Q. 99 – R. 45
COMMISSION INTERNATIONALE
DES GRANDS BARRAGES------VINGT-CINQUIÈME CONGRÈS
DES GRANDS BARRAGES
Stavanger, Juin 2015-------RETROFITTING AND CHANGE IN OPERATION OF CASCADE DAMS TO
FACILITATE SEDIMENT SLUICING IN THE MIMIKAWA RIVER BASIN (*)
Tetsuya SUMI
Disaster Prevention Research Institute, Kyoto University
Takeshi YOSHIMURA and Katsuyuki ASAZAKI
Mimikawa Hydro Power Development Office, Kyushu Electric Power Company
Mutsuhiro KAKUEngineering Division, Kyushu Electric Power Company
Josuke KASHIWAI
Japan Dam Engineering Center
Takahiro SATO
Central Research Institute of Electric Power Industry
JAPAN

1.

INTRODUCTION

The Mimikawa River in the Miyazaki Prefecture, in the southeast of Kyushu,
Japan, is flowing into the Pacific Ocean with a length of 94.8km and a watershedarea of 884.1km2. It is one of the largest ‘Class B’ rivers which are operated by
prefectural government. Making use of an abundant volume of water and large
water head, 7 dams and hydropower stations were developed between the 1920’s
and 1960’s as shown in Fig. 1, and currently have a combined generating powerof 340 MW, and an output of 900 million kWh, making it one of the most important
areas in the Kyushu region for hydropower production.
(*) Rééquipement et modification d’exploitation de la cascade de barrage pour faciliter lavidange des sédiments dans le bassin de la Mimikawa.

597

Q. 99 – R. 45
In September 2005, Typhoon Nabi hit Japan, causing large scale damage invarious parts nationwide, including Miyazaki Prefecture. In the Mimikawa River,rainfall and water volume flowing into dams also exceeded their designed dam
flood flow discharges.
Flood damage was extensive in the central area of Morotsuka village
located on the upstream edge of the Yamasubaru Dam. Moreover, flood damage
was amplified by mountain slope failures in approximately 500 locations, large
and small, causing a huge amount of sediment and driftwood flow into the riverand dam-regulating reservoirs in the Mimikawa River Basin.

Mimikawa
River

Kyushu3. Tsukabaru
63 MW[1938]③ 塚原

1.Kamishiiba
93 MW
② 岩屋戸
[1955]① 上椎葉

4. Morotsuka
50 MW④ 諸塚[1961]⑤ 山須原

5. Yamasubaru
41 MW
2. Iwayado[1932]
51 MW[1942]
Stream length: 94.8km
Watershed area: 884.1km2

7. Oouchibaru
16 MW[1956]⑦ 大内原⑥ 西郷

6. Saigou
27 MW
日向市[1929][ ] indicates year ofcommissioning

Fig.1
Mimikawa river basin, dam and power plant
Bassin de la rivière Mimikawa, barrages et centrales
In October 2011, Miyazaki Prefecture, the river administrator, compiled the“Mimikawa River Basin Integrated Sediment Flow Management Plan” which is
showing the current status of the complex Mimikawa River sediment problems
and possible approaches to solve these problems while balancing flood control,
water usage and environmental conservation. As part of the Management Plan,
the Kyushu Electric Power Company, KEPCO, which is responsible for dam
installation is aiming to restore the original sediment flow which has been trappedby dam reservoirs up until now, and has drawn up a plan for sediment sluicing
operation at Yamasubaru, Saigou and Oouchibaru Dams.
This paper reports on related matters including the following, 1) thenecessity for sediment flow management at dams in the Mimikawa River Basin, 2)
the appropriateness of choosing sediment sluicing at dams as a measure to
manage sediment flow, 3) numerical simulation carried out with the objective ofconfirming the effect of sediment sluicing and formulating optimal rules for
operation, 4) hydraulic model studies carried out to determine functional
characteristics of sediment sluicing operation, 5) details of dam retrofitting work
and innovative techniques to prepare for sediment sluicing operation, 6)
assessment of environmental impact below dams after commencement of
sediment sluicing operation and 7) adoption of auxiliary methods for sediment flow598

Q. 99 – R. 45
management being carried out in advance of commencement of sediment sluicing
operation.

2.

NECESSITY FOR SEDIMENT FLOW MANAGEMENT AT DAMS IN THE
MIMIKAWA RIVER BASIN

2.1.

