Q. 92 – R. 38
COMMISSION INTERNATIONALE
DES GRANDS BARRAGES------VINGT-QUATRIÈME CONGRÈS
DES GRANDS BARRAGES
Kyoto, Juin 2012-------PERFORMANCE OF MIWA DAM SEDIMENT BYPASS TUNNEL:
EVALUATION OF UPSTREAM AND DOWNSTREAM STATE AND
1
BYPASSING EFFICIENCY
Tetsuya SUMI
Professor, Dr. of Eng., DPRI, Kyoto University
Sameh A. KANTOUSH
Senior Researcher, Dr. of Eng., DPRI, Kyoto University
Shoji SUZUKI
Director, Mibu River Comprehensive Development Work Office, MLIT
JAPAN

1.

INTRODUCTION

Japan and Switzerland are the leading countries for sediment bypass
tunnels (SBPT) technique. Currently in Japan, there are 3 SBPT in operation and4 planned [1]. In Switzerland, there are 5 SBPT in operation and one of Solis dam
under constructions [2], [3]. Worldwide, limited numbers of sediment bypass
tunnels have been constructed because of topographical, hydrological or
economical conditions. In Japan, sediment bypass tunnels have been studied
most exhaustively. Although this technique involves high cost caused by tunnel
construction, there are many advantages such that it is applicable to existing
dams; it does not involve drawdown of reservoir water level and therefore no
storage capacity loss; and it has relatively small impact on environment because
sediment is discharged during natural flood events compared to sedimentdrawdown flushing which discharge accumulated sediment in a short period.

1

La performance de la galerie de dérivation des sédiments au barrage de Miwa:évaluation de l'état en amont, aval, et efficacité des dérivations.

576

Q. 95 – R. 38
The subjects of designing sediment bypass tunnels are to secure the safetyof sediment transport flow inside tunnels and to take countermeasures forabrasion damages on the channel bed surface. Among factors that significantly
relate to these problems are grain size, tunnel’s cross-sectional area, channel
slope, and design velocity. Table 1 shows examples of existing and underconstruction sediment bypass tunnels in Japan and Switzerland. In Switzerland,
five bypass tunnels have been constructed and proved an effective means tocounter reservoir sedimentation [2]. It is found that problems may arise with tunnel
abrasion, particularly if the sediment has a considerable quartz rich component.
In Japan, sediment bypass tunnels at Nunobiki Dam, completed in 1908,
and at Asahi Dam, completed in 1995, have been successfully introduced torealize sustainable reservoir management [4]. Sediment bypasses at Miwa Dam
have been completed in 2004 [5], ones at Matsukawa Dam and Koshibu Dam areunder construction, and one of Sakuma Dam is under-planning. For the purposeof designing these bypass systems, hydraulic characteristics of tunnel anddiversion weir have been studied ([6], [7], [8]).
Table 1
Sediment Bypass Tunnels in Japan and Switzerland
Galeries de dérivation des sédiments au Japon et en SuisseNo

Name of Dam

Country

TunnelCompl
etion

TunnelShapeTunnelCross
Section(B×H(m))TunnelLength(m)

GeneralSlope(%)

DesignDischarge(m3/s)

DesignVelocity(m/s)

Operation
Frequency

1

Nunobiki

Japan

1908

Hood

2.9×2.9

258

1.3

39

-

-

2

Asahi

Japan

1998

Hood

3.8×3.8

2,350

2.9

140

11.4

13 times/yr

3

Miwa

Japan

2004

Horseshoe

2r = 7.8

4,300

1

300

10.8

2 times/yr-

4

Matsukawa

Japan

Under
constr.

Hood

5.2×5.2

1,417

4

200

15

5

Koshibu

Japan

Under
constr.

Horseshoe

2r = 7.9

3.982

2

370

9

-

6

Egshi

Switzerland

1976

Circularr = 2.8

360

2.6

74

9

10days/yr

7

Palagnedra

Switzerland

1974

Horseshoe

2r = 6.2

1,800

2

110

9

2～5days/yr

8

Pfaffensprung

Switzerland

1922

Horseshoe

A= 21.0m2

280

3

220

10～15

～200days/yr

9RempenSwitzerland

1983

Horseshoe

3.5×3.3

450

4

80

～14

1～5 days/yr

10

Runcahez

Switzerland

1961

Horseshoe

3.8×4.5

572

1.4

110

9

4 days/yr

2.

2.1.

