Annual Journal of Hydraulic Engineering, JSCE, Vol.55, 2011, February
Annual Journal of Hydraulic Engineering, JSCE, Vol.55, 2011, February

EVALUATION OF SEDIMENT BYPASSEFFICIENCY BY FLOW FIELD AND SEDIMENTCONCENTRATION MONITORING TECHNIQUES
Sameh A. KANTOUSH1, Tetsuya SUMI2 and Mitsuhiro MURASAKI3
1Member of JSCE, Dr. of Eng., Senior researcher, Water Resources Research Center, Disaster Prevention ResearchInstitute, Kyoto University (Goka-sho, Uji-shi, 611-0011, Japan)
2Member of JSCE, Dr. of Eng., Professor, Water Resources Research Center, Disaster Prevention Research Institute,
Kyoto University (Goka-sho, Uji-shi, 611-0011, Japan)
3Engineer, Oyo Corporation, Miyukigaoka 43, Tsukuba, Ibaraki, 305-0841, Japan.

This paper evaluates the performance of the sediment bypass tunnel that diverts the suspended
sediment from Lake Miwa to downstream reaches below Miwa dam. In such a complex processes duringbypassing, it is difficult to predict the sediment bypassed efficiency numerically. Therefore, the digital
image techniques were developed and implemented to measure the flow pattern and suspended sedimentconcentration SSC for the first time in Miwa dam. Where, field measurements were conducted during
flood seasons in June and July 2010. Surface flow velocity and SSC were analyzed to clarify theperformance before and after the commencement bypassing operation mode. The measured sediment
bypassed efficiency assists the decision making on optimization of bypassing performance and validates
numerical prediction. The facility proved its effectiveness in mitigating reservoir sedimentation in Miwa
dam and turbidity of river water contributing towards restoration of downstream environment.

Key Words: Miwa Dam, sediment bypass tunnel, removal efficiency, digital image techniques, LSPIV, turbiditymeter

1. BACKGROUND
Often, the sedimentation problem is a criticalelement in the economic feasibility of a project,particularly when each year large quantities ofsediment material have to be dredged and disposed
at far-field locations. Worldwide sedimentmanagement techniques are classified and someexamples of representative dams from Japan andEurope are listed1). Sediment bypass tunnel in Miwa
dam is the first experience for multipurpose dams inJapan, as sediment management technique which
diverts mainly suspended sediment concentration.
Miwa Dam located on the Mibu River, a tributaryof the Tenryu, about 20 Million Cubic Meter(MCM) of sediments were deposited by 2002 since
the completion of dam. About 5 MCM of deposited
sediments were excavated by gravel quarrying and 2
MCM were excavated and transported by trucks2).
The bypass scheme includes upstream check dam of
140 m long, a 4.3 km long sediment bypass tunnel, a
230 m long diversion weir, and some auxiliary

facilities (trapping weir and two training dikes were
provided). Asahi sediment bypass tunnel is anotherexample that divert total sediment load (coarse andfine sediment). The number of bypass tunnel is
expected to increase in the future, which are underconstruction (Koshibu and Matsukawa dams), or
under planning (Yahagi, and Sakuma dams)3).
The issues of sediment bypass technique to beresolved in the future are the clarification of thehydraulic behavior of flow and sediment in tunnels
for the purpose of designing safe and economical
sediment tunnels, as well as the establishment ofcountermeasures for abrasion damages on channel
bed surface4).
Several parameters were monitored namely,
precipitation in the upstream, turbidity, water
quality, fish, benthic animals, attached algae, andetc2). These data can be used for switching operation
modes as shown in Fig.1. The first normal mode isthat all incoming flood flow will overflow thediversion weir into main reservoir (Mode 1). While
the second mode is some part of incoming flood

S_169flow will be diverted to bypass tunnel after opening
the bypass main gate (Mode 2). The last mode isrefilling mode by closing the bypass main gate
(Mode 3, Fig.1). Generally we can use inflow
discharge to design and guide the timing forswitching these operation modes. But we should pay
attention to sediment concentration, because theflow discharge and sediment concentration are notlinear relationship including sometime hysteresis.
For example, sediment concentration during
discharge increasing stage in the hydrograph isusually higher than that during decreasing stage.
Therefore, in order to increase the performance of
sediment bypassing and removal efficiency, we
focus on surface flow patterns and suspended
sediment concentration in the approach flow area tothe bypass tunnel according to different modes.
Incoming flood
flow