DAMAGE CAUSED BY TYPHOON NABI

Typhoon Nabi passed slowly northwards over the sea to the west of Kyushu
by maintaining its large-scale and strong power. This caused warm moist air toflow for a long period bringing heavy rainfall to mountains in Miyazaki Prefecturethat exceeded 1,300mm in total. In the Mimikawa River Basin, inflow discharge
exceeded previous the maximum recorded flows. Because of this extreme
discharge, power plants at Kamishiiba, Tsukabaru, Yamasubaru and Saigou were
flooded rendering power generation impossible, while Tsukabaru, Yamasubaru
and Saigou Dams were overtopped and their dam control facilities flooded [1].
Morotsuka Dam
IwayadoDam

Tsukabaru Dam

Central area of Morotsuka
Yamasubaru Dam

Kamishiiba Dam

Saigou Dam(b)Oouchibaru Dam

(a)
：①
：②
：③
：④

Fig. 2
Damage in the Mimikawa river basin area resulting from the Typhoon Nabi, 2005
Dégâts du typhon Nabi en 2005 dans le bassin de la rivière Mimikawa(a) Many slope failures in upstream(b) Extensive flood damage indownstream basin① Slope failure② Flood damage in residentialbuildings③ Road, embankment damage④ Other damage to residentialbuildings (flow of soil and rocks, etc.)(a) Nombreux éboulements en amont(b) Dégâts considérables des crues dans lebassin en aval
Éboulement
Inondations dans des bâtiments
résidentiels③ Routes et berges endommagées④ Autres dégâts aux bâtiments résidentiels(laves torrentielles, etc)
①
②

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Width 400m

Fig.3
Slope failure downstream of
the Tsukabaru dam
Éboulement du versant en aval dubarrage de Tsukabaru

Fig.4
Flooding in central area of theMorotsuka village
Inondation au centre de Morotsuka

Fig. 2 shows mountain slope failures and flood damage caused over theentire the Mimikawa River watershed. Within the watershed area, mountain slope
failures occurred in 491 locations and 424 residential buildings were flooded. The
cost of damage sustained over the whole of the Miyazaki Prefecture amounted to
130 billion yen (1.2 billion USD) which was the greatest amount of damage everrecorded for the prefecture. In the Mimikawa River Basin a total of 10.6 million m 3of sediment flowed into rivers as a result of mountain slope failures, withapproximately half of this amount (5.2 million m3) being deposited in
dam-regulating reservoirs. There is concern about an additional 26.4 million m3 of
sediment which is also in danger of flowing into rivers. Fig. 3 shows the largest
slope failure which formed a land slide dam. In addition, as shown in Fig. 4, in thecentral area of Morotsuka located on the upstream edge of the Yamasubaru Damregulating reservoir, damage caused by flooding was extensive with 70 buildings
being flooded, the largest number ever recorded. Over and above the record
rainfall, the large quantities of sediment flowing into the Yamasubaru Damregulating reservoir, which caused rapid aggregation in the vicinity, contributed tofurther damage.
2.2.

APPROACHES TO THE MIMIKAWA RIVER BASIN INTEGRATED SEDIMENT FLOW
MANAGEMENTWith the opportunity that arose due to the damage by Typhoon Nabi, Miyazaki
Prefecture, the river administrator, established the Mimikawa 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 aproper 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 sediment flow, while balancing flood control,
water usage and environmental conservation, formulated a policy to advance
integrated sediment flow management [2].

600

Q. 99 – R. 45
In October 2011, the Mimikawa 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 causedby sediment in the basin [3].
2.3.

APPROACHES TO SEDIMENT FLOW MANAGEMENT IN DAM-REGULATINGRESERVOIRSKEPCO had repeated discussions with Miyazaki Prefecture, national andacademic institutions as well as basin stakeholders, to draw up an action plan
regarding reservoir sedimentation problems which had become increasingly serious.
As a result, KEPCO formulated an action plan focusing on sediment sluicing operation
at Yamasubaru, Saigou and Oouchibaru Dams, with the aim of restoring the original
sediment flow which has been trapped by dam reservoirs up until now. Sedimentsluicing operation at dams, which is carried out at times of river flooding due totyphoons, is the temporary lowering of reservoir water levels to make the flow of waterin dam-regulating reservoirs close to the original, natural state of the river, thereby
allowing inflow sediment to be transported below dams (Fig. 5) [2].
①

(a)
(b)
②

Free flow

Fig .5
Sediment Sluicing
Vidange de sediments(a) Dam modification (reservoir water leveldrawdown)(b) Sluicing of sediment (early realization of flood
control + environmental measures)① Process of sediment accumulation undercurrent operation② Sediment sluicing operation

Modification de barrage (niveau de réservoir
abaissé)(b) Vidange de sédiments (réalisation rapide demesures de contrôle des crues etenvironnementales)① Processus d'accumulation de sédiments avec
l’exploitation actuelle② Gestion des vidanges de sédimentsa)601

Q. 99 – R. 45

3.

