RESERVOIR SEDIMENTATION DATABASE AND SELECTING SUITABLE
SEDIMENT MANAGEMENT OPTIONS IN JAPANTetsuya Sumi1
ABSTRACT
Approximately 3,000 dams over 15 meters in height have been constructed so far inJapan, but the total reservoir storage capacity is only 23 billion m3. Following thewidespread recognition of sedimentation problems, all dams having a storage capacity
over one million m3 have been obliged to report sedimentation progress to the authority
every year since the 1980s. As of 2006, from 971 dams accounting for approximately 1/3of all dams in Japan, annual changes in sedimentation volume and the shape ofaccumulated sediment were reported to the central government. It is probably only Japanthat established such a nationwide survey system, and such accumulated considerably
valuable records on a global basis. Based on these data, we can estimate regional or river
basin scale reservoir sedimentation progress such as total storage loss and its annual
speed. Classification of reservoirs both by reservoir sedimentation index and reservoir
turnover index can be useful to understand the current situation and selecting suitable
management options. In this paper, history of reservoir sedimentation survey in Japan is
explained, and how we have established and how we are trying to effectively utilize thedatabase is discussed.
INTRODUCTION
The sediment yields of the Japanese rivers are high due to the topographical, geologicaland hydrological conditions. This has consequently caused sedimentation problems to
many reservoirs constructed for water resource development or flood control purposes.
Under such circumstances, studies on estimation of sediment volume andcountermeasures for sedimentation have been conducted for a long time.
Currently, the reservoir sedimentation management in Japan is embarking on new stages
from two points of view. One is, in contrast to the emergent and local conventionalcountermeasures such as dredging and excavation, the active promotion of introductionof sediment flushing using sediment flushing outlets and sediment bypass systems, which
aim at radically reducing the sediment inflowing and deposition. Unazuki dam andDashidaira dam on the Kurobe River (e.g. Sumi and Kanazawa 2006, Sumi et al. 2009,
Kantoush et al. 2010), and Miwa dam on the Tenryu River (e.g. Sumi and Kantoush
2011a, Sumi et al. 2012) and Asahi dam on the Shingu River (e.g. Fukuroi 2012) are
advanced examples of using sediment flushing and sediment bypass techniques, which
are placed as permanent measures for sedimentation at dams.
The other is, considering a sediment movement zone from mountains through coastal
areas, the initiation of a comprehensive approach to recover a sound sediment transport in
the sediment routing system. However, these advanced techniques for sediment
1

Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, sumi.tetsuya.2s@kyoto-u.ac.jp

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management aiming at long life of dams have only been applied to a limited number, andtherefore continuous study is required. It is also important to solve the social issues, suchas consensus building on the need for sediment management throughout the basin people,
establishment both of legal system and cost allocation system. In this paper, history ofreservoir sedimentation survey in Japan is explained, and how we have established andhow we are trying to effectively utilize the database is discussed.
PRESENT STATE OF RESERVOIR SEDIMENTATION GUIDELINE IN JAPANHistory of guidelines on reservoir sedimentation design can be summarized as follows.
1) Dam Design Guideline by the Ministry of Interior in 1936 did not describe any
information on reservoir sedimentation.
2) Design and Research Conclusion of Gravity Dam in 1939-1943 by the Japan
Energy Council, the Japan Electric Generation and Transmission Company andthe Committee on Dam Design.
3) Design Standard of Agricultural Land Improvement Project in 1954 indicateddesign value for sedimentation depth base on the location in the river basin,
reservoir water depth and storage volume.
4) Technical Criteria for River Works by the Ministry of Construction in 1958
indicated that necessary volume for reservoir sedimentation is 100 years andshowed some examples of recorded specific sediment yield rate ranging from 25
to 600 m3/km2/yr.
5) Technical Criteria for River Works: Practical Guide for Planning by the Ministryof Construction in 1976 showed updated examples of recorded specific sedimentyield rate ranging from 49 to 5,257 m3/km2/yr. 100 years sedimentation volumes
has been also described but some exceptions in special case where sediment canbe discharged through spillways or excavated periodically were also accepted.
6) Design Standard of Agricultural Land Improvement Project in 1981 indicateddesign sedimentation volume for designing allocation of reservoir storage
volumes, calculating upstream flood water level and defining sedimentation depth
for dam stability analysis. Tentative specific sediment yield potential map was
introduced.
Technical Criteria for River Works: Practical Guide for Planning by the Japanese
Ministry of Land, Infrastructure, Transport and Tourism (hereafter MLIT) in 2005 is
describing necessary storage capacity for sedimentation as follows.
Normally the sediment deposition estimated for the next 100 years is used as the storage
capacity for sedimentation. However, the design sediment storage can be reduced in thecase of a facility that flows sediment from a flood spillway, one that removes inflow