Incoming flood
flow

Check dam

Check dam

diversion
weir

diversion
weir

Miwa Dam
Mode 1Mode 2Training dikes
upstream of themain gate

Check dam

Trap weir

Log boom
Gate todecreasewater level

Study area 1

Diversion weirFishpassage

Main
gate

Camera 1, 2

SMDP station SS2

Sub‐
gate

Study area 2
Camera 3, 4

RefillFishpassageAll

Bypass flow

Some

All

management, a sediment bypass system is plannedand completed in March 2004 to reduce
sedimentation of the reservoir. Fig.2 shows theschematic diagram of the bypass systemcomponents combined with the locations of themonitoring instrumentation and studied areas.

Turbidity metermeasuring SS1

Miwa Dam
reservoir

Flood bypass
tunnelFig.2 Sediment management facility at Miwa dam and cameras
and suspended sediment measurement locations.

Mode 3

Fig.1 Switching operation modes during flood at Miwa Dam.

2. OBJECTIVES AND TARGET PROJECT
The main purposes of this paper are, to evaluate
the effectiveness of sediment bypassing system, anddevelop digital image techniques to measure SS andflow field. These will assist in operating of sediment
bypass tunnel efficiently for managing diverted
flood with wash load from reservoir. The observed
efficiencies of bypassed sediment reported in thispaper will provide a basis for determining the best
management practice for existing and future planned
bypass tunnels.

3. FIELD SITE AND METHODOLOGY(1) Outlines of Miwa dam bypass system
The multipurpose reservoir Miwa Dam iscomposed of a 69 m high gravity concrete dam and29.95 MCM gross storage reservoir volumes with
311 km2 catchment area. Since the dam wascompleted, several extreme runoff events caused
serious disasters and sediment yield. From the view
point of the eternal reservoir sedimentation

Based on some theoretical considerations andrecorded field data, the annual sediment inflow wasevaluated at an average amount of 0.68 MCM, ofwhich 0.525 MCM is wash load and 0.16 MCM is
bed load combined with suspended load. The
complex sediment countermeasure system is shownin Fig.2 and consists of the following installations: Check dam of 0.22 MCM sediment storage
capacity; the sediment is periodically excavatedand transported by private gravel agencies.
 Diversion weir of 0.51 MCM combines thefunctions of training and trapping the sediment.
Wash load that flows down through the checkdam, is directed together with the flood waterfrom a bypass channel into a bypass tunnel.
Where two cameras 3 and 4 are installed tomonitor the flow velocity in the bypass channel(Fig.2). During high flood the facility allows thetraining dikes and trap weir (Fig.2), to trap thecoarse grain sizes that overflow from the checkdam, in order not to flow into the tunnel. The
approach flow in front of the main gate ismonitored by camera 1 and 2 for flow field
analysis before and after the gate opening for

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switching modes (Fig.2). Moreover, for SS
monitoring two points with two different
instruments are installed as shown in Fig.2.
 Flood bypass tunnel of 4.3 km long, 7.5 m
diameter, 1% slope, and 300 m3/s capacity.
 Auxiliary reservoir sediment discharge facility isunder planning to discharge wash load that flows
into the reservoir together with floodwater, into
the downstream reaches as a flood control
measure.
(2) Image monitoring techniquesa) LSPIV and Fx-8100
The development of image processing hardware
and associated software tools has led to a revolution
in the scope and quantity of quantitative informationthat may be gathered from laboratory and field
experiments5). More recently, the standard PIV
method has been applied to velocity measurements
for larger scales, commonly named as Large Scale
Particle Image Velocimetry (LSPIV)6). The opticalflow Fx-8100 developed by Mitsubishi was applied
during sediment flushing of Unazuki dam in July
20067). Large-Scale Particle Image Velocimetry(LSPIV) is an extension of a quantitative imaging
technique to measure water surface velocities5). In
the present applications we utilized the CCTV
(CIT-7430) camera from Mitsubishi Coro. The flow
videos recorded at 30fps and 740 by 480 pixels.
The accuracy of the LSPIV method was tested
using two simple test cases. First, fully developed
laminar flow in the bypass approach channel withdimensions of 12 m width was measured. A
comparison of the average velocity profile in themain flow direction obtained with LSPIV andFx-8100 technique will be illustrated after. Raw
pixel displacement fields were computed using astandard cross-correlation algorithm. The first stepof the post-processing involved identification andcorrection of spurious displacement vectors using a
local-median scheme.
Fx-8100 developed by Mitsubishi Electric
Corporation which is a Video Front End Processor(FEP) and a multipurpose image recognition
platform. This is a platform aiming to recognize thevideo-image, and have a lot of image processing
functions logic and a network interface. FEP'sfunction can be enhancing by updating program
according to different applications. More details
about the technique and some applications can befound in a previous study8).
b) Suspended sediment concentration by SMDP,
turbiditymeter, sampling, & image processing
In case of SS monitoring, various existing SS
measurement techniques were compared by Wren etal.9). SS monitoring systems based on a differential
pressure transmitter (SMDP) are implemented in
Miwa dam and Kurobe River10). The turbidimeterthat uses optical backscatter and relates
nephelometric turbidity unit NTU to SS is known
for continuous and automated field sampling11).
In the present paper digital optical camera
recorded the sediment-water mixture in-situ, todetermine the SS. SMDP and INFINITY-Turbi by
JFE-Advantech (Miniature Super-High Turbidity
Data Logger with Wiper)12), were installed for
sediment monitoring in the river as shown in Fig.2.
Furthermore, river water samples were taken
constantly to calibrate these monitoring systems.
Third instrument is INFINITY-Turbiditymeter by
JFE-Advantech Company. Regarding the image
processing we analyzed a method to determine theconcentration by measuring the reduction in theintensity of light transmitted through Mode 1, as
compared to that transmitted through Mode 2. Lightattenuation technique that developed by Dalziel
research group13), were used to obtain SSC resultsfrom images. A simple calibration procedure relates
the light intensity observed at each point on thelayer surface to the corresponding SSC at that point.

