12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Sediment Management Option by Sediment Sluicing in the Mimi River, Japan
Tetsuya Sumi, Sameh Kantoush
Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University
Kyoto, Japan

ABSTRACT
Currently, the reservoir sedimentation management strategy in Japan ischanging drastically from just receiving and depositing to releasing andsupplying to downstream river channels. In contrast to the emergent
and local conventional countermeasures such as dredging andexcavation, sediment bypassing, sediment sluicing and sedimentflushing measures which aim at radically reducing the sedimentinflowing and deposition are implemented. Unazuki and Dashidairadams in the Kurobe River, Miwa, Koshibu and Matsukawa dams in the
Tenryu River and Asahi dam in the Shingu River are well known asadvanced examples. Recently, sediment sluicing has been introduced
by retrofitting existing dams with deep spillway gates in severalhydropower dams in the Mimi River. This paper shows these present
state of the art methodologies of reservoir sedimentation in Japan andthese effects on sediment passage based on sediment grain size
distribution.

KEY WORDS: Reservoir sedimentation; Sediment flushing;
Sediment bypassing; Sediment sluicing;
INTRODUCTION
In Japan, there are more than 3,000 dams with one or multiple functions,which become essential for such modern society. Storage reservoirs
provide important functions such as flood mitigation, energy
production, water supply and so on. However, their side effects arestrong modifications in flow and discontinuity in sediment anddeclining river ecosystem health. In the past few years, in some ofJapanese dams’ serious sedimentation issues and loss of reservoir
capacity have been exceeding than estimated. Consequently, more
sediment has already accumulated than the design sediment yield andactive storage capacity is decreasing year by year. Therefore, without
immediate action plan some of these dams might have problems withmaintaining dam functions and reservoir operation.
Japan is one of the current leaders in sediment management
applications with nearly 25% of its dams restoring to sedimentexcavation, and a growing use as downstream supply (Kantoush et al.
2011). In order to sustain the downstream ecological functionality, wemust design optimal sediment volumes and sizes, as coarser substrates
are beneficial for benthic communities. As the sediment management
was not considered at the early stage of dam planning and designing, it
is a challenge to sustain reservoir functions, and to minimize downstream impacts of sediment starvation (Kondolf et al. 2014).

Fig. 1 Sediment management options (Kondolf et al. 2014)
There are various publications that classified techniques of reservoir
sediment management (Morris & Fan 1998, Kantoush & Sumi 2010,
Annandale 2013, Kondolf et al. 2014(Fig. 1)). Sediment bypass tunnels
avoids reservoir sedimentation and replenishing sediments below damwith a wide range of rates and timing similar to pre-dam conditions(Sumi et al. 2012). During sediment sluicing (or routing) water level at
dam has to be lowered sufficiently during high flows to maintain highvelocities. While, sediment sluicing through retrofitted dams of Mimi
River case study permits sediment to be transported through thereservoir rapidly to reduce siltation (Sumi et al. 2015, Peteuil et al.
2016). At the Kurobe River, both sediment drawdown flushing andsluicing are performed every year, where flushing channel is formed
and re-suspending sediments deposited in the reservoir flushed throughbottom outlets to downstream (Sumi et al. 2009). This paper overviews
these sediment management options and discusses the effects of uniqueretrofitting project in the Mimi River basin for sediment sluicing.

BASIC CONCEPT OF SEDIMENT PASSAGE ANDRECOVERING STAGE OF SEDIMENT TRANSPORT RATE
Sediment management techniques are classified by effects of sedimentpassage and recovering stage of sediment transport rate. Table 1
indicates the frequency of sediment passages from reservoirs todownstream reaches that are composed of fine and coarse sediments.
There are different sediment pulses replenished to the downstream
reach with each different reservoir sediment management method,where the reservoir pool is completely, partially or not lowered.