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sediment in the reservoir, or other facilities for which special measures have been
implemented.
Regarding reservoir sedimentation database, the first guideline was released in 1966 and1967 to collect necessary information on riverbed aggradation in order to prevent
flooding risks upstream of reservoirs caused by sedimentation. In 1982, more detailed
guideline was released which all dams having a storage capacity over 1 million m3 are
obliged to report sediment condition to the Ministry of Construction every year. Data
includes measured sedimentation volume both in dead storage (B), lower than L.W.L.,and active storage (A), between N.W.L. and L.W.L., volumes described in Figure 1.

Figure 1. Definition of sedimentation in (B) dead and (A) active storage volumes.
As of 2006, from 971 dams accounting for approximately 1/3 of 3,000 dams over 15
meters in height, annual changes in sedimentation volume and the shape of accumulated
sediment were reported. It is probably only Japan that established such a nationwide
survey system, and such accumulated data is regarded as considerably valuable records
on a global basis.
Figure 2 shows characteristics of those dams regarding years after completion, purposesof dams and total storage volumes. Many dams are listed after the 1950s and storage
volumes are mainly less than 10 million m3.

Figure 2. Dams of reservoir sedimentation database.
(a)Years after completion, (b)Purposes of dams and (c)Total storage volumes.

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Figure 3 shows reservoir storage losses by sedimentation depend on regions andcategories based on purposes. Regions are illustrated in Figure 4. Hydropower dams
show higher ones because they were mainly constructed before the 1960s and have
longer operation records. Dams in Chubu region is showing higher one because major
tectonic lines are passing where a large amount of sediment is produced.
Figure 4 shows storage loss by sedimentation of each multipurpose dam belongs to MLIT,
prefectural government and Japan Water Agency (JWA). Expected reservoir life
(CAP/MAS) of almost dams is ranging more than 200 years but some dams are very
short. Several countermeasures such as sediment flushing, sluicing or bypassing havebeen already taken in these dams.

Figure 3. Reservoir storage loss by sedimentation in all regions in Japan.

Figure 4. Reservoir storage loss by sedimentation in multipurpose dams.
(MLIT, Pref. Government and Japan Water Agency)(CAP: Reservoir Total Storage Capacity, MAS: Mean Annual Sediment Inflow)
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Figure 5(a) shows actual specific sediment yield of dams belong to MLIT. Some dams in
Chubu region are more than 2,000 m3/km2/yr. Using these actual sedimentation records
and, geologic and topographic conditions, sediment yield potential map have been
prepared by the committee in the Water Resources Environment Technology Center(WEC) as shown in Figure 5(b). This map is currently used to check sedimentation
planning for newly constructed dams and to estimate future sedimentation predictions forexisting dams.

Figure 5. Specific sediment yield; (a) Actual record and (b) Sediment yield potential map.
More detailed information has been reported such as sedimentation volumes in eachelevation. Figure 6 shows vertical profile of sedimentation and reservoir storage volume
changes in every 10 years at Koshibu dam in the Chubu region. We can easily
understand sedimentation progress depend on reservoir operation. At Koshibu dam, in
Baiu, Japanese rainy season in June – July, and in Typhoon season in August – September,reservoir water level is slightly drawn down at EL. 592 m and 604.8 m respectively.
These periodical partial draw down have been effectively transport sediment into moredeep area. Furthermore, in Koshibu dam, two check dams have been installed after dam
completion to trap and excavate mainly coarse sediment periodically as in Figure 7.
These check dams have effectively minimized sediment deposition. On the contrary, justbelow L.W.L. at EL. 588.7 m, sedimentation delta is still progressing by sandy materials
and finer materials are accumulated at density current bed below EL. 570 m.

