Q. 89 – R. 6
COMMISSION INTERNATIONALE
DES GRANDS BARRAGES------VINGT TROISIÈME CONGRÈS
DES GRANDS BARRAGES
Brasilia, Mai 2009-------THE EFFECT OF SEDIMENT FLUSHING AND ENVIRONMENTAL MITIGATIONMEASURES IN THE KUROBE RIVER *Tetsuya SUMI
Associate professor, Dr. of Engineering
Department of Civil and Earth Resources Engineering, Kyoto University
Shin-ya NAKAMURA
Former Head of the Kurobe River Work Office, Dr. of Engineering
Ministry of Land, Infrastructure, Transport and Tourism
Kyuichi HAYASHI
Chief Manager, Engineering Group
Hokuriku Branch, Kansai Electric Power Co., Ltd.
JAPAN

1.

INTRODUCTION

The Kurobe River in the eastern region of Toyama Prefecture is a
representative steep river in Japan which stretches over 85 km in a 682 km2
drainage basin (Figs. 1 and 2). The Unazuki Dam (completed in 2001, height:
97 m, gross capacity of reservoir: 24,700,000 m3), which is under the control of
Ministry of Land, Infrastructure, Transport and Tourism, is located at the farthest
point downstream of the Kurobe River. The Dashidaira Dam (completed in 1985,
height: 76.7 m, gross capacity of reservoir: 9,010,000 m3) owned by Kansai
Electric Power Co. Ltd. is located at the upstream of the Unazuki Dam. These twodams have an extremely large amount of sediment inflow compared to their grossstorage capacity of reservoir; therefore, they were the first in Japan which was*

Les effets de la chasse de sédiments et des mesures de gestion
environnementales dans la rivière kurobe

Q. 89 – R. 6
built with full-scale sediment flushing facilities (sediment flushing gates). Sedimentflushing has been conducted on the Dashidaira Dam since 1991. Since 2001,
when the Unazuki Dam was completed, sediment flushing and sluicing have been
conducted coordinately for the two dams (Photo 1).
① Kurobe river② Kurobe dam③ Dashidaira dam④ Unazuki dam⑤ Toyama bay⑥ Catchment area 617.5km2① Rivière Kurobe② Barrage de Kurobe③ Barrage de Dashidaira④ Barrage d’Unazuki⑤ Baie de Toyama⑥ Superficie du basinhydrographique 617,5km2

Fig.1

Kurobe river basin

Bassin de la rivière Kurobem1,600-

Kurobe dam
1,448.000

1,400-

1,000-

Sen-nindani dam
851.000
800-

Koyadaira dam
529.400

600-

400-

200-

Unazuki dam
245.000

Dashidaira dam
343.000

Shimokurobe
-

-

-

-

10

-

Aimoto0-

Elevation

1,200-

20

30

40

50

60 km

The distance from the sea

Fig. 2Longitudinal profile of the Kurobe River

Profil longitudinal de la rivière Kurobe

Q. 89 – R. 6

Unazuki dam

Dashidaira dam

Barrage d’Unazuki

Barrage de Dashidaira
Photo 1

Unazuki and Dashidaira dam during coordinated flushing

Barrages Unazuki et Dashidaira lors de chasses coordonnées de sédiment

Kokubo[1], Liu[2] and Sumi[3] and others have presented an overview of
coordinated sediment flushing operations at the Dashidaira and Unazuki Dams,
which are typical sediment flushing sites in Japan. In this paper we analyze
sediment flushing operations to date, and describe the effects of sediment flushingon the sediment continuity in the river basin and physical environment on the river.
We also describe technological initiatives to make the sediment flushing
operations more sophisticated, and environmental protection initiatives tominimize the impacts of sediment flushing on the downstream river environment.

2.

ANALYSIS ON COORDINATED SEDIMENT FLUSHING OPERATIONS

While it is important to maintain high sediment flushing efficiency in asediment flushing operation, it is also necessary to minimize the downstream
impact of flushed sediment.

2.1.