MIWA DAM SEDIMENT MANAGEMENT TECHNIQUEOUTLINES OF MIWA DAM

Miwa dam sediment bypass tunnel is the first experience for multipurpose
dams in Japan, which diverts mainly suspended sediment. Miwa dam wasconstructed in 1959 by the Ministry of Construction, located on the Mibu River, atributary of the Tenryu. Miwa Dam is composed of a 69 m high gravity concrete577

Q. 92 – R. 38
dam and 29.95 MCM gross storage reservoir volumes with 311 km2 catchment
area. The flood control plan at the time of constructing Miwa Dam assumed thatthe design flood discharge for 100-year return period to be 1200 m3/s and the
maximum outflow to be 300m3/s [5]. Also, original sedimentation capacity of about
6.6 million m3 was planned and secured, which corresponds to a 40-year
sedimentation volume.
In August 1959, right before the completion of construction, a heavy flood
which recorded flood discharge that almost equal to the design flood discharge
with a maximum flow of 1,182 m3/s and large volume of sediment deposition
occurred. After that, sediment excavation works from reservoir have been started
from 1965. In 1982, the largest flood of Mibu River that recorded the flood
discharge of 1321 m3/s exceeding the design flood discharge at the Miwa Dam
occurred, resulting in about 4.3 million m3 new sedimentation only within this year,
and the succeeding flood in 1983 delivered of about 1.6 million m3 sedimentation.
Such repeated floods have brought an enormous quantity of sediment into the
Miwa Dam reservoir, which has amounted up to 20 million m3 since thecompletion of the dam.
Check Dam
Entrance of the
Bypass Tunnel
Reservoir of
Miwa Dam

Mibu Weir(Diversion Weir)

Miwa Dam
Bypass Tunnel)
Miwa Dam

Outlet of Bypass
TunnelFig. 1
Outline of permanent sediment management measures in the Miwa Dam
Schéma des mesures permanentes de gestion des sédiments
au barrage de Miwa
Based on these conditions, Mibu River Comprehensive Development
Project including redevelopment of Miwa dam and another construction of Tokusa
Dam located upstream of Miwa Dam has been started since 1989. In 1994, the578

Q. 95 – R. 38
check dam was completed as a tentative facility of Miwa Dam, and the large
volume of excavation of the reservoir sediment started in 2000. In 2001,
construction of the permanent sediment management facilities was commenced
and a sediment bypass tunnel and a diversion weir were completed in 2005. An
overview of the present and future sediment management facilities of Miwa dam isshown in Fig. 1. 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.

2.2.

SWITCHING OPERATION MODES FOR MIWA DAM BYPASS TUNNEL

After completion of sediment bypass tunnel, several parameters have beenmonitored such as precipitation in the upstream, turbidity, water quality, fish,
benthic animals, attached algae and so on. These data can be used for switching
operation modes as shown in Fig. 2. The first normal mode is that all incoming
flood flow will overflow the diversion weir into main reservoir (Mode 1). While thesecond mode is some part of incoming flood flow will be diverted to bypass tunnel
after opening the bypass main gate (Mode 2). The last mode is refilling mode byclosing the bypass main gate (Mode 3).

Incoming flood
flow

Incoming flood
flow

Check dam

Check dam

diversion
weir

diversion
weir

Miwa Dam
Mode 1Mode 2

RefillAll

Bypass flow

Some

All

Mode 3Fig. 2
Switching operation modes during flood at Miwa Dam
Commutation des modes de fonctionnement de la galerie de dérivation pendantles crues au barrage de Miwa
The inflow discharge can be used to design and guide the timing forswitching these operation modes. But we should also pay attention to sedimentconcentration because the inflow discharge and sediment concentration are not in
the linear relationship but having sometime hysteresis. For example, sedimentconcentration during discharge increasing stage in the hydrograph is usuallyhigher than that during decreasing stage. Therefore, in order to increase theperformance of sediment bypassing and removal efficiency, we should focus on579

Q. 92 – R. 38
surface flow patterns and suspended sediment concentration in the approacharea to the bypass tunnel.
The main purposes of this paper are, to evaluate the effectiveness of
sediment bypassing system, monitor the bed and suspend loads in the upstream
reaches, and discuss the environmental impacts of SBPT on the downstream
reaches. These will assist in operating of sediment bypass tunnel effectively for
managing diverted flood with wash load from reservoir. The observed efficiencies
of bypassed sediment will provide a basis for determining the best management
practice for existing and future planned bypass tunnels. Finally we will discuss
how to increase the sediment bypassing efficiency through changing in the timingof the opening and closing the gate of the tunnel.

3.

3.1.

