River Sedimentation – Wieprecht et al. (Eds)© 2017 Taylor & Francis Group, London, ISBN 978-1-138-02945-3

The aging of Japan’s dams: Innovative technologies for improving dams
water and sediment management
S.A. Kantoush & T. Sumi
Disaster Prevention Research Institute (DPRI), Kyoto University, Kyoto, Gokasho, Ujishi, Japan

ABSTRACT: Dams are probably the most important hydraulic structure that are expected to be operated forthe longest life. The major challenge to extend dams’ life is siltation in reservoirs. In order to sustain dam
functionality for long-life and solve associate issues as dam safety, flood and sediment management, we must
consider the necessity of upgrading and retrofitting of such aging dams. As they have long service life, if we
can solve the associated issues of sediment management, extreme flood, monitoring dam structure, operationand gate control system, the dam will extend life with proper management. The main purpose is to explain, how
to select appropriate sediment management strategy in each reservoir according to sedimentation conditions
within reservoir and in downstream reaches. The other is how we can recovering, increasing, extend reservoir
functions. It is possible to increase the reservoir life by implementing proper sediment management method,
and they are important in dealing with dams’ life cycle cost. The paper highlight the potential benefits of theasset management and coordinating reservoirs management in the river basin scale. However, still missing are
the necessary cost benefits assessment to evaluate the sediment management investments.

1
1.1

INTRODUCTION
Necessity of dam redevelopment

Dams are probably the most important hydraulic structure that are expected to be operated for the longest
life. The aging of Japan’s dams and continuous loss ofstorage capacity due to reservoir sedimentation, coupled with increasing environmental needs, will cause
the social, economic, environmental, and politicalimportance of dams to constantly increase. Removing such stored sediments is often recommended asa potentially better way to recover reservoirs storage
capacities than dam heightening or building new dams.
The rapid reservoir sedimentation not only decreases
its storage capacity, but also increases the flood risk inthe upstream reaches and e greatly reduce the sedimentload in the river.
In order to sustain functions of dams for long-lifeand solve associate issues as dam safety, flood andsediment management, we must consider the necessity of upgrading and retrofitting of such aging dams.
As they have long service life, if we can solve theassociated issues of sediment management, extreme
flood, monitoring dam structure, operation and gate
control system, the dam will extend life with proper
management. Figure 1 presents a proposed approach
to increase dam service life by recovering, improving, and extending dam functions. This approach iscomprised of three assessment levels of dam functions(Fig. 1). The first for damaged functions and priority
investment is needed to recover the original function

Figure 1. Necessity of upgrading and retrofitting aging
dams.

and the second for additional function newly requiredby improvement and retrofitting the dam, and finallythe third by extend dam life.
1.2

Management of siltation in reservoir

The major challenge to extend dams’ life is siltation inreservoirs. A comprehensive paper review from fivecontinents experiences in managing reservoir siltationand mitigation downstream impacts summarized by
Kondolof et al., (2014). There are a wide range of publications classified sediment management techniques
as Morris and Fan, (1998) and Kantoush and Sumi,(2010). They categorized such techniques for threestrategies based on reducing sediment delivery fromupstream, maintain reservoir capacity, and provide
sediment to downstream reaches. Most of these strategies are planned and implemented without any consideration for the necessity of upgrading by improvement,
recovery, and extend dam life. This paper categorizing sediment management technologies based on theconceptual diagram in Figure 1 from dam reservoir

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Figure 2. Necessity of upgrading and retrofitting aging
dams.

