Proceedings of the Ninth International Symposium on River Sedimentation
October 18 – 21, 2004, Yichang, China

RESERVOIR SEDIMENTATION MANAGEMENT WITH BYPASSTUNNELS IN JAPANTetsuya SUMI
Graduate School of Civil and Earth Resources Engineering, Kyoto University, Kyoto, 606-8501, Japan
E-mail: sumi@basewall.kuciv.kyoto-u.ac.jp
Masahisa OKANO
Water Resources Environment Technology Center, Tokyo, 102-0083, Japan
Yasufumi TAKATACTI Engineering CO., LTD., Osaka, 540-0008, Japan

Abstract: This paper focuses on sediment bypass tunnels for reservoir sedimentation management. Worldwide,
number of constructed sediment bypass tunnels is very small because of topographical, hydrological or
economical conditions. Bypass tunnels, however, have many advantages such as they can be constructed even atexisting dams and prevent a loss of stored reservoir water caused by the lowering of the reservoir water level.
They are also considered to have a relatively small impact on environment downstream because inflow discharge
can pass through tunnels very naturally during flood times.
In Japan, sediment bypass tunnels of the Nunobiki Dam completed in 1908, and of the Asahi Dam completed in
1995, have been successfully introduced to realize sustainable reservoir management. Sediment bypasses of the
Miwa Dam, Koshibu Dam and Matsukawa Dam on the Tenryu River are expected to be next leading projects inJapan. In this paper, we discuss design criteria and lessons learned from existing facilities, and future challengesof planning projects. The focus is on tunnel geometries such as diameter, slope, bottom shape etc., and an
abrasion-resistance design of the tunnel invert.

Keywords: Reservoir sedimentation, Sediment bypass, Diversion tunnel, Abrasion damage
1 INTRODUCTION
Reservoir sedimentation is surely proceeding at dams in the world. Current gross storage
capacity in the world is 6,000 km3 with 45,000 large dams (higher than 15m, ICOLD), andtotal storage loss and annual sedimentation rate are about 570km3 (12%) and 31km3-year(0.52%-year) respectively. If additional new development projects are not considered, total
capacity will be decreased even to less than half by 2100.
In many countries, various countermeasures have been implemented to decrease sedimentaccumulation and loss of storage capacity. They are (1) Reduce sediment inflow by erosion
control and upstream sediment trapping, (2) Route sediments by sediment sluicing, off stream
reservoirs, sediment bypass, and venting of turbid density currents, and (3) Sediment removal
by hydraulic flushing, hydraulic dredging or dry excavation. Sediment bypassing and flushingare considered to be as permanent remedial measures.
Worldwide, limited numbers of sediment bypass tunnels have been constructed because of
topographical, hydrological or economical conditions. Bypass tunnels, however, have many
advantages such as they can be constructed even at existing dams and prevent a loss of stored
reservoir water caused by the lowering of the reservoir water level. They are also considered
to have a relatively small impact on the environment downstream because inflow discharge
can be passed through tunnels very naturally during flood time.
In Switzerland, five bypass tunnels have been constructed and proved an effective means tocounter reservoir sedimentation (Visher et al., 1997). It is found that problems may arise withtunnel abrasion, particularly if the sediment has a considerable quartzite component.
In Japan, sediment bypass tunnels at Nunobiki Dam, completed in 1908, and at Asahi Dam,1036