METHODOLOGYFIELD SITE

Fig. 3 shows the schematic diagram of the bypass system components
combined with the locations of the monitoring instrumentation and studied areas.
Based on some theoretical considerations and recorded field data, the annual
sediment inflow was evaluated at an average amount of 0.68 MCM, of which
0.525 MCM is wash load and 0.16 MCM is bed load combined with suspended
load. The complex sediment countermeasure system is shown in Fig. 3 andconsists of the following installations:
1) Check dam of 0.22 MCM sediment storage capacity; the sediment isperiodically excavated and transported by private gravel mining company.
2) Diversion weir of 0.51 MCM combines the functions of training and trapping
sediment. Wash load that flows down through the check dam is directed
together with the flood water through the main gate to the bypass tunnel.
Where two cameras are installed to monitor the flow velocity in the bypass
channel. During high flood, the submerged training dikes and the trap weir trap
the coarse sediments that overflow from the check dam. The approach flow infront of the main gate is monitored by another two cameras for flow field
analysis before and after the gate opening for switching modes.
3) Flood bypass tunnel of 4.3 km long, 7.5 m diameter, 1% slope, and 300 m3/s
capacity.

580

Q. 95 – R. 38

Training dikes
upstream of themain gate

Check dam
Turbiditymetermeasuring
SS1

Trap weir

Logboom
Gatetodecreasewaterlevel

Study area 1

Main
gate

Diversion weir
Camera 1, 2
Fishpassage

SMDPstationSS2

SubgateStudy area 2

Camera 3, 4
Fishpassage

Miwa Dam
reservoir

Flood bypass
tunnelFig. 3
Sediment management facility at Miwa dam and cameras and suspended
sediment measurement locations
Structures hydrauliques de gestion des sédiments au barrage de Miwa etemplacement des caméras et des points de mesure de la turbidité

3.2.
MONITORING TECHNIQUES OF FLOW FIELD AND SUSPENDED SEDIMENTCONCENTRATION

The development of Particle Image Velocimetry (PIV) has been studied forsurface velocity measurement in laboratory and field experiments [9]. Morerecently, the standard PIV method has been applied to velocity measurements forlarger scales, commonly named as Large Scale PIV (LSPIV) [10]. The optical flow
Fx-8100 developed by Mitsubishi Electric Co. was also applied during sedimentflushing of Unazuki Dam in July 2006 [11]. In the present applications, we utilized
the CCTV (CIT-7430) camera from Mitsubishi Electric Co. which recorded at
30fps and 740 by 480 pixels.
In case of SS monitoring, various existing SS measurement techniques
were compared by Wren et al. (2000). SS monitoring systems based on adifferential pressure transmitter (SMDP) are implemented in Miwa dam andKurobe River [12]. SMDP and INFINITY-Turbiditymeter by JFE-Advantech Co.,
Ltd. (Miniature Super-High Turbidity Data Logger with Wiper) [13], were installed
for sediment monitoring in the river as shown in Fig. 3. Furthermore, river water
samples were taken constantly to calibrate these monitoring systems.

581

Q. 92 – R. 38
4.

RESULTS OF EVALUATION AND MONITORING MEASUREMENTS IN
THE UPSTREAM REACHES OF MIWA DAM

4.1.

SEDIMENT YIELD AND INFLUENCE OF SEDIMENT

Base on the master plan, the annual average of sediment inflow is
685 000 m3. 160 000 m3 of larger particles (bed load and suspended load) aretrapped at the check dam, which has a capacity of about 200 000 m3, andexcavated for construction materials. In case a large flood occurs and the checkdam is up to the capacity, the diversion weir located downstream having acapacity of about 500 000 m3 prevents the inflow of sand and gravel into Miwa
Dam to the possible extent. Consequently, the 525 000 m3 of wash load reaching
the diversion weir consists only of fine particles, of which an annual average of
399 000 m3 is discharged through the sediment bypass tunnel. For reservoir waterlevel recovery for water utilization purposes and flood control, an annual averageof 126 000 m3 of wash load flows into the main reservoir over the diversion weir.
Fig. 4 shows the Miwa Dam sediment bypass system and sediment budgetbalance (deposited and bypassed) during actual three flood events (July 2006,
July and September 2007). During these three flood events, the bypassed
sediment was 320 000 m3 of fine sediment. The diversion weir and check dam at
the upstream of Miwa Dam trapped 520 000 m3 of coarse sediment during 2 years
as shown in Fig. 5. Total inflow sediment including that flowed over diversion weir
into the reservoir was estimated 1 180 000 m3. Therefore, 840 000 m3, 70% of thetotal inflow sediment was prevented from sedimentation in the reservoir.

◆ Downstream Miwa
SS：700mg/l

Sediment deposition: 520,000 m3 (during
October 2005- September 2007)
Miwa Dam

Tokiwa Bridge

Lake Takato
TakatoDam◆ Diversion weir and Check Dam◆ Miwa Power plants
SS： 480mg/l

Lake Miwa◆ Ijima Weir
SS：440mg/l
Diversion
weir◆ Miwa Dam Sediment Bypass Tunnel
Sediment bypassing：320,000 m3(combined total during July 2006, July
2007, and September 2007)◆Diversion
weir
SS：680mg/l

Check Dam

Prevents
sediment inflow
of 840,000m 3

Legends
Miwa Dam Sediment Bypass Tunnel
Spillway of Miwa power plants◆ Miwa Dam
SS： 280mg/l