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Table 1 Classification and characteristics of sediment passages forreservoir management strategies
Sediment Replenishment
Methods

Sediment Passages

Morotsuka Dam

Yamasubaru Dam
Saigou Dam
N
Ouchibaru Dam

Lowering
Reservoir
Pool

Frequency of
sediment
passage

Coarse pulses

Fine pulses

Storage Dam

No

No

No

Nearly No

Drawdown
Flushing

Complete

Once a year

Sediment Bypass
TunnelNo

Several times
per year

Almost all（Frequent）

All（Frequent）

Fig. 3 Location map of dams along Mimi River Basin

Sediment Sluicing

PartialSeveral times
per year

Almost all（Frequent）

All（Frequent）

Table 2 Dams and reservoir sedimentation in the Mimi River

Adding ExcavatedSediment

No

Several times
per year

PartialPartial(Concentrated) (Concentrated)

Sediment Transport Rate {STR} (Qs)

1 Original state3

Partial STR

4

Improved STR

Beforedam 1

5 Nearly recovered STR

5
4

3
2 After dam
construction

Flow Discharge (Qf)

Fig. 2 Effectiveness levels of sediment management methods to recover
the sediment transport rates below dams

SEDIMENT SLUICING IN THE MIMI RIVER
Mimi River (Mimi-kawa) system is 94.8 km in length flowing into the
Pacific Ocean, located in the southeast of Kyushu in Miyazaki
prefecture of Japan with a catchment area of 884.1 km2 (Fig. 3). There
are seven dams which are Kamishiiba, Iwayado, Tsukabaru, Morotsuka,
Yamasubaru, Saigo, and Ouchibaru dams. They are operated by the
Kyushu Electric Company (KEPCO). Dam reservoirs are normallydesigned to have a capacity 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 for all seven dams. Flood damages occurred wereextensive as four power plants were flooded and overflow of Tsukabaru,
Yamasubaru and Saigou dams. Moreover, damages were enlarged as
10.60 Million cubic meter (MCM) of sediments and driftwood flowing
to the river and the seven reservoirs as mountain slope failures invarious locations occurred.

Hyuga City

Tsukabaru Dam
Iwayado Dam

AllAll(Concentrated) (Concentrated)

Figure 2 shows that each Sediment Management method has different
effects levels on downstream reaches below dams and recovery of river
health. Adding excavated sediments is suitable method in the normal
stage among other sediment management techniques. However, during
flood stage the downstream recovery level is very limited. In order toachieve higher recovery level to the downstream conditions, we have toshift to other methods as sediment bypassing, sediment sluicing or
drawdown flushing according to the reservoir size. Therefore, optimal
design of sediment transport rate by suitable combination of flow
discharges, and quantity and quality of sediment is needed to improvemorphological effects.

2 Severe reduction in STR

Kamishiiba
Dam

Name ofDam

Damheight(m)/ Yearof opening
Kamishiiba 110/1958
Iwayado/1972
Tsukabaru
87/1975
Morotsuka/1952
Yamasubaru 29.4/1981
Saigo
20/1984
Ouchibaru 25.5/1957

Total
storage
Capacity(103m3)91,5508,310
34,330
3,480
4,190
2,450
7,490

Pacific
Ocean

Total
sedimentation volume(103m3)
12,600
5,560
6,980
1,060
2,590
1,010
1,930

Annualsediment
volume(103m3)217.80
77.20
91.80
20.30
32.00
12.00
33.80

Sediment accumulation and dam characteristics are shown in Table
2.The table shows that sedimentation in all seven dams has progressed
faster than that of the original plan. Approximately 5.2 MCM havebeen 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 willcontinue to be deposited in the reservoirs and aggregation will occur.
Therefore, new investment for creating new function by sedimentsluicing 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 improvement of dam
function through sediment replenishment as well by excavating sandand gravel and supply to the downstream of river reaches.
Dams in Mimi river are losing original functions due to sever
sedimentation occurred recently after Typhoon. KEPCO, which is
responsible for dam installations, is as part of the Management Planaiming to restore the original sediment flow, which has beenintercepted by dams, and has drawn up a plan for sediment sluicing,incorporating Yamasubaru, Saigou, and Oouchibaru Dams. In recent
years, various countermeasures have been studied. As shown in Fig. 4,
it has been proposed that using CAP/MAR (Total capacity/Meanannual runoff) and CAP/MAS (Total capacity/Mean annual inflow
sediment) as parameters, sediment measures can be classified, which
will assist in selecting an appropriate countermeasure. According to
Sumi 2008, with an increase in CAP/MAR (that is, a decrease in dam
regulating reservoir turnover rate), the appropriate sediment measure
will vary between: sediment flushing, sediment bypass, sedimentsluicing, sediment check dam, excavating and dredging, or no necessity
for a sediment measure to be applied. The reason for this is that thesevarious sediment measures depend largely on the amount of water thatcan be used for sediment management, which in turn depends on thescale of the dam regulating reservoir. CAP/MAR and CAP/MAS data
for each of the 7 dams in the Mimi River Basin is shown in Fig. 4.
According to this, it can be seen that compared to Kamishiiba,
Tsukabaru, and other dams upstream, Yamasubaru, Saigou and12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Oouchibaru Dams 1) have low CAP/MAS and therefore have asubstantial need for sediment measures and 2) dam-regulating reservoir
turnover rate is high (CAP/MAR is low), making sediment flushing andsediment sluicing appropriate. Based on this, the retrofitting of dams
for sediment sluicing operation was initiated.