functions. The main purpose is to explain, how to
select appropriate sediment management strategy ineach reservoir according to sedimentation conditions
within reservoir and in downstream reaches. The otheris how we can recovering, increasing, extend reservoir
function. It is possible to increase the reservoir life byimplementing proper sedimentation countermeasures,and they are important in dealing with dams’ life cycle
cost. In this paper, the results of study in Japan thatused redevelopment and asset management are presented. The examples provided demonstrate the use of
sediment excavation and retrofitting.
This paper focuses on one critically important element of these challenges: the need to retrofit ormodify the design and operation of existing dams. The
retrofitting of dams for modified operation by sluicing
refers to cutting dam body and installing (a) New spillways, (b) bottom outlet, (c) bypass tunnel, (d) modify
and enlarge existing spillway gates. Moreover, understanding of the necessity of upgrading and retrofittingof aging dams in Japan will be necessary to support
the difficult management decisions that ultimately will
have to be made. Figure 2 categorizing managing of theaging dams from the standpoint of reservoir recovery,
extending, and adding innovative new functions.
Possible management issues for aging dams include
sediment management techniques to increase thereservoir capacity, extend dam life, dam modification, dam removal, changes in reservoir operational
practices, habitat availability and quality, and the environmental consequences of any actions taken. The
sustainable development have proliferated, but the following concepts are most relevant from the standpointof retrofitting dams:• Guaranty security and acceptable functioning of thereservoir for its purposes;• Today’s patterns of infrastructure development
should preserve the capacity of the reservoir andmaximum possible time of functioning;

Figure 3. Proposed approach to design effective sedimentmanagement strategy.

• Produce minimum impact to the environment andmaintain biological diversity;• Minimize the potential for catastrophic disasters
resulting from dam failure;• Avoid activities that create a legacy of environmental restoration or dam rehabilitation obligations that
disproportional on future generation;• Consider all processes: hydrological regime, erosion, climate change, environment and associatedecosystems, water and sediment continuity.
Requiring a reservoir life measured in terms ofthousands years instead of decades will demand new
methods of analyzing costs and benefits.
For all these reasons, developing new techniquesto evacuate the fine and coarse sediment to maintainthe functionality, and at the same time ecologicallyre-habilitating the involved landscape would be economically and environmentally beneficial for all typesof reservoirs. Figure 3 shows the proposed procedure
for setting up sediment management strategy. At first,
we prioritization the selection of the dam that needs
countermeasures for sediment management purpose,
set its lifecycle for redevelopment project for such
dam to calculate investment cost and benefit and theduration to implement the sediment countermeasures.
Then, based on historical measurements of sedimentation, we can estimate future expected annual averageamount of inflow sediment and total inflow sediment.
Then choose the best scenario based on this in termsof maintaining both dam original functions and theenvironment of the rivers and coasts. The other is todesign sound river environment de-scribed representatively such as by river bed elevation, grain seize andmorphological dynamics and design necessary sediment supply volume and grain size. Finally, we should
combine these two needs and decide appropriate scenario to dis-charge sediment from dams. Based on the
proposed scenarios, we can select optimal sedimentmanagement measures from several possible options
such as reservoir flushing, dry excavation, sediment1031

Table 1. Dams and reservoir sedimentation characteristics
in Mimi River Basin.

Name ofdam

Figure 4. Location map of dams along Mimi River Basin.

bypass tunnel, sluicing, density current venting, dredging, siphoning, and HSRS. If technical significant
problems may arise, we should modify the scenario.
Selected sediment management measure should be
evaluated by benefit/cost analysis and, if the project is
not economically feasible, we should revise the projectfrom the master planning stage. Sediment management of reservoirs is very much complicated project
which will affect reservoir sustainability and also
maintaining downstream river health. So we should
start to implement step by step basis by conducting
filed monitoring under the adaptive management concept. Two case studies are presented in this paper
to discuss implementation of sediment management
techniques from recovering and improving functionsin Mimi River System. The second case study discussing the asset management of dam reservoirs byimplementing sediment management techniques toextend dam life in Kizu River System.

2

RECOVERING AND IMPROVING DAM
FUNCTIONS IN MIMIKAWA RIVER SYSTEM

2.1 The study area
Mimi River (Mimi-kawa) system is 94.80 km in length
flowing into the Pacific Ocean, located in the southeast of Kyushu in Miyazaki prefecture of Japan witha catchment area of 884.1 km2 (Figure 4). There areseven completed dams in the Mimi River System,
Kamishiiba, Iwayado, Tsukabaru, Morotsuka, Yamasubaru, Saigo, and Ouchibaru dams as shown in
Figure 4.
Dam reservoirs are normally designed to have acapacity to store 100 years, worth of sediment in
the deepest parts close to the dams. Recently, rainfall and water volume flowing into dams in September
2005 exceeded the designed dam reservoir flood forall seven dams. Flood damages occurred were extensive as four power plants were flooded and overflowof Tsukabaru, Yamasubaru and Saigou dams and their
dam control facilities flooded as well. Moreover, damages were enlarged as 10.60 Million Cubic meter(MCM) of sediments and driftwood flowing to the riverand the seven reservoirs as mountain slope failuresin various locations occurred. Sediment accumulation and dam characteristics are shown in Table 1.
The table shows that sedimentation in all seven dams
has progressed faster than that of the original plan.