SELECTION OF SEDIMENT FLOW MANAGEMENT MEASURE

In recent years, various countermeasures for sediment flow management
policies have been studied. As shown in Fig. 6, it has been proposed that using
CAP/MAR (Total capacity/Mean annual 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 [4].
According to Sumi, with an increase in CAP/MAR (that is, a decrease in
dam-regulating reservoir turnover rate), the appropriate sediment measure willvary between: sediment flushing, sediment bypass, sediment sluicing, sediment
check dam, excavating and dredging, or no necessity for a sediment measure tobe applied. The reason for this is that these various sediment measures depend
largely on the amount of water that can be used for sediment management, which
in turn depends on the scale of the dam-regulating reservoir.
CAP/MAR and CAP/MAS data for each of the 7 dams in the Mimikawa River
Basin is shown in Table 1. According to this, it can be seen that compared to
Kamishiiba, Tsukabaru, and other dams upstream, Yamasubaru, Saigou andOouchi Dams 1) have low CAP/MAS and therefore have a substantial need for
sediment measures and 2) dam-regulating 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.
Table 1
Reservoir sedimentation of dams in the Mimikawa River Basin
Dam Name

Total Storage
Volume CAP（103m 3 )Annual Runoff
Volume MAR（106m 3 )Total
Sedimentation
Volume （103m 3)Year of
Operation

AnnualSedimentation
Volume MAS（103m 3 )CAP/MAR CAP/MAS

Kamishiiba

91600

672

12600

58

217.8

0.136

Iwayado

8310

876

5560

72

77.2

0.00948

420
108

Tsukabaru

34300

1071

6890

75

91.8

0.032

374

Morotsuka

3480

151

1060

52

20.3

0.0231

172

Yamasubaru

4190

1410

2590

81

32

0.00297

131

Saigo

2450

1610

1010

84

12

0.00152

204

Ouchibaru

7490

1830

1930

57

33.8

0.00409222

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Q. 99 – R. 45

Fig. 6
Selection of sediment management options and dams in the Mimikawa River Basin
Sélection des options de gestion des sédiments et barrages dans le bassin de larivière Mimikawa(a) Reservoir Life=CAP/MAS(b) Capacity-inflow ratio=CAP/MAR① Sediment check dam
Sediment replenishment② Sediment bypass, Sluicing③ Sediment flushing, Sediment scoringgate④ Impoundment option⑤ Check dam⑥ Flushing⑦ Scoring gate
⑧ Scoring pipe
⑨ Bypassing⑩ Excavating⑪ Dredging⑫ Mimikawa

4.

(a) Vie d’un réservoir=CAP/MAS(b) rapport capacité/débit entrant = CAP/MAR① Barrage de contrôle②③

Remplissage en sédiments
Dérivation/vidange de sédiments
Chasses de sédiment, vanne dégraveur

④
⑤
⑥
⑦
⑧
⑨
⑩
⑪
⑫

Option d’endiguement
Barrage de retenue
Chasses
Vanne dégraveur
Tuyau dégraveur
Dérivation
Excavation
Dragage
Mimikawa

NUMERICAL SIMULATION OF SEDIMENT FLOWIn the study of sediment management measures for dam-regulating
reservoirs and river channel sections, as well as optimal rules for operation of
sediment measures, it is vital to understand sediment flow in the river basin, whichis affected by factors such as the relationship between river flow volume andamount of fluctuation in the river. In order to do this, one-dimensional analysis of603

Q. 99 – R. 45
riverbed variation was carried out, and the effect of sediment sluicing at dams
verified for the approximately 58km stretch of the main river, covering the threedams where sluicing is to be carried out, from the area upstream of the
Yamasubaru Dam regulating reservoir down to the river mouth. Table 2 shows
investigation conditions for an analysis of a 33 years period. Fig. 7 shows acomparison between riverbed elevation for Yamasubaru Dam after 33 years
under current operation (Case 1) and sediment sluicing operation (Case 2). Fig. 8
shows the effect of calculation for annual inflowing and outflowing sedimentvolume under sediment sluicing operation.
Table 2
Investigation conditions
Investigation conditions

Yamasubaru
Dam upstream

Saigou Dam
upstream

Oouchibaru Dam Oouchibaru Dam
upstream
downstream river

Initial riverbed conditions

Riverbed survey after Typhoon 0514

River flow rate conditions

Actual flow rates at each dam 1994-2004

Post-calculation conditions

Calculation until dam-regulating reservoir riverbed stabilization confirmed

Sediment
inflowconditionsFirst 10 years
3
2(m /km /year）

1,092

1,056

791

791

From year 11
3
2(m /km /year)

606

742

521

521

Existing
Case1 operation Reflects dam operation results
Water
case
level
Sluicing of
3
conditions
Using actual flow rates, while flow rate at each dam exceeds 200m /s,
Case2 sedimentcalculate water level during sluicing of sediment (all gates free flow)
case

130

(a)