CRITERIA FOR CONDUCTING SEDIMENT FLUSHING AND SLUICING

Since the completion of the Unazuki Dam in 2001, seven sediment flushingand seven sediment sluicing operations have been conducted. When a flood inexcess of 300 (250 in some special case) m3/s of inflow occurs at the Dashidaira
Dam at the first time of the year between June and August, a coordinatedsediment flushing is performed. Whenever a flood in excess of 480 m3/s occurs at
the Dashidaira Dam after sediment flushing, a sediment sluicing is performed.
These sediment flushing and sluicing practices have been taken place byagreement in the Kurobe River Sediment Flushing Evaluation Committee and theQ. 89 – R. 6
Kurobe River Sediment Management Council, considering the natural flow regime
in the Kurobe River as well as the impacts of sediment discharge downstream.
Fig. 3 shows the frequency distribution of discharges when the flood peaks
in each year since 2001. Fig. 4 shows the average rainfall in the river basin andthe inflow discharge to the Dashidaira Dam in 2006, as an example. The floods
occur primarily due to melting snow in May and the rainy season from June toAugust. The flushing period of June to August was chosen by considering thesenatural flow regime and the impacts on fishing and agricultural water use both in
the downstream river and the connected sea shore area. In recent years, waterflow in excess of 300 (250 in some special case) m3/s of inflow at the Dashidaira
Dam has been occurring several times a year, and a flood in excess of 480 m3/s
has been occurring about once a year. The threshold values are considered to beappropriate to perform sediment flushing operations regularly.
4.5
Over 250m3/s

4.0Over 480m3/s

3.5Monthly frequency

Over 300m3/s

3.0
2.5
2.0
1.5
1.0
0.5
0.0
Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Fig. 3Monthly frequency for every inflow rates to the Dashidaira Dam since 2001

Fréquence mensuelle pour chaque débit entrant dans le barrage
Dashidaira depuis 2001

800

20

600

Rainfall30

Inflow discharge

400

40

200

50
60
2006/1
2006/1

0
2

3

4

5

6

7

8

9

10

11

Fig. 4

Average rainfall in the river basin and inflow discharge
to the Dashidaira Dam in 2006

Précipitation moyenne dans le bassin de la rivièreet évacuation du débit entrant au barrage Dashidaira en 2006

12

3

0
10

Q. 89 – R. 6

0
5
10
15

Average hourly rainfall in Dashidaira basin [mm]20335Dashidaira

600

330

Inflow discharge[m3/s]
Outflow discharge[m3/s]
Water level[EL.m]

500
400

325
320

300

315

200

310

100

305

0

300

700

3

Discharge(m /s)

Waterlevel(EL.m)

3Discharge(m /s)

700

250

UnazukiInflow discharge[m3/s]
Outflow discharge[m3/s]
Water level[EL.m]

600
500

245
240

400

235

300

230

200

225

100

220

0

215

Water
level(EL.m)

Average hourly
rainfall(mm)The basic sequence of operations for sediment flushing is: drawing down
the reservoir water level, keeping free flow state in several hours and recovering
water level. The amount of time for the free flow sediment flushing depends
largely on the target amount of sediment to be flushed, which is planned prior to asediment flushing operation. Fig. 5 shows actual sediment flushing operation
performed in July 2006, as an example. Free flow state was continued for 12hours to flush out deposited sediment of 240,000 m3.

30,000SS(mg/l)25,000SS(Dashidaira dam)[mg/l]
SS(Unazuki dam)[mg/l]
SS(Shimokurobe)[mg/l]

20,00015,00010,0005,000
002006/7/1

3

6

9

12

15

18

21

0
7/2

3

6

9

12

15

18

21

0
7/3

Fig.5

Coordinated sediment flushing operation in 2006

Opération coordonnée de chasses de sédiments en 2006

2.2.

ACTUAL SEDIMENT FLUSHING AND SEDIMENT FLUSHING EFFICIENCY

Table 1 shows the actual amount of sediment flushed out of Dashidaira Damsince 2001. Flooding events when sediment flushing took place were those with
about 500 m3/s of discharge for sediment flushing, and about 700 m3/s for
sediment sluicing. The flushing volume data was obtained by measuring the
cumulative amount of sediment in the dam reservoir after the previous year’s
sediment flushing operation to the end of May. This quantity fluctuates dependingon flooding events out of the sediment flushing season from autumn to spring.
Fig. 6 shows the relationship between the quantity of flushed sediment andthe quantity of water used during sediment flushing operations. To measure thequantity of water used, we calculated the total discharge in two periods: (1) fromQ. 89 – R. 6
the start of drawing down reservoir water level to the completion of recovering
water level, and (2) from the start to the end of the free flow flushing phase. As
shown in Fig. 6, duration of sediment flushing namely the total discharge isplanned to be longer depending on the quantity of sediment to be flushed as seenat 2005 flushing example.