completed in 1995, have been successfully introduced to realize sustainable reservoir
management. Sediment bypasses at Miwa Dam have been almost completed, and ones at
Matsukawa Dam and Koshibu Dam are under construction and planning. For the purpose ofdesigning these bypass systems, hydraulic characteristics of tunnel and diversion weir havebeen studied (Ando et al., 1994, Kashiwai et al., 1997, Harada et al., 1997).
In this paper, we discuss design criteria and lessons learned from existing facilities, andfuture challenges of planning projects. We focus on tunnel geometries such as diameter, slope,bottom shape etc., and an abrasion-resistance design of the tunnel invert.
2 RESERVOIR SEDIMENTASTION AND ITS MANAGEMENT IN JAPANIn Japan, approximately 2,730 dams over 15 m in height have been constructed for water resource
development or flood control purposes, but the total reservoir storage capacity is only up to 23
billion m3. The sediment yields of the Japanese rivers are relatively high due to the topographical,
geological and hydrological conditions and this has consequently caused sedimentation problems tothose reservoirs.
Based on the annual research
conducted for 877 reservoir which havegross storage capacities of over one
million m3, specific sediment yields inthese catchment areas are ranging fromseveral hundred to several thousandm3·(km-2·yr-1), and the specificsedimentation volume becomes higher
in central high mountainous regions
along the Median Tectonic Line andthe Itoigawa-Shizuoka Tectonic Line asshown in Fig. 1.
Fig. 2 shows the relationshipbetween reservoir sedimentation rates,e.g. sedimentation volumes to grossstorage capacities, and reservoir ages.
Concerning the dams constructed
before World War II (ended in 1945)
Fig. 1 Specific Sedimentation Volume inJapanese Reservoirs
and used for more than 50 years,
sedimentation proceeded in the range
from 60% to beyond 80 % in some hydroelectric reservoirs. For the dams constructed
approximately between 1950 and 1960 and used for more than 30 years, sedimentation rates beyond
40 % were found in many cases. Following this period, meanwhile, large numbers of multi-purpose
dams gradually came to be constructed. This type of dams does not have high sedimentation rates
compared to the hydroelectric type, though, the rates of 20% to beyond 40 % were found in somedams. Since maintaining storage capacities for water supply and flood control is much more
important, the influence of sedimentation in those multi-purpose reservoirs becomes large.
Annual storage capacity losses in those reservoirs are ranging approximately from 1.0% to
0.1 %. In other words, it is noted that the reservoir lives are approximately 100 to 1,000 years.
Reservoir sedimentation problems are represented by such as the siltation of intake facilities,
aggradations of upstream river bed, reservoir storage capacity loss and, sometimes, lack ofsufficient sediment supply to downstream which may cause river bed degradation or
coastal erosion.
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Fig. 2 Relationship Between Reservoir Sedimentation Rates and Years after Dam Completion

Sediment management in reservoirs is largely classified into the three approaches: (1) toreduce sediment inflow to reservoirs. (2) to route sediment inflow so as not to accumulate inreservoirs, and (3) to remove sediment accumulated in reservoirs. In Japan, check dams to
reduce sediment yield from catchment area, sediment bypassing, draw down sedimentflushing, excavating and dreging are selected for management measures.
3

SEDIMENT BYPASS TUNNELS IN JAPANIn Japan, sediment bypass tunnels have been studied most exhaustively. Although thistechnique involves high cost caused by tunnel construction, there are many advantages suchthat it is applicable to existing dams; it does not involve drawdown of reservoir level andtherefore no storage capacity loss; and it has relatively small impact on environment because
sediment is discharged not so rapidly as sediment flushing which can be considered as theother countermeasure.
The subjects of designing sediment bypass tunnels are to secure the safety of sedimenttransport flow inside tunnels and to take countermeasures for abrasion damages on thechannel bed surface. Among factors that significantly relate to these problems are grain size,
tunnel’s cross-sectional area, channel slope, and design velocity. Table1 shows examples ofexisting sediment bypass tunnels in Japan and Switzerland.
Table 1

Sediment Bypass Tunnels in Japan and Switzerland

No Name of Dam Country

TunnelTunnelCompletion ShapeTunnelDesignTunnel General Design
Operation
Cross
Length Slope
Discharge Velocity
Frequency
Section(m·s-1)(m)(%)(m·s-1)(B×H(m))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-1