125

Central area of Morotsuka

Yamasubaru Dam

120

Reservoir upstream

①
②
③

115
110
0.0

1.0

2.0

3.0
(b)

4.0

5.0

6.0

Fig.7
Results of riverbed fluctuation analysis (Yamasubaru Dam)
Résultats des analyses au niveau du lit de fleuve
(a)
(b)
①
②
③

Elevation (EL.m)
Distance from dam (km)Initial elevation of riverbed
Case1 : Riverbed under continued
existing operation
Case2 : Riverbed under sedimentsluicing operation

604

(a)
(b)
①
②
③

Hauteur (EL.m)
Distance du barrage (km)Hauteur initiale du lit du fleuve
Cas 1 : Lit du fleuve avec continuation
de l’exploitation actuelleCas 2 : Lit du fleuve en cas de vidangede sédiments

Q. 99 – R. 45

Unit: ‘000m3/year

Silt and clay〔d ≦ 0.075mm〕

0〔0〕

Yamasubaru Dam
(46k)

Sand
〔0.075mm ＜ d ≦ 2mm〕

30

-1
〔-11〕

30
〔0〕

12

0
〔0〕

Saigou Dam
(36k)

42
〔0〕

17

0
〔0〕

21

+2
〔0〕

Estuary (0k)

78
〔0〕
0
〔0〕
78
〔0〕

+6
〔-10〕

28

0
〔-8〕

5

0
〔-24〕

8

1
1.2 k

132
〔+34〕
+3
〔+1〕
129
〔+33〕

+ 11
〔+1〕

+ 12
〔+10〕

137
〔+37〕
+ 14
〔+3〕

+2
〔-2〕

11
〔+11〕

+ 10
〔+6〕
39

Decreasing tendency
Somewhat increasing tendency
Increasing tendency

14
〔+12〕

108
〔+43〕

14 k

17

Notes: 〔〕indicates difference from existing case.

11
〔+10〕

80
〔+19〕

+1
〔0〕
79
〔0〕

58

59
〔+11〕

Oouchibaru Dam 59
〔0〕
(23k)

21

Gravel〔2mm ＜ d ≦ 75mm〕

7
9

6
〔+1〕
+6
〔+1〕

2

2
〔0〕
+2
〔0〕
0
〔0〕

Fig. 8Results of calculation for inflowing and outflowing sediment volume in Case 2
Résultats des calculs des sédiments entrant et sortant dans le cas 2
As a result, it has been confirmed that if current operations are continued,
sediment will continue to be deposited in the reservoirs and aggregation will occur.
Whereas if sediment sluicing operation is carried out, sediment upstream ofregulating reservoirs will be transported downstream, which will cause riverbed
forms upstream of regulating reservoirs to be stabilized at lower than existinglevels. In particular, it was confirmed that there would be improvement in thefeeding of sediment below dams, which have been accumulated in reservoirs up
to now. Therefore, it is expected that this will lead to mitigation of the flood risks
associated with rapid aggregation of dam-regulating reservoirs in upstream areas
during river flooding. Also, it is expected that it will contribute to creating ahealthier river environment, including the ecosystem, by controlling degradation
and beach erosion in the dam downstream and in littoral areas, as well as bypromoting diversification of riverbed materials. Using the tools mentioned,
sediment flow simulations are being conducted under the assumption of various
future risks, and specific methods for sediment sluicing at dams is being studied.

5.

EXPERIMENTAL INVESTIGATIONS USING HYDRAULIC MODEL

For dam retrofitting to be carried out with the objective of sediment sluicing it
is necessary to understand details such as the relationship between the form of a605

Q. 99 – R. 45
modified dam and sediment sluicing functions. Additionally, changes in theriverbed with time associated with the three-dimensional flow in the area
immediately upstream of a dam, which is difficult to represent by numericalsimulation, should be clarified. For this reason, a 1:50 scale model that recreatedtopographical form approximately 1.3 km up- and downstream from dams wasused to conduct hydraulic experiments related to sediment sluicing functions forthese dams. Fig. 9 shows the plan and cross-sectional views of the model
together with a photograph taken while an experiment was in progress.
3.ZφOV
3.ZφOV

①

ピット

ピット

3

③

3



直角三角堰



模型縮尺 









⑤




















3.ZφOV

全幅堰④

Scale 1:50

3.ZφOV

②

3

直角三角堰

3



3

3.ZφOV

ダム模型

ラジアルゲート門オーバーフローブロック段







沈砂水槽





ベルトコンベヤー⑥











取水口模型



角落とし

ブロック段ブロック段ブロック段3

Fig. 9Plan and cross-sectional views of hydraulic model, and photographs ofexperiment in progress (Yamasubaru Dam)
Plan et vue en coupe du modèle hydraulique et photos d’expérimentations en
cours
①Pit
②Suppress weir
③Right-angle triangular weir
④Right-angle triangular weir
⑤Dam model radial gates⑥Sluice gates①Fosse②Déversoir sans contraction latérale
③Déversoir triangulaire④Déversoir triangulaire⑤Vannes segments du modèle⑥Écluses