Table 1

Actual sediment flushing and sluicing operations at the Dashidaira Dam

Opérations réelles de chasse et d’écoulement au barrage Dashidaira

Maximum Discharge Average Discharge Flushing Volume
Maximum SS (mg/l) Average SS (mg/l)
Inflow (m3/s)(103 m3)
Inflow (m3/s)
2001 Flushing
333
277
590
90,00015,0002001 Sluicing
491
273
29,0006,700
2002 Flushing
362
215
60
22,0004,500
2003 Flushing
777
217
90
69,0007,100
2004 Flushing
356
229
280
42,00010,0002004 Sluicing
1,152
281
16,0007,300
2005 Flushing
958
290
510
47,00017,0002005 Sluicing 1835
275
90,00016,0002005 Sluicing 2790
250
40,0007,300
2006 Flushing
308
246
240
27,0006,500
2006 Sluicing 1378
203
12,0002,500
2006 Sluicing 2685
264
27,0005,200
2006 Sluicing 3
529
196
7,400
1,800
2007 Flushing
418
245
120
25,0003,500
Average of Flushing
502
246
270
46,0009,100
Average of Sluicing
694
249
31,6006,700
Average of All Data
598
247
270
38,8007,900

Fig. 7 shows sediment flushing efficiencies at the Dashidaira Dam, which
are calculated by the quantity of flushed sediment and the quantity of water useddefined by the above two periods. The sediment flushing efficiency which is
calculated by the quantity of water used during the free flow phase was about 0.01
to 0.03. As shown in the Figure, the sediment flushing efficiencies became muchhigher in 2007. In this year, relatively lower water level operation was maintained
for a long time prior to the sediment flushing. This operation pushed theaccumulated sediment to the dam prior to the free flow operation, which made theflushing more efficient than usual during the free flow operation.

Q. 89 – R. 6

(a)
(a)
70

60

50

40

30

2,000

20

10

0

0

0
5,000
(b)
(b)
10,000

①

②

(b)
(b)

15,00014,00012,00010,000

8,000

6,000

4,000

2001 Flushing
⑤

④

③

70

80

90

2002 Flushing2003 Flushing
2004
Flushing+Slucing
2005
Flushing+Slucing
2006 Flushing2007 Flushing

100

60

50 (a)
(a)

40

30

20

10

0

Fig. 6

Relationship between the quantity of water used and the quantity of flushed sediment

Relation entre la quantité d’eau utilisée et la quantité de sédiment chassé(a) Quantity of flushed sediment (×104m3)(a) Quantité de sédiment chassé (×104m3)(b) Quantity of water used (×104m3)(b) Quantité d’eau utilisée (×104m3)① From start of water level draw down to the end of① Du début de la baisse du niveau amont jusqu’à lawater level recovery

fin du remplissage du réservoir② From start to the end of free flow phase②Du début à la fin de la phase d’écoulement libre③ Quantity of flushed sediment③ Quantité de sédiment chassé④ From start of water level draw down to the end of④ Du début de la baisse du niveau amont jusqu’à lawater level recovery⑤ From start to the end of free flow phasefin du remplissage du réservoir⑤Du début à la fin de la phase d’écoulement libre

Q. 89 – R. 6

0.10

100

0.09

①

90

0.08

②

80

0.07

③

70

30

0.02

20

0.01

10

0.00

0

(b)

(b)

2007 Flushing

0.03

2006 Flushing

40

2005
Flushing+Slucing

0.04

2004
Flushing+Slucing

50

2003 Flushing(a)(a) 0.05

2002 Flushing60

2001 Flushing

0.06

Fig. 7

Sediment flushing efficiency

Efficacité de la chasse de sédiment(a) Sediment flushing efficiency(Flushed sediment / Used water)(b) Quantity of flushed sediment (×104m3)① Quantity of flushed sediment② From start of water level draw down to theend of water level recovery③ From start to the end of free flow phase

2.3.

(a) Efficacité de la chasse de sédiment(Sédiment chassé / eau utilisée)(b) Quantité de sédiment chassé (×104m3)① Quantité de sédiment chassé② Du début de la baisse du niveau amont

jusqu’à la fin du remplissage du réservoir③ Du début à la fin de la phase d’écoulement

libre

SS DISCHARGE AND CONTROLLING FACTORS

Fig. 8 shows the changes in SS discharge during each sediment flushing
operation plotted with the peak time set to zero, while Table 1 shows the maximum
and average amount of SS. Unlike sediment sluicing, sediment flushing flushes
the accumulated sediment since the last year’s sediment sluicing all at once, withhigh SS discharge peaks.