3

Miwa

Japan

Horseshoe

2r = 7.8

4,300

1

300

10.8

-

4

Matsukawa

Japan

2004
Under
construction

Hood

5.2×5.2

1,417

4

200

15

-

5

Egshi

Switzerland 1976

Circularr = 2.8

360

2.6

74

9

10d·yr-1

6

Palagnedra

Switzerland 1974

Horseshoe

2r = 6.2

1,800

2

110

9

2d·yr-1-5 d·yr-1

7

Pfaffensprung Switzerland 1922

Horseshoe

A= 21.0m2 280

3220

10～15 -200d·yr-1

8RempenSwitzerland 1983

Horseshoe

3.5×3.3

450

4

80

～14

1d·yr-1-5d·yr-1

9Runcahez

Switzerland 1961

Horseshoe

3.8×4.5

572

1.4

110

9

4 d·yr-1

1038

3.1 Bypass Tunnel Examples In Japan
3.1.1 Example Nunobiki Dam
The municipal water supply reservoir Nunobiki Dam completed in 1900, the oldest gravity
concrete dam in Japan, is located at Kobe city. The scheme is composed of a 33.3m highgravity concrete dam and 0.76 million m3 gross storage volume reservoir with 9.8km2
catchment area. The Rokko Mountains, its catchment area, is characterized by high sedimentyield rate because of both surface geology of deep weathered granite and steep slopes. Aftercompletion of the project, reservoir sedimentation had extremely proceeded from the above
reasons. In order to reduce inflow sediment to the reservoir, a sediment bypass system(comprised of a 3.0m high diversion weir and a 258m long bypass tunnel with a maximum
discharging capacity of 39m3·s-1) is constructed in 1908, just 8 years after the dam completion.
Fig. 3 shows the schematic diagram of the bypass system. The tunnel is designed to divert
inflow discharge over 1.11 m3·s-1 and the effect of the system can be estimated as follows.
Daily sediment inflow in ca.100 yrs can be calculated by the following stream power
equation (Ashida and Okumura 1977).
β(1)
D = α (A ⋅ R ⋅ I )3
2
where, D ：sediment yield during a flood event(m )， A ：catchment area(km )， R ：daily
rainfall(mm)， I ：average riverbed slope in 200 m upstream from the calculating point，2
α , β ：constants. All catchment area and the one downstream diversion weir are A1=9.83km
and A2=0.47km2 respectively. Daily rainfall R is estimated by the historical records at the
Kobe Ocean Meteorological Station from 1897 and riverbed slope is defined as I=0.044 by
the geographical survey.
Since constant β is usually fixed as 2.0 (Ashida and Okumura 1977), constant α isinversely estimated as 6.0 by real accumulated sediment volumes in the reservoir measured in
1938, 1967, 1978 and 1990.
By using A1 and A2, mean annual sediment inflow before and after a sediment bypass
completion is estimated ca.30,000 m3·yr-1and ca.1,500 m3·yr-1 respectively.
From the view point of sedimentation management, reservoir life has extended from ca.30
years to ca.500 years.

Fig. 3 Sediment Bypass Scheme at Nunobiki Dam

3.1.2 Example Asahi Dam
The lower regulating reservoir Asahi dam completed in 1978, located in the Shingu river
system in Nara Prefecture, belongs to the Okuyoshino Power Plant (pure pumped-storage type,1039

1,206 MW) operated by the Kansai Electric Power Co. Inc. The lower regulating scheme iscomposed of a 86.1m high arch dam and 15.47million m3 gross storage volume reservoir with
39.2km2 catchment area.
Since completion of the dam, the prolonged turbidity problem has been getting noticeable
due to the upstream condition changes by the collapse of mountain slopes and the devastationof forests caused by large-scale runoffs(Kataoka 2003). Moreover, because of the frequent
typhoons in 1989 and 1990, the mean annual accumulated sediment volume from 1989 to
1995 also increased sharply to 85,000m3·yr-1 which is more than four times of that from 1978
and 1988.
Following the above view points, a sediment bypass system (comprised of a 13.5m highdiversion weir and a 2,350m long bypass tunnel with a maximum discharging capacity of
140m3·s-1) was constructed in April 1998 to reduce the prolonged turbidity and reservoir
sedimentation. Fig. 4 shows the schematic diagram of the bypass system.

Fig. 4 Sediment Bypass Scheme at Asahi Dam

The bypass system was designed to flow both suspended load and bed load, and the design
discharge, 140 m3·s-1, was determined considering a scale of one-year return period flood witha peak discharge of ca. 200 m3·s-1. The cross-section of the bypass tunnel was determined on
the basis of uniform flow calculation, and it can release the maximum design discharge at awater depth of 80% of the tunnel height. Hood type cross section of the tunnel was selectedfrom the viewpoint of economy and easy maintenance.
During the four years from 1998 to 2002, sediment bypassing was performed about 16
times a year and about 40 percentage of the annual run-off was diverted through the tunnel. It
is found that bypass could have reduced both turbidity period and sediment inflow. It isestimated that 10% to 20% of the annual inflowing sediment is deposited in the reservoir,
while the remaining 80% to 90% is bypassed downstream through the bypass system. Fig. 5
shows the effect on the reduction of reservoir sedimentation by the bypass tunnel.
From just after the tunnel operation, local abrasion damages was found on the tunnel invert.
A total of 400m3 of abrasion on all tunnel invert (area 9,000m2) and an average abrasion depthof 45 mm, locally up to maximum 200mm, was found in 1998. From a study on therelationship between the mean abrasion depth and the estimated bypassed sediment from 1998
to 2001, it can be found that the abrasion quantity is almost proportional to the bypassed
sediment volume.
Though these abrasion damages are within the range forecast at the design stage, locations
where invert concrete of the tunnel with design strength of 36 N/mm2 is seriously damaged
are repaired during non-flood season.
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Fig. 5