Capacity/Mean annual Sedimentat Yield
Reservoir Life [Year] = CAP/MAS

100000

Flushing
Scoring gate
Bypass Tunnel
Dry Excavation
×No measures
Mimikawa

Check Dams
Scoring pipe
Dredging

10000

4- Density
current venting,storage
operation

SaigouOhuchibaru Kamishiiba
1000

Morozuka

3- Check dams,
Replenishment

100

10

2- Bypass,sluicing,
excavation
1- Flushing,dredging,
HSRS

Yamasubaru

Tsukabaru

Fig. 5 Retrofitting works of Yamasubaru and Saigou dams
Existing Operation

Iwayado

0.1
1
0.001
0.01
10
Retention time [Year]= Capacity/Inflow = CAP/MAR
Fig. 4 Proposed sediment management techniques and dams in theMimi river system

The river administrator established the Mimi River 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 thesevarious 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 sediment flow, while balancing
flood control, water usage and environmental conservation, formulated
a policy to advance integrated sediment flow management. In October
2011, the Mimi River Basin Integrated Sediment Flow Management
Plan was compiled. The plan established matters such as the work to be
carried out and stakeholder roles, with the aim of resolving problems
caused by sediment in the basin.
KEPCO formulated an action plan to recover the original state.
Retrofitting works on Yamasubaru and Saigou dams (Fig. 5) havestarted in November 2011. In Japan, it will be the first time that anexisting dam will be modified by the addition of a new sluicing
function after 80 years of commissioning. Figure 6 shows the conceptof sluicing operation at Yamasubaru, Saigou and Oouchibaru Dams, bytemporarily lowering of reservoir water levels to make the flow ofwater in dam regulating reservoirs close to the original, natural state of
the river, thereby allowing inflow sediment to be transported below
dams. It has to be noted that due to the ongoing retrofitting works,changes have been already experienced regarding the quantity of
sediments passing the reservoirs. In the particular case of the Saigou
dam, a significant rise of coarse sediment fluxes has been observed
since the beginning of the works as a result of the water depth drop andflow velocity increasing. It is important to understand and monitor the
sediment input from the river basin that is affected by factors such as
the relationship between river flow volume and amount of fluctuation.

Dam(a) Dam modification (reservoir level reduction)
Upgrading(b) Sluicing of sediment(early realization ofcleansing effects + environmental measures)

Dam sediment sluicing management (ideal image)Sluicing Operation after dam modification

Free flow
Sediment
sluicing

Fig. 6 Schematic view of sediment sluicing
In order to do this, one dimensional analysis of riverbed variation was
carried out, and the effect of sediment sluicing at dams verified for theapproximately 58 km stretch of the main river, covering the three dams
where sluicing is to be carried out, from the area upstream of the
Yamasubaru Dam regulating reservoir down to the river mouth. Figure
7 shows a comparison between riverbed elevation for Yamasubaru
Dam after 33 years under current operation (without sluicing) and withsluicing operation. It has been confirmed that if current dam operations
are continued, sediment will continue to be deposited in the reservoirs.
Whereas if sediment sluicing operation is carried out, sediment
upstream of regulating reservoirs will be transported downstream,
which will cause morphological changes in the downstream reaches.