Dam
Total
Total
Annualheightstorage sedimentation sediment(m)/Year capacity volume
volume(103 m3 )of opening (103 m3 ) (103 m3 )Kamishiiba 110/1958
Iwayado57.5/1972
Tsukabaru 87.0/1975
Morotsuka 59.0/1952
Yamasubaru 29.4/1981
Saigou20.0/1984
Ouchibaru 25.5/1957

91,5508,310
34,330
3,480
4,190
2,450
7,490

12,600
5,560
6,980
1,060
2,590
1,010
1,930

217.80
77.20
91.80
20.30
32.00
12.00
33.80

Approximately 5.2 MCM have been deposited in reservoirs of Mimi river system. It is clearly noticed that,
if the group of dams will continue with the current operations rules without recovering or adding
new dam function, the sediment will continue to bedeposited in the reservoirs and aggregation will occur.
Therefore, new investment for creating new function
by sediment sluicing is proposed, where sediment
upstream of reservoirs will be drawdown and passing
through dam to the downstream reaches. In particular, it was confirmed that there would be improvementof dam function through sediment replenishment aswell by excavating sand and gravel and supply to thedownstream of river reaches.

2.2 Sustainable selection of sediment techniques
Dams on Mimi River are losing original functions(flood control, water supply, and recover the downstream environmental conditions) due to the sever sedimentation problems occurred recently after Typhoon.
KEPCO, which is responsible for dam installations,is as part of the Management Plan aiming to restore
the original sediment flow, which has been intercepted by dams up until now, and has drawn up aplan for sediment sluicing, incorporating Yamasubaru Dam, Saigou Dam, and Oouchibaru Dam. In
recent years, various countermeasures for sedimentflow management policies have been studied.
As shown in Fig. 5, it has been proposed thatusing CAP/MAR (Total capacity/Mean annual runoff)
and CAP/MAS (Total capacity/Mean annual inflow
sediment) as parameters, sediment measures can beclassified, which will assist in selecting an appropriate countermeasure. According to Sumi 2010, withan increase in CAP/MAR (that is, a decrease in damregulating reservoir turnover rate), the appropriate sediment measure will vary between: sediment flushing,sediment bypass, sediment sluicing, sediment checkdam, excavating and dredging, or no necessity for asediment measure to be applied. The reason for this isthat these various sediment measures depend largelyon the amount of water that can be used for sedimentmanagement, which in turn depends on the scale of the1032

Figure 5. Proposed sediment management techniques anddams in the Mimi River system.

dam-regulating reservoir. CAP/MAR and CAP/MAS
data for each of the 7 dams in the Mimikawa River
Basin is shown in Figure 5. According to this, it canbe seen that compared to Kamishiiba, Tsukabaru, andother dams upstream,Yamasubaru, Saigou and Oouchi
Dams 1) have low CAP/MAS and therefore have asubstantial need for sediment measures and 2) damregulating reservoir turnover rate is high (CAP/MAR
is low), making sediment flushing and sediment sluicing appropriate. Based on this, the retrofitting of dams
for sediment sluicing operation was initiated.
Basin Integrated Sediment Flow Management
Technical Committee, in which river basin stakeholders participate. Rather than focusing on each problem
separately, the prefecture came to a proper understanding of these various sediment-related problems
over the entire river basin, including the mountainous areas, dams, the river itself and the coastal areas.
And with the aim of restoring the original sedimentflow, while balancing flood control, water usage andenvironmental conservation, formulated a policy toadvance integrated sediment flow management.
2.3

Recovering dam function

This class includes the recovery or rehabilitation ofreservoir functions, i.e. reservoir storage volumes andlevels from losing the functionality due to reservoir sedimentation. For dams in such losing function condition, sediment management techniques are
implemented to restore the effective reservoir capacity needed to regulate flood events or water supply
and to guarantee live storage. There are various structural and nonstructural methods can be engaged are:(a) dry excavation, (b) permanent dredging facility,(c) redevelopment of old spillway by replacing gate
and valves, (d) recovery environmental functions forriver by constructing fish passage and sediment supply, (e) water quality conservation measures for thethree major issues of long-term turbidity, temperature,and eutrophication in reservoirs.
It is assumed that in response to the change in theamount of flow of sediment due to sediment sluicing