Q. 89 – R. 6

2001 SF
2006 SF

2001 SS
2006 SS1

2002 SF
2006 SS2

2003 SF
2006 SS3

2004 SF+SS
2007 SF

2005 SF

2005 SS1

100,000

2005 SS2

SF: Sediment Flushing
SS: Sediment Sluicing

SS(mg/l)

10,000

1,000

100
-10

0

10
Time(hr)20

Fig. 8Changes in SS (Dashidaira dam)

Modification des MES (barrage Dashidaira)We will now discuss the factors that affect the SS. Figs. 9 (a) to (c) show the
relationship between the average SS and the quantity of flushed sediment and theaverage flow rate during the sediment flushing operation, and the maximum SS
and reservoir water level draw down speed, respectively. Generally speaking, as
Fig. 5 shows, the amount of flushed SS during a sediment flushing operation
increases from starting point when the reservoir water level is drawn down to thestart of the free flow phase. As shown in Fig. 9, the average SS is higher for alarger quantity of flushed sediment, and for a larger average flow rate during a
flushing operation. We attribute this to the fact that a larger quantity of sediment isflushed downstream, and that the sediment concentration flowing in is also higher.
The maximum SS values, although they fluctuate, tend to be high when the speedof drawing down the reservoir water level is high. This is presumably due to thefact that, as the water level is drawn down, part of a reservoir becomes a riverstate again, which increases the flushing power, pushing the accumulated tiny
sedimentary particles downstream all at once. To plan sediment flushing
operations that are less taxing on the environment, we need to forecast theseeffects and adjust for them with dam operations such as controlling the speed ofdrawing down the reservoir water level.

30

Q. 89 – R. 6
Sediment flushing
20,0003.

(1) Relationship between
average SS and quantity offlushed sediment

18,000
Average SS (mg/l)

16,00014,00012,000y = 232.19x + 2816.5
2
R = 0.869

10,0008,000

Relation entre la moyenne
des MES et la quantité desédiment chassé

6,000
4,000
2,000
0
0

20

40

60

80
4

3

Quantity of flushed sediment (×10 m )Sediment flushing

Sediment slucing

20,000(2) Relationship between
average SS and average
flow rate during sedimentflushing operation

18,000

Average SS (mg/l)

16,000y = 153.43x - 28549
2
R = 0.6517

14,00012,00010,000Relation entre la moyenne
des MES et la moyenne dedébit durant l’opération dechasse de sédiment

8,000
6,000
4,000
2,000
0
0

100
200
300
400
Average flow rate during sediment flushing operation(m3 /s)

Sediment flushing

Sediment slucing

100,000(3) Relationship between the
maximum SS and waterlevel draw down speed

90,000Maximum SS (mg/l)

80,000y = 3927.1x + 18550
R2 = 0.7121

70,00060,000Relation entre le maximum
des MES et la vitesse debaisse du niveau d’eau

50,00040,00030,00020,00010,000
0
0

5

10

15

Water level draw down speed (m/hr)

Fig. 9Relationship between SS during a sediment flushing operationand controlling factors

Relation entre les MES pendant une opérationde chasse de sédiment et les facteurs contrôlant

Q. 89 – R. 6
3.

EVALUATION OF SEDIMENT FLUSHING EFFECTS

The effects of sediment flushing in the Kurobe River should be evaluated inthree locations: the river downstream, at the mouth of the river, and at theneighboring sea shore. Also, since the Dashidaira and Unazuki Dams werecompleted in different years, we need to consider each of the following time
periods: (a) before the completion of the Dashidaira Dam (before 1985), (b) before
the start of sediment flushing at the Dashidaira Dam (1985-1990), (c) before thecompletion of the Unazuki Dam (1990-2000), and (d) the present period whencoordinated sediment flushing has been performed. Items to be evaluate include(1) physical environment (riverbed level (average and deepest point), compositionof the riverbed materials, amount of sediment transport, change in the shoreline,etc.), (2) water environment (water quality, quality of the bottom sediment, etc.),and (3) biological environment (algae, benthos, fish, birds, etc.). Suchconsiderations are in progress by the Kurobe River Dam Sediment Flushing
Evaluation Committee.
In addition to these items, we will discuss the sediment balance to maintainthe continuity of sediment transport in the river basin, change in physicalenvironment in the river channel, and the environmental conditions at the bottomof reservoir.

3.1.