Mibu River

Ijima
Weir

Fig. 4
Scheme of Miwa Dam bypass system and sediment budget until 2007
Schéma du système de dérivation des sédiments au barrage Miwa et budget dessédiments jusqu'en 2007582

Q. 95 – R. 38
4.2.

SEDIMENTATION CONDITIONS UPSTREAM OF MIWA BYPASS TUNNEL

To evaluate the performance of the facilities installed upstream of the
bypass tunnel, sedimentation of bed and suspended loads were surveyed. Fig. 5
shows the deposited sediment height and grain size distributions at different crosssections. During flood, a complex classification process for trapping coarse andfiner sediments were conducted by check dam, submerged and diversion weirs.
First, the coarse sediment was trapped at check dam after Ijima weir asshown in Fig. 5 (sections 27 and 26). The mean diameters of these coarse
sediments were about 7 and 3 mm at cross sections 27 and 26, respectively. The
suspended sediment load passed over the check dam was divided into threeparts: some deposited behind the check dam, some part overflows the diversion
weir toward the main reservoir, and some part diverted by the bypass tunnel todownstream reaches behind Miwa Dam. The actual deposited volume of washload was 50 000 m3 between cross section 23 and 22, which are in a good
agreement with the expected value (see table in Fig. 5). The average grain size
diameter was around 0.08 mm which was finer than that deposited at crosssections 27 and 26. The middle class sediment sizes with average diameter of
0.4mm were deposited at cross section 24 at the right side bank of the two training
dikes which were installed at the left bank before the tunnel.

Flood date

Measured
Deposited volume

EstimateddepositedvolumeSeptember
2007

About 50,000 m3

57,000 m3

累加百分率(％)
Grain size distributions of the deposited sediment100
No.22-C
No.23-C80
No.22
No.24-C
No.23
No.25
No.25-C60
No.26
No.26-C40
No.27-C% Passing

The amount of Wash load after September 2007 flood

No.24

20
0
0.001

No.27
0.01

0.1
1
Grain size diameter [mm]

10

100

Deposited height

Fine sediment (wash load) was depositedCoarse sediment as suspended and
bed load was deposited

Fig. 5
Monitoring of sediment deposition in the upstream reaches of sediment bypass
tunnel after September 2007 flood event
Surveillance des dépôts de sédiments dans les tronçons en amont de la galeriede dérivation des sédiments après l'inondation de Septembre 2007583

Q. 92 – R. 38
5.

5.1.

EVALUATION OF SEDIMENT BYPASS TUNNEL PERFORMANCEBYPASS OPERATION RESULTS

In Miwa dam, there were eight times of bypass operation after 2006 till 2011.
These data are summarized in Table 2. The maximum trial operation was on
September 2007 with the maximum wash load bypassed about 155 000 m3 during
48 hours bypass operation time. During the three floods of 2006 and 2007,
approximately 710 000 m3 of fine sediments (wash load) were transported withfloodwater into the reservoir, while the bypassed sediment was 320 000 m3. The
bypassing efficiency was around 45%.
In July 2010, we have middle class flood event which was peak flow of
230 m3/s. The bypassing efficiency was up to 60% since several preliminary small
flood events filled up the storage volume of Miwa reservoir and then the peaks of
SSC could effectively pass through tunnel on time without delay.
Table 2
Sediment bypass operation records after 2006
Archives d’opération de la galerie de dérivation des sédiments après 2006
Flood Date

July July September June June July May May
2006 200720072008 2010 2010 2011 2011
253 117254
144 87214 138 99

Average Rainfall (mm)Max. Inflow Discharge
366 166(m3/s)
Max. Bypass Discharge
242 136(m3/s)
Bypass Operation time
47
35
(hrs)
Max. Inflow Sediment
16 900 3610
Concentration (mg/l)Max. Bypass Sediment
16 900 2810
Concentration (mg/l)
Total Inflow Wash Load
326
37(1000 m3)
Total Bypass Wash Load150
14(1000 m3)
Bypassing efficiency (%) 46.0 37.8

584

568

107

145

229

293 140

264

30

56

198

204 101

48

7

13

145

50

27

25 0002100 2990 12300 8680 1660

20 2001030 1880 12100 8270 1940

461

12

16

131

67

13

155

0.3

3

80

42

5

33.6

2.5

18.8 61.1 62.7 38.5

Q. 95 – R. 38
5.2.

FLOW PATTERN BEFORE AND AFTER BYPASSINGLSPIV and Fx-8100 techniques were applied for two different study areas infront of the bypass tunnel approach flow in the reservoir of Miwa dam as shown in
Fig. 3. Recorded images at two modes were treated to evaluate the approach flowin three zones, considering the possibility of the vortex formations that can
influence the bypass efficiency. Flow field measured by LSPIV are shown in Fig .6
and 7. The main flow is directed towards the diversion weir at Mode 1 and acirculation in front of the bypass tunnel was formed (see Fig. 6(a)).
The velocity magnitudes with vectors shows low velocity inside the reservoir
around 30 cm/s as shown in Fig. 6(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. 6(a)on the left), reveals the effect of the bypass operation in Mode 2. The separationof 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 theleft side of the reservoir.