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Figure 6. Sedimentation progress and storage volume changes at Koshibu dam.

Figure 7. Sedimentation profile and two check dams at Koshibu dam.
In case of Koshibu dam, new sediment bypass tunnel system is now under construction inorder to reduce all sediment loads to the reservoir. Dimensions of the tunnel are tunnel
length: L=1982 m, diameter: R=3.95 m, A=49 m2, Slope: s=0.02 %, design discharge=370 m3/s. Inlet of the tunnel is installed upstream of the upstream check dam.
SELECTING SUITABLE SEDIMENT MANAGEMENT OPTIONS
Following a recommendation by River Council of Japan in 1997, comprehensive
sediment management in order to recover a sound sediment transport regarding not only
quantity but also quality point of view in the sediment routing system is now being
advanced earnestly. Sediment management in reservoirs is largely classified into the three
approaches: 1) to reduce sediment inflow to reservoirs, 2) to route sediment inflow so as
not to accumulate in reservoirs and 3) to remove sediment accumulated in reservoirs.
Sumi and Kantoush (2011b) has classified selected examples in Japan and Europ to thesemethodologies.

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Reduction of Sediment Inflow into Reservoirs
There are two techniques to reduce the amount of transported sediment: 1)
countermeasure to control sediment discharge which covers entire basin including theconstruction of erosion control dams; and 2) countermeasure to forcibly trap sediment byconstructing check dams at the end of reservoirs. Although the catchment areas of dams
have high forest cover rates, a remarkable amount of sediment is produced in thewatershed where landslides frequently occur due to the topographical and geologicalconditions.
Since an attempt to trap sediment using check dams is found effective for the reservoirs
where bed load of relatively coarse grain size accounts for a large percentage of sedimentinflow, many dams have proceeded in constructing them recently. In this technique, a low
dam is constructed at the end of reservoir to trap transported sediment, and then thedeposited sediment is regularly removed. The accumulated sediment can be excavated onland except for flood time, and the removed sediment is utilized effectively as concrete
aggregate. As of 2006, the check dams have been constructed at 57 out of the dams underjurisdiction of MLIT.
Recently, sediment augmentation, or replenishment, has been carried out in some dams inJapan (Kantoush et al. 2010, Kantoush and Sumi 2011). Trapped sediments in thesediment check dam upstream of the reservoir are excavated and transported to thedownstream of the dam. These sediments are put on the downstream river channel
temporarily and washed out by the natural flood flows.
Routing of Sediment Inflow into Reservoirs
Another possible approach to sediment management, next to the reduction of sedimentinflow itself, is to route sediment inflow so as not to allow it to accumulate in reservoirs.
In Japan, the following techniques are adopted: 1) sediment bypass by directly diverting
sediment transport flow, and 2) density current venting by using a nature of highconcentration sediment transport flow.
In Japan, sediment bypass tunnels have been studied most exhaustively. Although thistechnique involves high cost caused by tunnel construction, there are several advantages
such that it is also applicable to existing dams; it does not involve drawdown of reservoir
level and therefore no storage capacity loss; and it has relatively small impact onenvironment because sediment is discharged not so rapidly as sediment flushing.
Nunobiki dam is the first example of the bypass tunnel in Japan. The reservoir to which
longevity is estimated to be only 25 years without bypass is prolonged to over 1,000
years. Recently, the bypassing effect is clarified after completion of Asahi and Miwa
dams and the bypass tunnels in Matsukawa and Koshibu dams are under construction.
Density current venting, on the other hand, is a technique to use a nature of highconcentration sediment transport flow, which runs through relatively deep reservoir withoriginal channel bed of steep slope as a density current with less diffusion, and to