Figure 6. Retrofitting concepts to sediment flow management in dam-regulating reservoirs by cutting down spillway
crest height.

operation at dams, that there will changes in the riverenvironment and it is therefore necessary to makean assessment of the impact. However, there are no
specific guidelines to assess the impact of sedimentsluicing on the river environment. Therefore, at first it
is necessary to implement adaptive management basedon an initially adopted plan that is subject to review
while monitoring is carried out, during which timea new plan for an assessment method appropriate to
the characteristics of sediment sluicing at dams on theMimi River will draw up.
When drawing up a plan for assessment methods,
it is important to appropriately set both the impactof sediment sluicing at dams, and the response. The
setting for impact is based on calculated fluctuations
in sediment flow, using a one-dimensional analysis
of riverbed variation. The setting for the assumed
response has been made based on the setting for
impact.

2.4 Adding new functions
By adding new functions to the existing dam, such as
flood control and disaster risk reduction, rehabilitation
activities have to be implemented. In order to create
addition of new function, the following upgrading andretrofitting means can be considered:
1. Increasing the capacity of the reservoirs by excavating/dredging the deposited sediments;
2. Increasing flood discharges by improving discharge
facilities and adding new outlet;
3. Increasing dam height will add extra effective
storage for flood control or other purposes;
4. Exchange reservoir storage functions among groupof dams by linking several single adjacent damsin the basin so that the storage capacity can beeffectively used;
5. Modify reservoir operation rules by preliminary
drawdown water level in the reservoir and keep
it flexible during flood period. Thus, by increasing flow volumes, an additional reservoir storage
capacity will be guaranteed;1033

Table 2.

Results of the FDO tracks for different flows.

Figure 7. Multipurpose dam operation system and flexible
dam operation system.

6. Increase dam safety and ability of the dam body toresist earthquake and other disasters;
7. Improving the existing functions for energy recreation and amenities;
The Flexible Dam Operation (FDO) is a dam
management method to enhance conservation andrestoration of the river environment located in thedownstream of a dam via utilizing “usable capacity”
without interrupting prime flood control.
The “usable capacity” is a vacant portion of dam’s
capacity, which is reserved for actual flood event during the rainy/typhoon seasons. The water stored in thecapacity is called “usable water”. The FDO requires
storing water temporarily up to its design level or“usable water level” within the flood control storage
capacity as illustrated in Figure 3.
This utilization storage capacity that has been newly
created is used for utilization discharge as follows:• Discharge to increase the instream flow: Its purpose

is to improve the scenery and the habitat for fishes.
• Flushing discharge: Its purpose is to stir up theriverbed and flush out silt and slack water on theriverbed.
Since 1997, a “Flexible Dam Operation” has been
underway at dam under the jurisdiction of MLIT withthe purpose of improving the river environment downstream of dams. The positive effects that have beenobtained by the investigation are summarized in Table
2 according to each discharge type.

3

EXTENDING DAM LIFE BY ASSETMANAGEMENT IN KIZU RIVER SYSTEM

3.1 Networking dam group for reservoir sedimentmanagement
Five dams in the Kizu river system are sustainedin group by Japan Water Agency (JWA) for water
resource development including Takayama, Nunome,
Hinachi, Shourenji, and Murou dams as shown in
Figure 8. The current conditions of reservoir sedimentations in all five dams has progressed faster thanFigure 8. Location map for networking reservoir sedimentmanagement on Kizu River upstream adjacent dam group.