Fig. 7 Results of riverbed fluctuation analysis (Yamasubaru Dam)

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.
Unit: ‘000m3/year

Silt and clay〔d ≦ 0.075mm〕

0〔0〕

Yamasubaru Dam
(46k)

Sand
〔0.075mm ＜ d ≦ 2mm〕

30

-1
〔-11〕

12

0
〔0〕
42
〔0〕

17

0
〔0〕

21

28

+2
〔0〕

Estuary (0k)

78
〔0〕
0
〔0〕
78
〔0〕

0
〔-8〕

5

14 k

0
〔-24〕

8

1
1.2 k

132
〔+34〕
+3
〔+1〕
129
〔+33〕

+2
〔-2〕

+ 11
〔+1〕

11
〔+11〕

distribution. For existing dams, retrofitting works in the Mimi River,Japan will be an unique reference. By modifying spillway gates,
sediment sluicing will increase the sediment fluxes released fromreservoirs and is to minimize upstream flood risks and likely to induce
morphological changes in the downstream river environment.
Assessing the impact of such measure prior to its implementation istherefore necessary. To achieve this objective and better define thecurrent state of the Mimi River in terms of sediment dynamics, a survey
is being carried out, covering issues such as water quality, bedmaterials characteristics, river channel morphology, as well as aquatic
fauna and flora. 1D and 2D numerical modelling are also important topredict short term and long term sediment transport and morphologicaleffects.

+ 12
〔+10〕

137
〔+37〕
+ 14
〔+3〕

Decreasing tendency
Somewhat increasing tendency
Increasing tendency

14
〔+12〕

+ 10
〔+6〕
39

17

Notes: 〔〕indicates difference from existing case.

11
〔+10〕

108
〔+43〕

+1
〔0〕
79
〔0〕

+6
〔-10〕

80
〔+19〕

Oouchibaru Dam 59
〔0〕
(23k)

21

58

59
〔+11〕

30
〔0〕

Saigou Dam(36k)

Gravel〔2mm ＜ d ≦ 75mm〕

79

REFERENCES

6
〔+1〕
+6
〔+1〕

2

2
〔0〕
+2
〔0〕
0
〔0〕

Fig. 8 Sediment budget change after sediment sluicing operation
Therefore, it is expected that the retrofitting will lead to reduce theflood risk and develop a healthier river environment, including theecosystem, by controlling degradation and beach erosion in the dam
downstream and in littoral areas, as well as by promoting
diversification of riverbed materials. Using the tools mentioned,
sediment flow simulations are being conducted under the assumption ofvarious future risks, and specific methods for sediment sluicing at dams
is being studied. These results are very encouraging and demonstratethat sediment sluicing is a promising measure for restoring the sedimentcontinuity throughout the river system for a wide range of particles.
Figure 8 is showing sediment budget change after retrofitting works.
Sediment sluicing operation is providing more passage for sand andgravel materials.
It has to be noted that due to the ongoing retrofitting works, changes
have been already experienced regarding the quantity of sediments
passing the reservoirs. In the particular case of the Saigou dam, a verysignificant intensification of coarse sediment fluxes has been observed
since the beginning of the works as a result of the water depth drop andflow velocity increasing. Those preliminary field evidences are very
encouraging and demonstrate that sediment sluicing is a promisingmeasure for restoring the sediment continuity throughout the river
system for a wide range of particles. Field monitoring appears to be an
essential management tool and is thus scheduled to continue forunderstanding and quantifying the impact of sediment sluicing
operations performed at dams on the river environment.

CONCLUSION
In case of reservoir sediment management, it is necessary to assess thecurrent situation and possible future scenarios from a reservoir
sustainability point of view. For sediment management plan,
combination of flow and sediment release should be appropriately
designed to meet demands of various functions based on data ofhydrology, water quality, river morphology and ecosystem, etc. Among
several updated methodologies, sediment sluicing is one of effective
and ecofriendly solutions. In order to maximize the benefit, selecting
suitable timing, duration, frequency and water level or velocity in thereservoir are important based on target sediment volume and grain seize

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