As a result of experiment, it was confirmed that 1) differences in the effect of
sediment sluicing depended on the height of the cut in the dam body, 2) when
there was a transition in spillway gate flow from orifice flow to free flow and 3) a
phenomena occurred in which there was a massive flow of sediment downstream
from the area immediately upstream of a dam. Fig.10 shows one example ofexperimental results. In addition, based on these results and as mentioned later inthis paper, it was confirmed that it is necessary to carry out auxiliary sedimentmanagement work, for example to remove accumulated sediment in the area
immediately upstream of a dam in advance of initial sediment sluicing operation.

606

Q. 99 – R. 45

ＥＬ①

98 100 102 104 106 108 110 112 114 116 118 120 122 124

(a)

60
40
20
0
-20
-40
0

100

200

300

400

500

600

700

(b)

④
ＥＬ

②③

98 100 102 104 106 108 110 112 114 116 118 120 122 124

(a)

60
40
20
0
-20
-40
0

100

200

300

(b)

400

500

600

700

⑤

Fig.10
Example results for hydraulic model experiment for Yamasubaru Dam(Riverbed before and after sediment sluicing)
Exemples des résultats de l’expérimentation sur modèle hydraulique(lit de la rivière avant et après la vidange des sédiments)
(a)
(b)
①
②
③
④
⑤

Transverse direction
Distance from reference point (m)Longitudinal direction
Distance from reference point (m)EL.114 Initial form of sand surface
before sediment sluicing
Form of sand surface after sedimentsluicing
Q=2,000m3/s, with sand feeding
Immediately upstream of dam(after sediment sluicing)
Condition of riverbed upstream ofdam (after sediment sluicing)

6.

(a)
(b)
①
②
③
④
⑤

Direction transversale
Distance du point de référence (m)Direction longitudinale
Distance du point de référence (m)EL.114 Forme initiale de la surface desable avant la vidange de sédiments
Forme de la surface de sable après lavidange de sédiments
Q=2 000m3/s, avec alimentation en
sable
Immédiatement en amont du barrage(après la vidange de sédiments)
État du lit de la rivière en amont dubarrage (après la vidange de sédiments)

DAM RETROFITTING PLAN AND INNOVATIVE RETROFITTINGTECHNIQUESFor Yamasubaru and Saigou Dams, with the existing structure it is notpossible to do the necessary drawdown in order to carry out sediment sluicing
operation. For this reason, sluicing functions are now being added to these dams
by partially cutting down their overflow spillway sections without causing structural
damage. It will be the first time in Japan that an existing dam, 80 years after607

Q. 99 – R. 45
commissioning, will be modified by the addition of a new sluicing function. For
Oouchibaru Dam, which is the furthest downstream, dam height is low, and bychanging dam operation, sediment sluicing will be possible with the existing
structure, so the dam will not be retrofitted.
6.1.

FORM OF DAM RETROFITTINGFor Yamasubaru Dam, retrofitting will be carried out by removing the existing
two central spillway gates, cutting down spillway crest height by approximately 9.3 m,
and installing a single large spillway gate. For the form of dam retrofitting chosen, theamount of sediment flow will be comparable to removal of the dam, and it was also
confirmed that this method is the optimal measure by which sediment flow
management cost will be minimised. For Saigou Dam, removing the four central
spillway gates and then cutting down spillway crest height by approximately 4.3 m
was confirmed to be the optimal measure. Fig. 11 shows the final appearances of
Yamasubaru and Saigou Dams after retrofitting.
Yamasubaru Dam

Saigou Dam【Current state】【Current state】【After dam modification (image)】【After dam modification (image)】①②

Fig. 11
Dam retrofitting
Modernisation des barrages① Of the eight existing radial gates, two① Des huit vannes segmentsin the center will be removed, and theexistantes, deux au centre seront
overflow section cut down by approx.
éliminées et la crête de déversoir
9.3m to install one radial gate.
réduite de 9,3m approximativement
pour installer une vanne-segment.
② Des vannes wagon existantes, quatre② Of the eight existing roller gates, fourau centre seront éliminées et la crête
in the center will be removed, and thede déversoir réduite de 4,3m
overflow section cut down by approx.
approximativement pour installer deux
4.3m to install two roller gates.
vannes wagon.

608

Q. 99 – R. 45

6.2.