assumed in the original plan (In Japan, the planned
sedimentation capacity is set by the sedimentation of
100 years).
Moreover, at the Takayama Dam, Shorenji Dam,and Muro Dam, that were constructed more than 30
years ago, sedimentation has reached 40 to 50% of
the planned sedimentation. By focusing on annual
variations of incoming sediments revealed that at
the Takayama dam, annual average sedimentation isapproximately 100,000 m3 , but the maximum sedimentation in a single year is about 620,000 m3 . The
maximum annual sedimentation at the other dams isabout 5% to 12% of sedimentation capacity, and thepast maximum sedimentation in a single year was
equivalent to that occurring in a 5 to 12 year period(Table 3)
Reservoir as an asset is generally predicted to havesemi-permanently long service lifetimes. Therefore, it
is important to evaluate its reliability, because deterioration of the dam could have serious consequences on
the society. A traditional common design concept ofreservoir is to provide a dead storage space for sediment deposition. This concept is incorrect as the delta
formation and siltation is prevalent in the active storage space from the early stage of the dam construction.
Moreover, sedimentation is handled by setting 100
year capacity as the planned sedimentation capacity.

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Table 3.

State of sedimentation in the 5 dams group.

Table 4. Categorization of facilities and management priorities by renewal period.

Table 5. Scenarios and options of studies sediment management techniques.

Such dead storage space for sediments is actuallya stagnant water storage with less deposition comparing to the active space deposition. It is necessary to
account for such loss in active reservoir storage space
early on during the life of a project and to recognize its impact on the reliability of water and power
supply, and on flood control. As the length of service life and priority of management activity vary
between facilities, it is necessary to apply asset management according to these differences as shown in

Table 4. It is possible to prolong service lifetimes
by creative dam design concept through appropriate
sediments and fish passages, including sediment management strategies which become relevant in the studyof the life cycle costs of dams. In Japan, as a common practice a periodical monitoring for reservoir
sedimentation is conducted before and after flood. In
cases of increased reservoir siltation and the dam lose
its functions, an immediate intervention for sedimentmanagement to recover the original dam functionality
or upgrading dam facility to ensure reservoir capacity.
When a large-scale sediment management techniques
is implemented, obtaining a large storage yard forremoved sediments is a challenge, and it is necessary
to reduce costs by using sediment material effectively
combined with flushing flow. Regarding the environment, blocking the continuity of sediment by a dam
impacts rivers, coastlines and the ocean, so it is necessary to restore sediment downstream from dams.
But the quantity that must be restored downstream
to conserve the environment, and the environmental
benefits of this downstream restoration have not been
evaluated. Judging from the above, clarifying cases
where applying asset management to sedimentation
countermeasures and taking sustained sedimentation
countermeasures are beneficial from the cost perspective, and it is possible to reduce life cycle costs and at
the same time, reduce the environmental impact of adam.
3.2 Effects of management strategies in extending
dam life
A case study was performed of sedimentation countermeasures at the furthest downstream end of thedam group: the Takayama dam with the dam group’s
largest reservoir. The effects of implementing eachsedimentation countermeasure hypothesized by thisstudy are organized considering the application to
the study of maintenance plans over long periods oftime, and hypothesizing the “annual sediment removal
rate (percentage of inflowing sediment load which
can be removed by taking the countermeasure)”. So
in addition to normal sedimentation countermeasuremethods, a comparative study of “dry excavation”
is also performed (see Table 5). Table 5 shows thesedimentation countermeasures and key issues in longterm management for the Kizu River dam group. Dry
excavation is performed by shutting down the dam for
one year at an interval of a specified number of years,
completely emptying the reservoir and removing sediment by dry excavation. Therefore, to perform dry
excavation, it is necessary to carry out a comparative
study with other methods to determine whether or not it
is an effective method considering the loss of reservoir
functions caused by draw down and relationship andmerits and demerits of removing sediment by low costdry excavation. The cost of dry excavation includes,
in addition to compensation for reduced power generation and excavation unit price, the loss caused bydraw down which is calculated treating the loss of the1035

counted on, so the sedimentation capacity should bemaintained to constantly ensure leeway from about
5% to 12%. So the target sedimentation rate for casestudies of each dam when operated independently wasset at 80%. (Assuming that sedimentation load in thereservoir is controlled within 80% of the planned sedimentation capacity).The study should focus on thefollowing points in order to achieve more efficient
sediment removal.
a) As a result of the case study of long-term maintenance of the Kizu River upstream dams, it has
been shown that more economical results at all
the dams are obtained by carrying out the dry
excavation with the reservoir emptying (predicting compensation for reduced power production
and compensation for water use loss) than bycountermeasure methods requiring initial facility investment (for example, sediment bypass,
flushing sediment etc.).
b) The above results suggest the possibility of “optimization of sediment removal methods as a damgroup (cheaply removing sediment by linking a
dam group)”.