Effect on the Reduction of Reservior Sedimentation by the Bypass Tunnel(Kataoka 2003)

3.1.3 Example Miwa dam
The multi-purpose reservoir Miwa Dam completed in 1959, located in the Tenryu river
system in Nagano prefecture, is operated for the purpose of flood control, irrigation watersupply and hydroelectric power generation by the Ministry of Land, Infrastructure andTransport. The scheme is composed of a 69m high gravity concrete dam and 29.95 million m3
gross storage volume reservoir with 311 km2 catchment area.
Since the dam was completed, extreme runoff events occurred in 1959, 1961, 1982 and1983 causing serious disasters and sediment yield in the catchment river basin that resulted inquick increasing of reservoir sedimentation. Since 1966, gravel have been constantly removed
by a maintenance plan and then approximately 5.32 million m3 sediment have been dredged in
33 years up to 1998. If those gravel is not removed, the total sedimentation is approximately
19.47 million m3, and estimated the mean annual sedimentation is 0.47 million m3·yr-1.
From the view point of the eternal reservoir sedimentation management, a sediment bypass
system (comprised of a 20.5m high diversion weir and a 4,300m long bypass tunnel with amaximum discharging capacity of 300 m3·s-1) is completed in March 2004 to reduce
sedimentation of the reservoir. Fig. 6 shows the schematic diagram of the bypass system.

Fig. 6 Sediment Bypass Scheme at Miwa Dam

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This bypass system was designed to flow mainly wash load since about 3/4 of the sedimentdeposited in the reservoir is wash load smaller than 74 μm. According to the master plan of
the redevelopment project, 685,000 m3 of sediment including 525,000 m3 of wash load, and160,000 m3 of bed load and suspended load will flow into the reservoir. The bypass tunnel
will divert 399,000 m3 of wash load and the remaining 126,000 m3 will flow into the main
reservoir. The bed load and the suspended load that flow in at an annual average of 160,000
m3 are captured by the 15.0m sediment trap weir that has sedimentation capacity of
200,000m3, then it is removed mechanically and transported for construction materials.

Design discharge (m3/s)

Design discharge (m3/s)

Design discharge (m3 /s)

3.2 Bypass Tunnel Design Against Abrasion Damages
Figs. 7 (a) -(c) shows relationship between
700(a)
Sediment bypass(Japan)
tunnel length, bottom slope, design velocity
600
Sediment bypass(Switzerland)and design discharge of bypass tunnels in
Diversion tunnel(Japan)
500
No.1 - 9 : Table-1
Japan and Switzerland. Since up to now,400
3
sediment bypass examples are very limited,300
7
4
diversion tunnels in Japan are also plotted for200
comparison in the figure. Regarding tunnel
9
2
5
100
8
6
length, some sediment bypasses are longer
10
than 1,000 m since inlet of bypass tunnels
0
1000
2000
3000
4000
5000
T unnel length (m)should be located upstream reach of reservoirs,
Sediment bypass(Japan)
700
Sediment bypass(Switzerland)
while almost diversion tunnels are shorter than(b)
Diversion tunnel(Japan)
600
1,000 m. Design discharge is ranging up to 300
500
m3·s-1 for sediment bypasses and also up to 600
No.1 - 9 : Table-1
3 -1
400m ·s for diversion tunnels. Since larger design
3300
discharge, e.g. larger tunnel cross section, and7
4200
longer tunnel length make tunnel design harder,
2
9
6
100
sufficient cost/benefit analysis is needed
5810considering the maximum design flood that should
0.00
1.00
2.00
3.00
4.00
5.00
be totally bypassed through the tunnel.
Slope (%)
It should be also understood that careful
Sediment bypass(Japan)
Sediment bypass(Switzerland)
700
design should be necessary if steeper bottom
Diversion tunnel(Japan)
600
slope, e.g. higher velocity, and larger grain size(c)No.1 - 9 : Table-1
500
will be expected. Regarding design velocity and400
slope, 10 m3·s-1 -15 m3·s-1 are expected in case of
3300
3% - 4 %.
7
4
200
6,9
Generally, the main problem of bypass
2
8
100
5
tunnels is abrasion along the invert (Visher et0al., 1997). In Switzerland, to counter abrasion,
0
5
10
15
20
Design velocity (m/s)
linings of steel or plates of granite, or evenFig. 7 Sediment bypass dimensions
molten basalt have been used. Moreover,selected concretes such as micro-silicate
concrete, roller-concrete, steel-fiber concrete, polymer-concrete and standard concrete(forreference) are tested. In Japan, several materials have been also tested in case of Asahi dam.
These materials show better performances against abrasion damages than conventional
concrete, while it will need more study to use them on wider areas because these works arevery much expensive. In Japan, from the view points of initial construction cost and easy
maintenance, selecting high strength concrete and preparing enough abrasion depth on top ofnecessary tunnel invert depth is recommended at the moment.