(a)

m/s

0.0

(b)

m/s

0.0

m/s

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

m/s

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

m/s

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. 6
Stationary flow fields and corresponding velocity in the reservoir of Miwa dam for(a) Mode 1, (b, c) Mode 2Champs d'écoulement stationnaire et vitesses correspondantes dans le réservoir
du barrage de Miwa (a) Mode 1, (b, c) Mode 2585

Q. 92 – R. 38

B ypas s channelm/sBypas s t unnel

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. 7
Actual field measurements by LSPIV, surface velocity vectors in the bypass
channel and tunnel of Miwa dam
Mesures réelles de site par LSPIV, champs de vitesse de surface dans le canalde galerie de dérivation du barrage de Miwa

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

2.5
LSPIV

Mitsubishi Fx-8100

2

1.5
zone A1zone A

0.5

0

0.0

0.0

0.164

0.328

0.492

0.655

0.819

0.983

1.147

1.311

1.475

1.638

1.802

0.259

0.518

0.777

1.036

1.295

1.554

1.813

2.072

2.331

2.590

2.849

3.108

1.966

Fig. 8Average velocity measured at zone A during sediment bypassing operation in the
Miwa dam bypass tunnel at selected periods
Vitesse moyenne mesurée à la zone A pendant la dérivation de sédiments dans letunnel de Miwa barrage à des périodes différents

This result in the flow separation on the right and left to the weir and bypass
can be illustrated clearly in Fig. 6(c). Interesting effects of the flow separations infront of the bypass, such as concentration of inflow to the bypass and increase of
velocities up, can be detected by the measurements Fig. 6(c) on the right. The
LSPIV measurements in zone 2 (see Fig. 6(c) on the right) reveal the impact of
the contraction due to the trap weirs. The area between the dikes on the left bankforces the main flow to the right and increases the main velocity. Fig. 7 shows thesurface velocity vectors and corresponding velocity magnitudes obtained by
LSPIV in the bypass channel.

586

Q. 95 – R. 38
To compare and validate the LSPIV ability, a small zone area A of 40 by
50 pixels (Fig. 8) in middle 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 at zone A are shown in
Fig. 8. Both techniques represented are able to monitor the behavior of averagesurface velocity during bypassing (mode 2). The surface velocity has accelerated
in two or more from about 1.25 m/s to 2.5m/s along the center of the bypass
channel at zone area A.
The flow is almost uniform as illustrated by the velocity magnitude andvectors at July 14, 13:59. Due to the increase of water surface profile at different
periods, the channel surface area covered with water is reduced. Two examplesof LSPIV flow field and velocity magnitudes on July 13th and 14th are shown in
Fig. 8. By comparing velocity measured by LSPIV and FX-8100, it can be said that
results are in a good agreement and LSPIV can be also applied to reservoir area.
The operation record for flood event in July was also well monitored. Inflow
and outflow discharges and suspended sediment concentration were recorded asin Fig. 9. Peak concentration was almost equal to inflow discharge peak and turbid
flow of from 2500 to 1000 mg/l was bypassed. Turbidity trend was well monitored
by several measurement data and very much helpful to switching from Mode 1,
Mode 2 and Mode 3.

200

I n f l o wD i s c h a r g(em3/s)
B y p a s sD i s c h a r g(em3/s)

J u n e1 9, 2 0 1 0

150
100
50
0
4,000

D i g i t a ima
lg et e c h n i q uSe S ( mg / l )S MDP S S ( mg / l )S a mp l i n gS S ( D i v e r s i owne i r ) ( mg/l)
S a mp l i n gS S ( B y p a stsu n n e l ) ( m
g/l)

3,000
2,000
1,000

Startb y p a s s i n g

End

0
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

T i me (hrs)Fig. 9Comparison of monitored SSC measured by four different techniques duringbypassing on 19 June 2010, Miwa dam
Comparaison des SSC mesurés par quatre techniques différentes pendant ladérivation contournant le 19 Juin 2010, du barrage Miwa587

Q. 92 – R. 38
5.3.

BYPASS OPERATION AND ITS OPTIMIZATION

Suspended Sediment
Concentration [mg/l]