4. RESULTS AND DISCUSSIONS(1) Bypass operation results
In Miwa dam, there were three times of bypass
operation after 2006. These data are summarized inTable 1. In 2010, we have two middle class flood
events which are peak flows of 150 and 250 m3/s.
Table 1 Sediment bypass operation records after 2006.
Flood date
AverageRainfall (mm)July 2006 July 2007 September 2007
253
117
254
366
166
568

3

Max. Inflow Discharge (m /s)
3Max. Bypass Discharge (m /s)
Bypass Operation time (hrs)Max. Inflow Sediment Concentration
Max. Bypass Sediment Concentration( /l)3
Total Inflow Wash Load (1000m )3

Total Bypass Wash Load (1000m )

242

136

47

35

264
48

16,9003,610

25,00016,9002,810

20,200

326

37

461

150

14

155

(2) Flow pattern before and after bypassing
LSPIV and Fx-8100 techniques were applied fortwo different study areas in front of bypass tunnel
approach flow in the reservoir of Miwa dam asshown in Fig.2. Recorded images at two modes
were treated to evaluate the approach flow in threezones, considering the possibility of the vortex
formations that can influence the bypass efficiency.
Flow field measured by LSPIV are shown in
Figs.3,4. The main flow is directed towards thediversion weir at Mode 1 and a circulation in frontof the bypass tunnel was formed (see Fig. 3(a)).

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(a)

m/s

0.0 0.033 0.065 0.097 0.129 0.162 0.194 0.226 0.258 0.290 0.323 0.355 0.387

(b)

m/s

0.0 0.042 0.084 0.126 0.168 0.210 0.251 0.293 0.335 0.377 0.419 0.461 0.503

(c)
m/s

0.0 0.054 0.107 0.161 0.214 0.267 0.321 0.374 0.428 0.481 0.535 0.588 0.642

Fig.3 Stationary flow fields and corresponding velocity in the reservoir of Miwa dam for (a) Mode 1, (b, c) Mode 2.

Bypass channel

Bypass tunnel
m/s

0.0 0.164 0.328 0.492 0.655 0.819 0.983 1.147 1.311 1.475 1.638 1.802 1.966

Fig.4 Actual field measurements by LSPIV, surface velocity vectors in the bypass channel and tunnel of Miwa dam.

The velocity magnitudes with vectors shows a
low velocity inside the reservoir around 30 cm/s asshown in Fig.3(a) right. Two zones in front of the
bypass were measured, before and after the log
boom. The flow field of zone 1 (see Fig.3(a) on theleft), reveals the effect of the bypass operation inMode 2. The separation of the flow in the reservoir
leads to a small retention in the immediate upstream
part of the reservoir and therefore to high turbidity.
This process is localized in the left side of thereservoir. This result in the flow separation on theright and left to the weir and bypass can beillustrated clearly in Fig.3(c). Interesting effects ofthe flow separations in front of the bypass, such asconcentration of inflow to the bypass and increaseof velocities up, can be detected by themeasurements Fig.3(c) on the right. The LSPIV
measurements in zone 2 (see Fig.3(c) on the right)
reveal the impact of the contraction due to the trap
weirs.