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Fig. 8 Bypass Tunnel Design against Abrasion Damages

Figs. 8 (a) and (b) show concrete strength and abrasion depth designed for sediment bypasses
and diversion tunnels in Japan. In case of design velocity higher than 10 m3·s-1, it can be foundthat concrete strength higher than 30 N·mm-2 and abrasion depth deeper than 10 mm-35 mm
have been selected. Since necessary abrasion depth is, off course, depend on the interval of
repairing works, more experiences for real operation and maintenance are needed.
4 CONCLUSION
The conclusions can be listed as follows:
Sediment bypass system in the Nunobiki dam and Asahi dam in Japan are contributing tosustain reservoir lives, and completion and realization of other projects are highly desired.
Data of existing sediment bypass tunnels in Japan and Switzerland are as follows: the tunnel
lengths are between 250 m and 4,300 m; the section areas are between 10 m2 and 60 m2; thegeneral bottom slopes are between 1 and 4%; the design discharges are between 40 and 300
m3·s-1; the design velocities are between 9 m3·s-1 and 15 m3·s-1.
The main problem of sediment bypass tunnels is abrasion along the invert. To counter
abrasion, selecting high strength concrete and preparing enough abrasion depth on top ofnecessary tunnel invert depth are recommended from the view points of initial construction
cost and easy maintenance.
In order to operate the sediment bypass system effectively, it is important to predict and to
perform real-time monitoring not only the flow discharge but also the concentration of
sediment in the inflowing water. Since Miwa dam has the flood control function, such kind ofoperational challenges is now under studying, and will be reported in the near future.
REFERENCES
Ando, N., Terazono, K. and Kitazume, R., 1994, Sediment removal project at Miwa dam, The 18th Congress of
ICOLD, Durban, Q.69, R.27, pp.421-441
Ashida, K. and Okumura, T., 1977, Analitical study of sediment yield rate during heavy rainfalls, Record analysis
of natural disasters 4, pp.85-91 (in Japanese)
Harada, M., Terada, M. and Kokubo, T., 1997, Planning and hydraulic design of bypass tunnel for sluicing
sediments past Asahi reservoir, The 19th Congress of ICOLD, Florence, C.9, pp.509-539
Kashiwai, J., Sumi, T. and Honda, T., 1997, Hydraulic study on diversion facilities required for sediment bypass
systems, The 19th Congress of ICOLD, Florence, Q.74, R.59, pp.957-976
Kataoka, K., 2003, Sedimentation management at Asahi dam, Session “Challenges to the sedimentation
management for reservoir sustainability”, The 3rd world water forum, Kyoto-Shiga, pp. 197-207
Morris, G.L. and Fan, J., 1997, Reservoir sedimentation handbook, McGraw-Hill, New YorkMorris, G.L., 2003, Reservoir sedimentation management: world wide status and prospects, Session “Challenges tothe sedimentation management for reservoir sustainability”, The 3rd world water forum, Kyoto-Shiga, pp. 97-108
Sumi, T., 2000, Future perspective of reservoir sediment management, International workshop on reservoir
sedimentation management, Toyama, pp. 145-156
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