In order to increase sediment bypass efficiency, we should understand therelation between inflow hydrograph and sediment concentration curve as shownin Fig. 10. In general, sediment concentration during discharge increasing stage ishigher than discharge decreasing stage. So it is important to optimize the bypass
efficiency by discharging the high sediment concentration as much as possible
and storing the low sediment concentration. We are studying rainfall-runoff andsediment concentration model to predict inflow sediment concentration in theflood events. Fig. 11 shows two different outflow discharges. Scenario B, Qin=Qout,
is higher than scenario A which was real record in Sep.2007. Bypass efficiency
were 65% in scenario B and 45% in scenario A as shown in Fig. 12.
30,00025,00020,00015,00010,000
5,000
0
0

100

200
300
400
Inflow discharge [m3/s]

500

600

600

30,000Inflow (m3/s)

In flow disch arge [m 3 /s]

500

Senario A (Real) (m 3/s)

25,000

Senario B （Qin=Qout） (m3/s)
400

Inflow SS (mg/l)

20,000300

15,000200

10,000100
009/06 12:00

5,000

09/07 00:00

09/07 12:00

09/08 00:00

09/08 12:00

0
09/09 00:00

Su spen ded Sedimen t Con centration [m g/l]

Fig. 10
Inflow discharge and suspended sediment concentration in September, 2007
Débit entrant et concentration de sédiments en suspension en Septembre 2007

Fig. 11
Inflow discharge, sediment concentration and bypass scenarios
Débit entrant, concentration des sédiments et scénarios de dérivation

588

Q. 95 – R. 38

Hourly suspended sediment volume [m3]40,00035,000Qs in (m3)30,000Qs bypass (Senario A (Real)) (m3)25,000Qs bypss (Senario B (Qin=Qout)) (m3)

20,00015,00010,0005,000
009/06 12:00 09/07 00:00 09/07 12:00 09/08 00:00 09/08 12:00 09/09 00:00

Fig. 12
Inflow and outflow SS by bypass scenarios
Débit entrant et sortant des sédiments en suspension selon lesscénarios de dérivation

6.

6.1.

EVALUATION AND MONITORING OF ENVIRONMENTALDOWNSTREAM IMPACTS

EVALUATION OF TURBIDITY OUTFLOW

Bypass tunnel outflow will result in more turbid water flowing downstream
compared with the case of discharge through dam outlet gates. On the other hand,
it is expected to reduce the turbidity of dam reservoir and control prolonged
turbidity of post-flood outflow for power generation or irrigation water. The effect of
the turbidity were evaluated with comparison of the changes in turbidity behavior
at different locations from upstream to downstream before and after thecommencement of the bypass tunnel operation at sites of tributary junctions, thesites of varying discharge and suspended sediments, as shown in Fig. 13.

589

Q. 92 – R. 38

Fig. 13Site map of Miwa Dam and its sediment bypass management techniquePlan du site du barrage de Miwa et ses techniques de gestion des sédiments par
galerie de dérivation

In order to verify changes in turbidity of rivers downstream after the bypass
operation, post-flooding changes in suspended sediment concentrations were
investigated as shown in Fig. 14. The observed turbidity in Fig. 14 are at different
sites of the outflow of the Miwa Power Plant (withdrawing from Miwa Dam) andoutflow of the Haruchika Power Plant (withdrawing from Takato Dam and outflows
to the Tenryu River) (these locations are shown in Fig. 13).
The bypass discharge is expected to let out water with higher turbidity todownstream compare to the conventional gate discharge via the reservoir. On theother hand, it is designed to lessen the turbidity in the reservoir and reduce
prolonged turbidity of discharge after the flood. These items are to be evaluated.
The floods in July 2006 and September 2007 were greater than the flood in
October 2004 before the implementation of bypass operation. Highly turbid water
discharged into Miwa Dam through the Ijima weir (Fig.1 4 on the top). The turbidity
changes were not found on outflow either from Miwa Dam or Hauchika Dam. It
was thus verified that the turbidity in the rivers downstream has no impacts.

590

Q. 95 – R. 38

Fig. 14
Observed suspended sediment concentrations at different sites upstream anddownstream of sediment bypass tunnel of Miwa Dam
Concentrations de sédiments en suspension observées à différents sites enamont et en aval de la galerie de dérivation des sédiments du barrage de Miwa

6.2.

ATTACHED ALGAE, CONDITION OF FISHES AND BENTHIC ANIMALSThe significant impacts on organisms living in the downstream areas inconnection with the change in turbidity of dam discharge are to be discussed.
Approximately a duration of 5 years from the commencement of trial run isplanned as the monitoring period, where the frequency or items to be monitored
shall be reviewed as needed. In order to verify the effects of bypass operation on
species, the restoration of attached algae before and after the bypass operation
was evaluated based on chlorophyll a as shown in Fig. 15. The restoration of
attached algae was observed before and after bypass operation both in Mibu andTenryu Rivers. No significant changes were found due to operation of sediment
bypass. Moreover, pre-operation monitoring has been conducted since 2004, a
year before the trial run, to have the Chubu Regional Dam and Estuary Barrage
Management Follow-Up Committee, organized by the Chubu Regional
Development Bureau, conduct scientific and objective evaluation of the acquired591

Q. 92 – R. 38
data by comparing it to the monitored results after the trial run. The growth ofsweet fish “Ayu”, Japanese trout, is temporarily delayed because of flood events
but it was growing thereafter. The population of benthic animals decreased rightafter the flood, but increased smoothly later on as before the implementation of
the bypass operation. Short-term effects were found on fishes and benthic
animals as shown in Fig. 16.