RETROFITTING W ORK PLAN

Dam retrofitting work on Yamasubaru and Saigou Dams was commenced in
November 2011. The work is in principle being carried out between November
and May, outside the flood season. While work is being carried out, a temporary
cofferdam is installed upstream of each dam, and the river diverted through aheadrace to allow power generation to be continued. In addition, there is the issue
that while retrofitting work is in progress, it is necessary to discharge water fromdams safely during flooding, in the same way as up until now. As shown in Fig. 12,
to resolve this, a 4 m high steel-rubber gate (SR gate) has been installed in theupper part of the temporary cofferdam, the first instance of such a measure being
taken in Japan. Fig. 13 shows the state of progress in work being carried out on
Saigou Dam, and the SR gate during current operation. Up to 2014, operations
had continued for 2 years, and in that period, the SR gate had been operated for 9
times.
②

①
④

③

⑤

Fig. 12
Temporary cofferdam installed upstream during dam retrofitting work
Bâtardeau provisoire installé en amont pendant les travaux de modernisation①②

River water
SR (Steel-Rubber) gate

①
②

③
④

Temporary cofferdam
Cut down by approx. 9.3 m(Yamasubaru)
Existing dam

③
④

⑤

⑤

Eau de la rivière
Vanne hybride en acier etcaoutchouc
Bâtardeau provisoire
Découpe de 9,3 m
approximativement (Yamasubaru)
Barrage existant609

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SR gateSR gateFig.13Coffer dam installed upstream for period of dam retrofitting work (Saigou Dam)
Bâtardeau installé en amont pendant les travaux du barrage de Saigou

7.

ASSESSMENT OF ENVIRONMENTAL IMPACT DOWNSTREAM FROMDAMS AFTER COMMENCEMENT OF SEDIMENT SLUICING

It is assumed that in response to the change in the amount of flow of
sediment due to sediment sluicing operation at dams, that there will changes in
the river environment and it is therefore necessary to make an assessment of the
impact. However, there are no specific guidelines to assess the impact of
sediment sluicing on the river environment. Therefore, at first it is necessary to
implement adaptive management based on an initially adopted plan that is subject
to review while monitoring is carried out, during which time a new plan for anassessment method appropriate to the characteristics of sediment sluicing atdams on the Mimikawa River will drawn up.
When drawing up a plan for assessment methods, it is important toappropriately set both the impact of 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. The monitoring to be
used in assessing the impact of sluicing sediment has been carried out
continuously for the river since 2007, and for coastal areas since 2009, andcovers items needed to assess the assumed response. An assessment of the
impact of sediment sluicing operation at dams will be made by making
comparisons before and after sluicing, as well as by making comparisons
between impact and control areas.

8.

ADOPTION AUXILIARY METHODS BEING CARRIED OUT IN ADVANCEOF SEDIMENT FLOW MANAGEMENT

In the transition to dam sediment flow operation, after completion of dam
retrofitting work, proper consideration must be given to flood control and sudden610

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changes in the river environment. As a measure to deal with this, after
appropriately assessing factors such as future riverbed form and riverbed grain
size within dam-regulating reservoirs through model experiments and numericalsimulations, relocation of existing accumulated sediment in reservoirs will be
carried out as shown in Fig. 14, in parallel with dam retrofitting work.

①
⑤

Appro
130,0 x,00m 3

Central area of
Morotsuka(a)⑦
Yamasubaru
Dam③

SaigouDam②

Oouchibaru
Dam

⑥

A m3
Immediately
upstream of
Yamasubaru
Dam

(b)

B m3
Immediately
upstream of
Saigou Dam

④

C m3
⑧

Ishitoge Lakeland Oouchibaru Dam: Possible routes to move sediment(c)Fig. 14
Sediment relocation plan in dam-regulating reservoirs
Plan de délocalisation des sédiments dans les réservoirs de régulation(a) Condition 1: Early demonstration offlood control effect(b) Condition 2: Preventing flow of existing
sediment at early stage of sedimentsluicing (measure for localized
scouring)(c) Condition 3: Preventing flow of existing
sediment at early stage of sedimentsluicing (measure for resuspension ofsilty sediment)① Move outside reservoir② Move within reservoir③ Move outside reservoir④ Cover sand
⑤ Move outside reservoir⑥ Move within reservoir⑦ Other effective uses, damping at
disposal sites⑧ Silty sediment accumulation(a) Condition 1: Démonstration rapide del’effet de contrôle des crues(b) Condition 2: Empêcher l’écoulement desédiments existants aux premiers
stades de vidange de sédiments (contre
l’affouillement localisé)(c) Condition 3: Empêcher l’écoulement desédiments existants aux premiers
stades de vidange de sédiment(mesure contre la remise ensuspension de sédiments limoneux)① Déplacement hors du réservoir② Déplacement dans le réservoir③ Déplacement hors du réservoir④ Sable de couverture⑤ Déplacement hors du réservoir⑥ Déplacement dans le réservoir⑦ Autre usages effectifs, dépôt dans dessites dédiés
⑧ Accumulation de sédiments limoneux611