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discharge it effectively through outlets in timing of reaching dam. In both techniques, themain target is fine-grained sediment such as suspended sediment and wash load. In
multiple-purpose dams in Japan that usually have high-pressure bottom outlets for flood
control, the effective operation of these facilities during flood season can increase achance to actively discharge fine-grained sediment.
Removal of Sediment Accumulated in Reservoirs
This approach is regarded as the last measure in case sediment is accumulated inreservoirs in spite of various efforts being done: 1) mechanically excavating sedimentaccumulated in the upstream region of reservoirs, 2) dredging sediment accumulated at
the middle and downstream regions, and 3) flushing out sediment with tractive force. As
for excavation and dredging techniques, it is important that the removed sediment shouldbe treated properly and reused.
On the other hand, sediment flushing is a technique to restore tractive force in a reservoir
beyond its critical force by means of drawdown of reservoir level, and flush the deposits
through bottom outlets in the dam body with inflow water, mainly in an open channel
flow condition, to the downstream of dam. When the amount of sediment inflow issignificantly large, man-powered techniques such as excavation and dredging are hard to
be adopted because of problems involving transportation and dump site. In such a case,however, sediment flushing can be a permanent measure if conditions are met.
Traditionally in Japan, sediment flushing facilities such as flushing sluices and outlets
were installed at small-scale hydroelectric dams or weirs for the purpose of discharging
sediment deposited in the vicinity of intake. In contrast, at Dashidaira-Unazuki dams in
the Kurobe river, where a large amount of sediment is discharged, sediment flushing is
implemented in coordination of upstream and downstream dams, e.g. coordinatedsediment flushing.
Sediment flushing is performed at many dams all over the world. Sediment flushing is
considered as an extremely effective technique for discharging sediment in terms ofharnessing tractive force in natural river channel. However, when this technique isintroduced, an extensive study is required in the planning stages, considering suchconditions as inflow, sediment inflow, storage capacity, grain size distribution andreservoir operation. At the same time, it is also required to consider measures concerning
environmental problems under sediment flushing process.
Moreover, HSRS (Hydro-suction Sediment Removal System) which can intake anddischarge sediment using only the water level differences without the mechanical force isdeveloped in some types in recent years. There are stationary and movable types in thesystem. In case of the stationary type, establishment of the measures to move sediment tothe system neighborhood in the reservoir and, in case of the movable type, securing
enough operation time corresponding to the target sediment volume to be discharged
during a year and a safe work environment are problems to be solved.

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Promotion Strategy of Reservoir Sedimentation Management
The problems to promote such reservoir sedimentation management in future are priority
evaluation of reservoirs where sediment management should be introduced andappropriate selection of reservoir sediment management strategies. Especially, whenseveral dams exist in a water system, it is realistically difficult to introduce the sedimentmanagement all together due to the limit of the budget under the present situation. Then,
it is necessary to evaluate priority according to some indices.
If the dam whose priority is high is selected, it is necessary to select a concrete sedimentmanagement strategy. In Figure 8, dams in the Japanese whole country are plotted by theparameter of the turnover rate of water (CAP/MAR=Total capacity/Mean annual runoff)and sediment (CAP/MAS=Total capacity/Mean annual inflow sediment). It is thoughtthat the selected sediment management measures are changed by these two parameters. It
is understood that measures actually selected have changed in the order of the sedimentflushing, the sediment bypass, sediment check dam and excavating, and dredging as
CAP/MAR increases (decrease in the turnover rate). This is because of greatly dependingon the volume of water to be able to use the sediment management measure that can beselected for the sediment transport. Figure 9 is focused on multipurpose dams which is
described in Figure 4. This information is very much useful to detect suitable actions.
C apacity-inflow ratio and R eservoir Life
(× No measure)100000No measures (from inner factors)