Fig. 15
Evolution of observed chlorophyll a. before and after bypass operation
Évolution de la chlorophylle a. observée avant et après opération de dérivation

Within 1 week (July 26, 2007)

Within 6 weeks (August 30, 2007)

Before flood (Sept.6, 2007) After flood (Sept.6, 2007) Within 4 weeks (Oct.18, 2007)

Fig. 16
Temporary change on sweet fish and benthic invertebrates before and after
flooding events
Modification temporaire de poissons Ayu et d’invertébrés benthiques avant etaprès les inondations.

592

Q. 95 – R. 38
CONCLUSIONS

The major conclusions of this paper are as follows.
1) As a result, the use of such evaluation processes is important to develop
a prediction model and effective monitoring techniques to optimize sediment
bypass operation. The check dam effectively trapped coarse sediment and thesubmerged trap weir effectively blocked large materials that may cause abrasion
damages for the tunnel surfaces. Sediment bypass system in Miwa dam iseffectively routing incoming fine sediment directly to reservoir downstream. The
bypassing efficiency depends on flood magnitude and bypass operation (timing).
It is necessary to formulate the most effective plan for such operations as whencommencing of bypass discharge or when operation ending flow rate, timing, andfor how long etc, by analyzing and feeding back the monitoring results obtained
from trial run. Moreover, the measured surface flow velocities and suspended
sediment concentrations in the context of sediment management in reservoirs areimportant for the validity of the numerical analysis of Miwa dam.
2) The image technique methods were improved in order to obtain flow
pattern and suspended sediment concentration despite water surface elevation
variations, windy, rainy and foggy weather conditions. The described methods canbe used to evaluate other sediment bypass systems.
3) Up to now, no major impacts on downstream ecological point of view(attached algae, benthic animals, and fishes) have been reported. Further studies
are needed to clarify long term impacts and find necessary improvement
measures for bypassing operation.

REFERENCES

[1]

[2]

[3]

[4]

[5]

SUMI T., KANTOUSH S.A. Comprehensive sediment management
strategies in Japan: sediment bypass tunnel. 34th IAHR biennial congress,
Brisbane, Australia, 2011.
VISCHER D., HAGER W.H., CASANOVA C., JOOS B., LIER P., MARTINI
O. Bypass tunnels to prevent reservoir sedimentation. Q. 74, R. 37, Proc.
19th ICOLD Congress, Florence, Italy, 1997.
AUEL C., BERCHTOLD T., BOES R. Sediment management in the Solis
reservoir using a bypass tunnel. Proc. of the 8th ICOLD European Club
Symposium, Innsbruck, Austria, 2010.
SUMI T., OKANO M., TAKATA Y. Reservoir sedimentation managementwith bypass tunnels in Japan. The 9th International Symposium on River
Sedimentation, ii, pp. 1036-1043, 2005.
SUZUKI M. Outline and effects of permanent sediment management593

Q. 92 – R. 38

[6]
[7]

[8]

[9]

[10]

[11]

[12]

[13]

measures for Miwa dam. The 23rd ICOLD Congress, Brasilia, Q.90-R.1.,
2009.
ANDO N., TERAZONO K., KITAZUME R. Sediment removal project at Miwadam. The 18th congress of icold, Durban, Q.69, r.27, pp.421-441, 1994.
KASHIWAI J., SUMI T., HONDA T. Hydraulic study on diversion facilities
required for sediment bypass systems. The 19th Congress of ICOLD,
Florence, Q.74, R.59, pp.957-976, 1997.
HARADA M., TERADA M., KOKUBO T. Planning and hydraulic design ofbypass tunnel for sluicing sediments past Asahi reservoir. The 19th
congress of ICOLD, Florence, c.9, pp.509-539,1997.
KANTOUSH S.A., SCHLEISS A.J. Large scale PIV- measurements at thesurface of different shallow reservoir geometries. Journal of visualization,
vol. 12, no. 4 pp. 361-373, 2009.
FUJITA I., MUSTE M., KRUGER A. Large-scale Particle Image Velocimetry
for flow analysis in hydraulic engineering applications, Journal of hydraulicresearch, Vol.36, No.3, pp.397-414, 1998.
SUMI T., MURASAKI M., TAIRA K., SHINBO K., NAGURA H., TAMAKI H.
Study on measurement of fine sediment discharge process during sedimentflushing operation using PIV. Ann. Journal of Hydraulic Eng., JSCE, vol.51,
1075-1080, 2006 (in Japanese).
SUMI T., MORITA S., OCHI T., KOMIYA H. Development of thesuspended-sediment concentration measuring system with differential
pressure transmitter in rivers and reservoirs, Proc. Hydraulics
measurements and experimental methods Conference, ASCE, 2002.
SUMI T., BAIYINBAOLIGAO， MORITA S. Characteristics of fine sediment
DISCHARGE during sediment flushing of Unazuki dam, JSCE, Journal of
Hydro-science and hydraulic engineering, VOL.25, NO.1, PP. 99-106, 2007.