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Fig.15
Mechanism of special ejector pump system and high pressure water injection
Mécanisme du système de pompe avec éjecteur spécial et injection d’eau① High pressure water② Suction③ Discharge④ Generation of negative pressure① Eau à haute pression② Aspiration③ Décharge④ Génération de pression négative②② 土砂吸引管③③ ｴｼﾞｪｸﾀｰﾎﾟﾝﾌﾟ① 土砂取込口①④ 土砂輸送管④⑤⑤ 土砂排出口Fig.16
Schema of special ejector pump system
Schéma du système de pompe avec éjecteur spécial
① Sediment intake (Crusher)② Sediment suction pipe
③ Ejector pump
④ Sediment conveying pipe
⑤ Discharge outlet
破砕機

612

超高圧ﾎﾟﾝﾌﾟ① Alimentation en sédiments (concasseur)② Tuyau d’aspiration de sédiments③ Pompe d’éjecteur④ Tuyau de transport de sédiments⑤ Décharge de sédiments
特殊ｴｼﾞｪｸﾀｰ

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sediment

Fig.17Work being carried out at Saigou Dam in 2013
Travaux en cours au barrage de Saigou en 2013
① Tuyau de transport de sédiments② Pompe avec éjecteur spécial

①Sediment conveying pipe
②Special ejector pumpConcerning the method for relocation of sediment, for sites characterized byfor example sediment composed mainly of coarse sand and gravel and buried
driftwood, or confined work spaces, it is difficult to apply conventional submersible
pump or grab dredging methods. For this reason, attention is being given to a
special ejector pump system as the most applicable technology. With this system,
using an ultrahigh-pressure pump, water is injected through a pipe at highpressure. This generates negative pressure that draws in sediment, which is thenconveyed by the pressure produced by the ultrahigh-pressure pump (Fig.15).
More specifically, as shown in Fig. 16, using a long arm backhoe aboard a barge
to maneuver an intake and suction pipe, submerged deposited sediment is drawn
in and then conveyed through a series of pipes. In addition, in the suction section,scree is broken down by a crusher into pieces around 150mm in diameter before
being drawn in. The suction pipe structure is simple, making it resistant toblockage and internal wear. The system makes possible what could not be tackled
by dredging with a conventional submersible water pump, which is limited tosediment particle size equivalent to that of sand. Fig. 17 shows the system in
actual operation, which in practice has conveying capacity of approximately
25 m3/h (over an average distance of 300 m).

9.

CONCLUSION

This paper covers the background to implementation of sedimentmanagement at dams in the Mimikawa River System, and explains the use ofnumerical simulation and a hydraulic model for predicting sediment flow. The
paper also provides an overview of specific retrofitting work for sediment sluicing
at dams, adoption of auxiliary sediment management work being carried out in613

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advance of commencement of sluicing, and methods for assessment ofenvironmental impact in the downstream. Main conclusions are follows.
1) Based on the expected sediment load into reservoirs after Typhoon Nabi in
2005, drastic change of reservoir sediment management strategy is
indispensable for reservoir sustainability in the Mimikawa River Basin.
2) Among several countermeasures, sediment sluicing option by installing highspillway gates and introducing patricianly drawdown operation during flood
flows is suitable for three dams in the basin. This selection has been guided
by two guiding parameters, CAP/MAR (Total capacity/Mean annual runoff)
and CAP/MAS (Total capacity/Mean annual inflow sediment) (Sumi, 2013).
3) One dimensional numerical simulation has been carried out to clarify theeffect of sediment sluicing by obtaining optimal operation rule including target
flood events and threshold discharges for starting and ending reservoir
drawdown. Differences in the effect of sediment sluicing depended on theheight of the cut in the dam body.
4) Physical model experiments have been carried out to confirm three
dimensional sediment transport phenomena such as scouring and depositing
within reservoir area and to determine efficiency of sediment sluicing. It wasalso confirmed that it is necessary to carry out auxiliary sedimentmanagement work, for example to remove accumulated sediment in the area
immediately upstream of a dam in advance of initial sediment sluicing.
5) Unique retrofitting work on two dams have been started such as installing
temporary cofferdam, 4m high steel-rubber gate (SR gate), which can keep
reservoir water level for power generation in normal time and discharge flood
water by falling down the gates. .
6) Environmental impact assessment downstream from dams after
commencement of sediment sluicing have been started by taking field data suchas turbidity, river bed change, bed grain size and aquatic species and so on.
7) In order to prevent fast sediment bed change during the first drawdown
operation causing large impacts on downstream by discharging high sedimentconcentration, adoption of auxiliary methods for sediment flow management
being carried out in advance of commencement of sediment sluicing.
With sluicing planned to commence in 2016, this project is the first in Japan
to incorporate large-scale dam retrofitting work and to investigate specificoperational methods for sediment sluicing at dams. Following the commencementof sluicing, investigation of optimal sluicing with consideration for flood control,
water usage and environmental conservation is scheduled to continue.