Figure 9. Schematic Diagram of integrated operation of
upstream Kizu River group of dams by an N+1th Dam.

capacity itself is as the loss-cost. So the loss of reservoir functions by dry excavation is reflected in the cost
as shown below.
If excavation is done during a non-flood period, only
the water use capacity is used. Therefore, the loss-cost
for the lost reservoir capacity is calculated by allocating the cost of water supply capacity to the operator ofa multi-purpose dam (allocation cost).
However, the following study uses the relationshipof the cost allocation with the water use capacity at
the Hinachi Dam that is the newest of the Kizu River
upstream dam group.
• Assuming that the water use function can be pro-vided for 100 years, the cost per unit capacity (1 m3 )per year is calculated as:(Cost of water use capacity)/ (water use capacity)/ (100 years).
• Based on the above, it is possible to calculate theloss-cost for the case of a dam shut down for one
year.
• If the cycle of emptying the reservoir is long, it ishighly likely that sudden sedimentation will occur
often, so considering the fact that there is a 1/10
year to 1/50 year probability of the occurrence of the
maximum annual sedimentation that has occurredin the past in the Kizu River dam group, thisstudy assumes that empty the reservoir every 10
years. Table 8 shows the cost-loss of dry excavation
with the reservoir emptying as calculated based on
conditions at the Hinachi Dam.
3.3

Optimization of sediment removal methods

As stated in Table 3, the actual maximum annual sedimentation (maximum value of inflowing sedimentload occurring unexpectedly) is approximately 5%
to 12% of the planned sedimentation capacity. So toensure stable reservoir functions in the event of an
unexpected inflow of sediment, when a dam is operated independently, backup by other dams cannot be

Based on the above results, considering the fact that
it is difficult to pay compensation for water use loss
which dry excavation actually causes, and that dry
excavation is performed very cheaply, in the future,long-term maintenance based on integrated operation(backing up water supply capacity, ensuring potential water resources, etc.) must be studied in order to“optimize sediment countermeasures as a dam group.”
Fig. 7 shows an image of coordinated dam group
operation by N+1 dams.
√
Dry excavation with the reservoir emptying (inexpensive because it is land excavation) is performedat all dams in rotation, and a dam is positioned as√ a “refresh dam” while the draw down.
The N+1th dam installed to backup reservoir
functions provides supplementary backup for the
decline in reservoir functions while the draw down.
(• means that maintenance is performed by (N+1)
dams: the minimum number of dams consideredessential (N) plus a maintenance use dam.)

4

CONCLUSIONSThis study has outlined the necessity of upgrading
and retrofitting the aged dams, asset management, andapproach to design effective sediment management
startgies required to construct future dams, maintainreservoir functions of existing dams, and contributeto total basin scale development. Tens if thousandsof fams are multifunction with tens of thousands of
sediment deposition volumes in deep, middle, andupstream tailwater parts of the reservoir. There are various sediment management methods siutable for eachpart of the reservoir as excavating, dredging, bypassing, flushing, and sluicing (Figure 9). These methods

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REFERENCES

Figure 10. Scenarios for sediment management options torecover dam functions and extend dam life.

often compete, and one of the most common tradeoffs involves choosing between no action or recovering
dam function, and extending dam life. Sediment management needs investment with different costs basedon evaluating the economical values of the selected
method. The costs and benefits of asset management
in coordination with changes in dam and downstream
are important to understand the importance of thesediment management.
A new concept and methodology should be conceived to design an intergenerational, sustainable,
self-supporting rehabilitation system for river basins
with manmade reservoirs. An efficient asset management and a variety of sediment management methods
in Kizu River upstream dams performed. Based on
the results, precautions to be followed to perform asset
management of dam reservoirs by sediment techniques
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it will be essential to study long-term maintenancein order to optimize sedimentation countermeasures
as a dam group: sedimentation countermeasures byintegrating the operation of a dam group, and the application of permanent sedimentation measures at the
Takayama Dam. Measuring the benefits and costs of an
improvement in water quality is often difficult. First,
for a complete analysis, all relevant benefits and costs
have to be measured.

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