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Velocity in the tunnel during sediment bypassed (m/s)

2.5
LSPIV

Mitsubishi Fx-8100

2

1.5
zone A1zone A0.5

0

0.0 0.259 0.518 0.777 1.036 1.295 1.554 1.813 2.072 2.331 2.590 2.849 3.108

0.0 0.164 0.328 0.492 0.655 0.819 0.983 1.147 1.311 1.475 1.638 1.802 1.966

Fig.5 Average velocity measured at zone A during sediment bypassing operation in the Miwa dam bypass tunnel at selected periods

The area between the dikes on the left bankforces the main flow to the right and increases themain velocity. In the area between the dikes, thehighest flow velocities are detected on the border ofthe dike. As a result, sedimentation occurs in before
this reach. Fig.4 shows the surface velocity vectors
and corresponding velocity magnitudes obtained by
LSPIV in the bypass channel.
To compare and validate the LSPIV ability, asmall zone area A of 40 by 50 pixels (Fig.5) inmiddle of the bypass channel, is measured by
FX-8100 as well. Image analysis for video recorded
from 13th of July at 11:02 to 14th of July at 14:00,
and the results of averaged velocity of 1 minute atzone A are shown in Fig.5. Both techniques
represented and are able to monitor the behavior of
average surface velocity during bypassing (mode 2).
The surface velocity has accelerated in two or morefrom about 1.25 m/s to 2.5m/s along the center ofthe bypass channel at zone area A. The flow isalmost uniform as illustrated by the velocity
magnitude and vectors at July 14, 13:59.
Due to the increase of water surface profile atdifferent periods, the channel surface area covered
with water is reduced. Two examples of LSPIV
flow field and velocity magnitudes on July 13th and14th are shown in Fig.5. By comparing velocity
measured by LSPIV and FX-8100, it can be said
that results are in a good agreements and LSPIV canbe applied within reservoir as well.
(3) Suspended sediment concentration
Inflow suspended sediment concentration isrecorded as Fig.6. Peak concentration is almost
equal to inflow discharge peak and turbid flow offrom 2500 to 1000 mg/l was bypassed. Turbidity

trend is well monitored by several measurement
data and very much helpful to switching from Mode
1, Mode 2 and Mode 3.
200
150

Inflow Discharge (m3/s)
Bypass Discharge (m3/s)

June 19, 2010

100
50
0
4,000

Digital image technique SS (mg/l)
SMDP SS(mg/l)
Sampling SS(Diversion weir)(mg/l)
Sampling SS(Bypass tunnel)(mg/l)

3,000

2,000

1,000

Start bypassingEnd0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0

Time (hrs)Fig.6 Comparison of monitored SSC measured by four different
techniques during bypassing on 19 June 2010, Miwa dam.

(4) Efficiency of sediment bypass system
The operation record for flood event in July was
also well monitored as shown in Fig.7. Moreover, toevaluate the removal efficiency of Miwa dam
bypass tunnel the measured rainfall, inflow andbypass discharges, and SSC measured by threemethods are depicted in Figs.7(a,b,c), respectively.
The calculated efficiency is defined as mass ratio of
sediment bypassed to sediment inflow. The
sediment discharges are the product of water
discharge and SSC measured by turbiditymeter. In
Fig.7(b) the bypass discharge is equal to inflow
discharge. Suspended sediment concentration was
up to 10,000 mg/l and high turbid flow was almost
diverted effectively through the tunnel. In this case,
bypass efficiency was estimated up to about 80 %.

S_173

REFERENCES

0510(a) 15300

Average rainfall（mm）

Inflow Discharge（m3/s)
Bypass Discharge (m3/s)

200
100

(b) 0
15,00010,000Turbidity (INFINITY) [ppm]Sampling SS(mg/l)
SMDP [mg/l]

Diversion Weir

5,000

(c) 0
2.0
1.5
1.0
0.5
(d) 0.0
150,000

Qs in (m3/s)
Qs bypass (m3/s)

Efficiency (%)

100,000

50,000(e)

012-Jul13-Jul

14-Jul15-Jul

90
80
70
60
50
40
30
Qs in (m3)20
Qs bypass (m3)10
Bypass efficiency (%)
016-Jul 17-Jul 18-Jul 19-Jul

Fig.7 (a) Rainfall, (b) inflow and bypass discharges, (c) SSC
measurements, (d) Calculated inflow and bypassed
sediment, (e) calculated removal efficiency for Miwadam bypass scheme on flood events of 13 and 14 July.