C heck dam

R eservoir Life=C A P /M A S

Flushing

10000

S coring gate
S coring pipe
B ypassing

1000
⑱
⑥

④

100
⑫ ⑤ ③

②

⑬

⑧

⑮
⑪

⑭
⑨

①

⑲

10
0.001

Excavating

⑦

⑰
⑳

⑩

D redging⑯ Sediment check dam,

Sediment replenishment

Sediment bypass, Sluicing
Sediment flushing, Sediment scoring gate

0.01
0.1
1
C apacity-inflow ratio=C A P /M A R

10

1 大井
Oi
D, SP
2 千頭
Senzu
SG
3 泰阜
Yasuoka SG, D, PF
4 笠置
KasagiSG, PF
5 大間
Ooma
SG, PF
6 常盤
Tokiwa
SG
7 三浦
Miura
SP
8 平岡
HiraokaD, PF
9 佐久間 Sakuma
D
10 八久和 Hakuwa
E, D
11 井川
Ikawa
SP, E, D, PF
12 山口
Yamaguchi SG, PF
13 秋葉
Akiba
E, D, PF
14 美和
Miwa
B, E, D, CD
15 牧尾
Makio
E, D, CD
16 小渋
Koshibu
E, CD
17 松川
Matsukawa B, E, CD
18 旭
Asahi
B
19 出し平 Dashidaira F
20 宇奈月 Unazuki
F
CD ：
F ：
PF ：
SG ：
SP ：
B ：
E ：
D ：

Check damFlushing (Draw down)
Flushing (Partial draw down)
Scoring gate
Scoring pipe
Bypassing
Excavating
Dredging

Figure 8. Representative sediment management dams in Japan.

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Figure 9. Representative sediment management dams in multipurpose dams.
(MLIT, Pref. Government and Japan Water Agency)
CONCLUSION
Reservoir sedimentation management in Japan is entering a new era. Although there arestill technical problems to be solved, we believe that the importance of sedimentmanagement will increase. Assessing issues, depending on each case, of dam security,
sustainable management of water resources and sediment management in a sedimentrouting system, an effective sediment management plan should be studied andmaterialized. Under such circumstances, collecting necessary data on reservoir
sedimentation to provide database nationwide and releasing suitable guideline forsustainable reservoir management is one of important tasks. We should share lessons
learned how to collect and how to utilize those data effectively worldwide through
ICOLD mechanism.
REFERENCES
Fukuroi, H., (2012). Damage from Typhoon Talas to Civil Engineering Structures for
Hydropower and the Effect of the Sediment Bypass System at Asahi Dam, Proc. of theInternational Symposium on Dams for a Changing World, The 80th Annual Meeting of
ICOLD, Kyoto.
Kantoush, S. A., Sumi, T., Suzuki, T. and Murasaki, M., (2010). Impacts of sedimentflushing on channel evolution and morphological processes: Case study of the Kurobe
River, Japan”, International Conference on Fluvial Hydraulics, River Flow 2010
Braunschweig, Germany, pp. 1165-1173.
Kantoush, S. A., Sumi, T. and Kubota, A., (2010). Geomorphic response of rivers below
dams by sediment replenishment technique”, International Conference on Fluvial
Hydraulics, River Flow 2010 Braunschweig, Germany, pp. 1155-1163.
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Kantoush, S. A., Sumi, T., and Takemon, Y., (2011). Lighten the load, International Water
Power & Dam Construction, May, 38-45.
Kantoush, S. A. and Sumi, T., (2011). Sediment Replenishing Measures for Revitalizationof Japanese Rivers below Dams, Proc. of the 34th IAHR World Congress, 2838-2846.
Sumi, T. and Kanazawa. H., (2006). Environmental study on sediment flushing in the
Kurobe River, 22nd international congress on large dams, Barcelona, Q.85-R.16, 219-242.
Sumi, T.，Nakamura, S. and Hayashi K., (2009). The Effect of Sediment Flushing andEnvironmental Mitigation Measures in the Kurobe River, ICOLD, 23rd Congress,
Brasilia, Q89-R6.
Sumi, T. and Kantoush, S. A., (2011a). Comprehensive Sediment Management Strategies
in Japan: Sediment bypass tunnels，Proc. of The 34th IAHR World Congress, 1803-1810.
Sumi, T. and Kantoush, S. A., (2011b). Sediment management strategies for sustainable
reservoir, Proc. of the International Symposium on Dams and Reservoirs under Changing
Challenges, The 79th Annual Meeting of ICOLD, 353-362.
Sumi, T., Kantoush, S. A. and Suzuki, S., (2012). Performance of Miwa Dam Sediment
Bypass Tunnel : Evaluation of Upstream and Downstream State and Bypassing Efficiency,
Proc. of ICOLD, 24th Congress, Kyoto, Q92-R38, pp.576-596.

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