator decided to build a SBT which started operation in 2012
(Figure 8). The length is 968 m, the cross section shape is archway with 4.40 m width and 4.68 m
height. In order to measure the sediment flow, eight geophones have been installed at the tunnel
outlet in a cross-sectional way (Figure 9). The geophones help to know temporal and transversal
distribution of sediment transport, and to improve the tunnel operation and quantify the abrasion in thetunnel. Furthermore, the geophones are part of an in-situ monitoring system including turbidity metersand geophones upstream of the reservoir in order to quantify the total sediment budget. As the resultof observation by geophones, uneven bedload transport rate in the cross sectional direction was
found. This result is considerable to explain a secondary flow currents due to a curving portion of thetunnel located 100m upstream of the outlet (Hagmann et al. 2015).

Figure 8. Outlet of the Solis SBT (courtesy of C Auel).

Figure 9. Installation of geophones at the tunnel outlet
3.2

Koshibu SBT

The Koshibu dam is located in the Koshibu river basin in Nagano prefecture, Japan, and is operated
by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (Figure 10). The dam was built in
1969, and is 105 m high with a width of 293.3 m. This reservoir suffered several severe floods withhigh sediment inflows causing rapid sedimentation progress. In 2015, the sedimentation volume
exceeds 15.6 106 m3 almost reaching to 20.0 106 m3 of original designed sedimentation capacity.
Tunnel construction works have been almost completed and operation will start from spring 2016. The

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length of SBT is 3,982 with a cross section of circular shape and a plain invert. The width and heightare 5.5 m and 7.9 m, respectively.
Similar to the Solis SBT, bedload transport rate measurements are planned in the Koshibu SBT.
Figure 11 shows the SBT outlet where the measurement devices will be installed. Figure 12 shows thearrangement of measurement devices at the outlet where the line AB is corresponded to the line AB in
Figure 11. Seven plate devices where both microphones and vibration sensors are mounted below, as
well as two pipe hydrophones will be installed. The plate thickness is 15 mm (t15) for all plates despite
one, where it is reduced to 12 mm (t12) in order to validate the detection sensitivity. The pipehydrophones are attached in order to directly compare the difference of detection sensitivity andphysical resistance to the plate hydrophones. Five t15 plates will be installed parallel in the spanwise
direction. Additionally, the last t15 plate in the tunnel centre line will be inclined with 10 degree. Twopipe hydrophones will be installed symmetrically perpendicular to the flow. The inclination is
introduced in order to verify an increase of detection rate due to the plate inclination toward the flow
direction (Auel and Boes 2011).
In order to calibrate the devices, the amount of transported sediment has to be correlated to thenumber of impacts recorded. Laboratory experiments, however, are limited to confirm high velocity
and large volume of sediment with mixed grain sizes. Hence, in-situ calibration experiments areplanned in the tunnel before the start of normal operation in 2016. Measured amount of sediment will
be put in the inlet of the SBT by trucks and then flushed through the tunnel by short gate opening. The
measurement devices will record the impacts by the transported sediment with using both the plate
microphone and the plate vibration sensor. With this test setup, precise calibration of the sensors will
be possible since the amount of transported sediment is exactly known.
Through these measurements, it is expected that; 1) the cross sectional bedload transport will beasymmetric on the inner side of the curve due to a right curve upstream of the outlet; 2) plate t12 may
detect finer sediment than t15; 3) plates with inclination should detect more amount of sedimentcompared to no inclination (Auel & Boes 2011b).

Figure 10. Map of Koshibu reservoir and SBT location

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A

B

Figure 11. Outlet of the Koshibu dam SBT

Figure 12. Plan view of hydrophone, plate microphone and vibration sensors installedon the Koshibu dam SBT cross sections
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4.

CONCLUSION

Around the world, there are a lot of existing dams with reservoir sedimentation problems. The current
and the next generation of engineers has to ensure to maintain their important functions. Due toclimate change, it is expected that the sedimentation problem will be more severe in the future withhigher rates of estimated incoming sediments. Many of aged dams in Japan are suffering from such aproblem. In order to maintain their functions and realize a sustainable use of dams, sediment bypass
tunnels are an effective method of a sediment routing in Japan, Switzerland and more and more
worldwide. Although a SBT is highly effective and ecologically favourable, one of serious problems is
the invert abrasion existing at many SBTs.
To estimate and establish countermeasures against the problem, it is essential to clarify the bedloadtransport rate. Moreover, establishing a suitable method to measure bedload transport is alsomeaningful from the aspect of comprehensive river basin management. As existing methods,
Japanese hydrophones and Swiss geophones are presented herein. Considering that both systems
have particular weak points about durability and minimum detectable grain size, the newly developed
plate microphone and plate vibration sensor are also introduced here. Even though their measuring
accuracy is not exclusively analysed yet, it is shown that the experimental results are promising.
Furthermore, both the ongoing bedload transport observation using geophones at the Solis SBT,
Switzerland and the newly planned observation at the Koshibu SBT in Japan are introduced.
Additional in-situ calibration experiments at Koshibu are to be conducted.
5.

ACKNOWLEDGEMENTS

The authors acknowledge the support of the Ministry of Land, Infrastructure, Transport, and Tourism(MLIT) by sharing the information about the Koshibu SBT. Many thanks also go to VAW of ETH Zurichfor excellent collaboration during the experimental calibration tests. The last author acknowledges thefinancial support of the Japanese Society for the Promotion of Science (JSPS). This work was
supported by JSPS KAKENHI Grant Number 26257304 and by regional research project of river andsediment management technology development by MLIT.
6.

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