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REFERENCES
[1]
[2]
[3]

[4]

YAMAGAMI Y. Approaches for Integrated Sediment Flow Management
at Dams In the Mimikawa River Basin, Proc. of International Symposium,
The 80th Annual Meeting of ICOLD, pp108-113, 2012.
MIYAZAKI PREFECTURE Mimikawa River Basin Integrated Sediment
Flow Management Plan, 2011 (in Japanese).
NAKAHARA M. et al. Integrated Sediment flow Management in the
Mimikawa River Basin, Through Collaboration Between Business, Local
Government, Academia and Local Population, Japan Society of Civil
Engineers, Global Environment Symposium, 2014 (in Japanese).
SUMI T. Reservoir Sedimentation Database and Selecting Suitable
Sediment Management Options in Japan, Proc. of International
Symposium, The 81st Annual Meeting of ICOLD，pp.1717-1727, 2013.

SUMMARY
With the huge sediment production in mountainous areas due to the
Typhoon Nabi disaster in the Mimikawa River Basin in 2005, full-scalecountermeasures to deal with reservoir sedimentation became necessary. To that
end, the “Mimikawa River Basin Integrated Sediment Flow Management Plan”,
which took into consideration the entire river basin from the mountainous areas tothe sea, was compiled in 2011 by the Miyazaki Prefecture. Sediment sluicing
operation at a group of dams in the main river in the Mimikawa River Basin willplay a central role in this plan, and is scheduled to commence in FY2016.
Sediment sluicing operation is planned for Yamasubaru, Saigou and Oouchibaru
Dams in the Mimikawa River, which are administered by Kyushu Electric Power
Company (KEPCO). In order that water level can be lowered sufficiently at timesof river flooding during typhoon period, crest gate retrofitting work on Yamasubaru
and Saigou Dams is currently being carried out.
This paper reports on related matters including the following, 1) thenecessity for sediment flow management at dams in the Mimikawa River Basin, 2)
the appropriateness of choosing sediment sluicing at dams as a measure to
manage sediment flow, 3) numerical simulation carried out with the objective ofconfirming the effect of sediment sluicing and formulating optimal rules for
operation, 4) hydraulic model studies carried out to determine functional
characteristics of sediment sluicing operation, 5) details of dam retrofitting work
and innovative techniques to prepare for sediment sluicing, 6) assessment ofenvironmental impact below dams after commencement of sediment sluicing
operation and 7) adoption of auxiliary methods for sediment flow management
being carried out in advance of commencement of sediment sluicing operation.

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RÉSUMÉ
Avec l'énorme afflux de sédiments dans les zones montagneuses en raison
du typhon Nabi dans le bassin de la Mimikawa, des mesures de grande enverguresont devenues nécessaires pour faire face à la sédimentation des barrages. Àcette fin, le «Plan intégré de contrôle des flux de sédiments dans le bassin de laMimikawa», qui a pris en considération l'ensemble du bassin fluvial des régions
montagneuses à la mer, a été conçu en 2011. La vidange des sédiments dans ungroupe de barrages sur la rivière principale dans le bassin de la Mimikawa jouera
un rôle central dans ce plan, et il est prévu de commencer en 2016. La vidange
des sédiments est planifiée sur la Mimikawa pour les barrages Yamasubaru,
Saigou et Oouchibaru, administrés par Kyushu Electric Power Company(KEPCO). Pour que le niveau de l'eau puisse être abaissé suffisamment lors decrues de la rivière, la modernisation des vannes de crête est actuellement encours aux barrages Yamasubaru et Saigou.
Ce article rend compte des questions associées, notamment : 1) nécessitéd'une gestion des flux de sédiments dans les barrages du bassin de la Mimikawa,2) pertinence du choix de vidange de sédiments dans les barrages en tant que
mesure pour gérer les flux de sédiments, 3) simulation numérique effectuée dansle but de confirmer l'effet de vidanges des sédiments et la formulation des règles
optimales de fonctionnement, 4) essais de modèles hydrauliques réalisés pourdéterminer les caractéristiques fonctionnelles de vidange des sédiments qui
résultera de travaux de modernisation, 5) détails des travaux de modernisation desbarrage et techniques de pointe de réaménagement pour la vidange dessédiments, 6) évaluation de l'impact environnemental en aval des barrages après
le début de vidange des sédiments et 7) adoption de méthodes auxiliaires pour lagestion des flux de sédiments devant être réalisée avant le début des vidanges desédiments.

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