5. CONCLUSIONS
Sediment bypass system in Miwa dam iseffectively routing incoming fine sediment directly
to reservoir downstream. The measured efficiencyof sediment bypassed by optical turbidity meter was
up to 80% of the total inflow sediment. Thisefficiency was also measured at selected events byimage processing which is a very useful tool incharacterizing the performance of bypass tunnel
operation. Moreover, the measured surface flow
velocities and suspended sediment concentrations in
the context of sediment management in reservoirs
are important for the validity of the numericalanalysis of Miwa dam.
The image technique methods were improved inorder to obtain flow pattern and suspended sedimentconcentration despite water surface elevation
variations, windy, rainy and foggy weather
conditions. The described methods can be used toevaluate other sediment bypass systems.

1) Kantoush, S., and Sumi, T.: River morphology and sedimentmanagement strategies for sustainable reservoir in Japanand European Alps, Annuals of Disas. Prev. Res. Inst.,
Kyoto Univ., No. 53 B, pp.821-839, 2010.
2) Enomura, Y.: Sediment management measures at Miwa
Dam, International symposium on Sediment Management
and Dams 2nd EADC Symposium, PP. 67-72, 2005.
3) Sumi, T. and Kantoush, S.A.: Stochastic sediment deposition
processes of large reservoirs in Japan, 11th International
Symposium on River Sedimentation (ISRS), South Africa,
PP. 1-11, September 2010.
4) Sumi, T.: Reservoir sedimentation management in Japan, The
3rd World Water Forum, Kyoto-Chiga-Osaka., Japan, 2003.
5) Kantoush, S.A., and Schleiss, A. J.: Large scale PIVmeasurements at the surface of different shallow reservoir
geometries, Journal of Visualization, Vol. 12, No. 4 pp.
361-373, 2009.
6) Fujita, I., Muste, M. and Kruger, A.: Large-scale particle
image velocimetry for flow analysis in hydraulicengineering applications, Journal of Hydraulic Research,
Vol.36, No.3, pp.397-414, 1998.
7) Sumi, T., Murasaki, M., Taira, K., Shinbo, K., Nagura, H.
and Tamaki, H.: Study on measurement of fine sediment
discharge process during sediment flushing operation using
PIV, Ann. J. Hydra. Eng., JSCE, Vol.51, 1075-1080, 2006.
8) Sumi, T., Murasaki, M., Taira, K., Shinbo, K., Nagura, H.
and Tamaki, H.: Study on measurement of fine sediment
discharge process during sediment flushing operation using
PIV, Annual Journal of Hydraulic Engineering, JSCE,
Vol.51, PP.1075-1080, 2006.
9) Wren, D. G., Barkdoll, B. D., Kuhnle, R. A., and Derrow, R.
W.: Field techniques for suspended-sediment measurement,
J. Hydrol. Eng., 126(2), 97–104, 2000.
10) Sumi, T., Morita, S., Ochi, T., and Komiya, H.:
Development of the suspended-sediment concentration
measuring system with differential pressure transmitter inrivers and reservoirs, Proc. hydraulics measurements &experimental methods conference, ASCE, 2002.
11) Kantoush, S. A., and Schleiss, A. J.: Optical backscatter
technique (OBS) for suspended sediment concentration
measurement in shallow reservoir, IAHR Proc. of theInternational Conference on Fluvial Hydraulics, Izmir,
Turkey, September 3 -5, 2008.
12) Sumi，T. and Baiyinbaoligao， Morita, S.: Characteristics offine sediment discharge during sediment flushing of
Unazuki dam, JSCE, Journal of Hydroscience andHydraulic Engineering, Vol.25, No.1, PP. 99-106, 2007.
13) Cenedese, C. and Dalziel, S.B.: Concentration and depth
fields determined by the light transmitted through a dyed
solution; Proceedings of the 8th International Symposiumon Flow Visualization, ed. Carlomagno & Grant. ISBN 09533991 0 9, paper 061, 1998.

ACKNOWLEDGMENT: This study is supported
by MLIT and Miwa Dam office. Authors are
expressing their great thanks to them.

S_174(Received September 30, 2010)