SUMMARY

Sediment bypass tunnels are very important and effective engineering
practices in reducing the sedimentation rate of reservoirs and extend the reservoir
lifespan. This type of sediment management technique is sometimes expensive,
applicable only in the case of medium size reservoirs. The medium size refers not
only to that of the reservoir, but also to the hydrological size of the catchment. In
Japan, Miwa Dam (Tenryu River) and Asahi Dam (Shingu River) are good
examples for sediment bypass tunnels where routing incoming sediments directly
to downstream reaches. These dams are practically using sediment bypass
tunnel successfully to realize a sustainable reservoir management.
The sediment bypass technique of Miwa Dam includes three topics to bestudied: (1) How to trap coarse sediment in the upstream reaches of reservoir bycheck dam, (2) How to effectively divert the suspended sediment and high turbid
flow around the reservoir basin through tunnel, and (3) How to minimize the594

Q. 95 – R. 38
downstream impacts on Mibu River. As results, the use of such evaluation
processes is important to develop a prediction model and effective monitoring
techniques to optimize sediment bypass operation. The check dam effectively
trapped coarse sediment, and the submerged trap weir effectively blocked largematerials that may cause abrasion damages for the tunnel surfaces. Sediment
bypass system in Miwa dam is effectively routing incoming fine sediment directly
to the reservoir’s downstream. The bypassing efficiency depends on the flood
magnitude and the bypass operation (timing). It is necessary to formulate themost effective plan for such operations, i.e. when to initiate the bypass discharge
or when to end the operation flow rate, define the timing, and duration etc…This ispossible by analyzing and feeding back the monitoring results obtained from thetrial run. No impact on downstream ecological point of view (attached algae,
benthic animals, and fishes).

RÉSUMÉ

Les galeries de dérivation des sédiments sont des pratiques techniques,très importantes et efficaces pour réduire le taux de sédimentation des réservoirs,
et pour prolonger leur durée de vie. Ce type de technique de gestion dessédiments est parfois cher, applicable uniquement dans le cas de réservoir detaille moyenne. La taille moyenne ne se réfère pas seulement aux réservoirs,
mais aussi à la taille hydrologique des bassins versants. Au Japon, le barrage
Miwa (sur la rivière de Tenryu) et le barrage d’Asahi (sur la rivière Shingu) sont debons exemples pour les galeries de dérivation, où les sédiments sont dérivés
directement en aval. Ces barrages emploient avec succès des galeries dedérivation pour gérer durablement leur réservoir.
La technique de dérivation des sédiments du barrage de Miwa comprend
trois aspects qui seront étudiés: (1) Comment retenir les sédiments grossiers
dans les tronçons en amont du barrage réservoir par recours aux barrages derétention des alluvions (2) Comment renvoyer efficacement les flux de sédiments
en suspension et courants de turbidité autour du réservoir par le tunnel, et (3)Comment réduire les impacts en aval sur le fleuve Mibu. Il résulte de cette étude
que l'utilisation de ces processus d'évaluation est importante pour développer unmodèle de prédiction ainsi que des techniques de surveillance efficaces pouroptimiser le fonctionnement de dérivation des sédiments. Le barrage de rétention
des alluvions a efficacement retenu les sédiments grossiers, et le déversoir
submergé a bloqué les matériaux de grande taille qui pourraient endommager lessurfaces du tunnel. Le système de dérivation des sédiments dans le barrage
Miwa conduit efficacement les sédiments fins directement vers l'aval du réservoir.
L'efficacité de la dérivation dépend de l'ampleur des crues et des opérations dedérivation (synchronisation). Il est nécessaire de formuler des plans de gestion
efficaces pour ces opérations, afin de définir quand commencer le débit decontournement, ou quand terminer les opérations, en synchroniser et définir la595

Q. 92 – R. 38
durée. Cela est possible en analysant et en exploitant les résultats de lasurveillance obtenus à partir des essais. Il n’y a pas d'impact sur le planécologique en aval (algues fixées, animaux benthiques et poissons).

596