SEDIMENT BALANCE DUE TO SEDIMENT FLUSHING

Fig. 10 shows the amount of flushed sediment from the Dashidaira Dam andthe amount of accumulated sediment at the Unazuki Dam for each year.
Due to regular sediment flushing, the Dashidaira Dam has been maintaining
present sedimentation volume equivalent to about 45% of the gross storage
capacity, which is its equilibrium state. In contrast, because the Unazuki Dam wascompleted only recently, the accumulation of sediment is currently still in progress
mainly with coarse sediment that was brought by sediment flushing from theDashidaira Dam as well as from the Kuronagi River which is a tributary of the
Kurobe River. The accumulation of sediment at the Unazuki Dam was being
reduced in 2006, and as the size of sediment materials is now being stabilized,
coarse sediments are reaching up to the dam. Therefore, these coarse sediments
are also beginning to pass through the dam in addition to the fine sediments
produced upstream.
A one-dimensional river bed evolution model has been constructed for the
Kurobe River in order to forecast the amount of sediment and to evaluate its effecton the downstream river bed, taking into consideration the sedimentation andsediment flushing at the reservoirs. Fig. 11 shows the inflows and outflows of
sediments for different grain sizes in July 2006, which contains one sedimentQ. 89 – R. 6

7,000

3

3

Cumulative sedimet volume（×10 m ）flushing and two sediment sluicing operations, by comparison with data such asmeasured SS values and change of sedimentation at the reservoirs

Dashidaira dam sediment flushed

6,000

Unazuki dam sedimentation volume

5,000
4,000
3,000
2,000
1,000

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

0

Fig. 10Amount of flushed sediment at the Dashidaira Dam and accumulated sediment at the
Unazuki Dams

Quantité de sédiment chassé au barrage Dashidaira et sédiment accumulé aubarrage Unazuki

RiverShimokurobe

AimotoUnazukiDashidaira

Mouth

⑥

⑤

④

Kuronagi
River
③

②
①
103 m3

①

②

③

④

⑤

⑥

70mm <

37

93

50

0

4

0

2-70mm

54

165

108

18

60

80

0.2-2mm

123

279

213

479

386

312

< 0.2mm

201

371

97

513

564

556

Total

416

909

468

1,010

1,013

948

Fig. 11
Inflows and outflows of sediment of different grain sizes in July 2006

Débits entrants et sortants de sédiment de différentes granulométries
en juillet 2006

According to this chart, the Dashidaira Dam flushes out 910,000 m3 of
sediment which includes 420,000 m3 of newly inflowing sediment. The Unazuki
Dam has an inflow of 1,380,000 m3 of sediment which includes 470,000 m3 fromQ. 89 – R. 6
the Kuronagi tributary. From this inflow, the dam is passing through 1,010,000 m3,
or 73%, which mainly consists of fine sediments smaller than 2 mm. The amountof coarse sediments larger than 2 mm passing through the Unazuki dam is only
20,000 m3, or 10%, and 90% is currently trapped at the reservoir. In the river
channel downstream, it is estimated that there is almost the same quantity of
sediment load as the one flushed out from the Unazuki Dam. Among this sedimentload, the wash load component, which is smaller than 0.2 mm, is washed out
directly to the sea. While a part of the 0.2-2 mm diameter component of thissediment load is trapped on the riverbed, we believe that some amount of river
bed materials larger than 2 mm is washed downstream. We further expect that thepassing of the coarse sediment component from the Unazuki Dam will increase in
the future, and accordingly it will further be supplied downstream.

3.2.

PHYSICAL ENVIRONMENT CHANGES IN THE DOWNSTREAM RIVER CHANNELIn the Kurobe River downstream, 0-6 km from the river mouth is braided
channel, and 6-13 km from the river mouth is single bar. Fig. 12 shows theaverage riverbed level for each section from year to year based on the one in1980 before the Dashidaira Dam was completed. While the riverbed degradation
had been occurred over all sections of the downstream river channel, after
sediment flushing was begun, the riverbed began to rise again, mainly in thesections closest to the river mouth. Slightly riverbed degradation started again
after the Unazuki Dam was completed, but we can now notice the riverbed
aggradations again, reflecting the coordinated sediment flushing.
Based on 1980
1.00

Dashidaira dam
completion 1985

0.50

Start of Dashidaira
dam flushing 1991

Unazuki dam
completion 2000

0.00
-0.50
-1.00
-1.50

Fig. 12

Relationship between river bed level and sediment flushing

Relation entre le niveau du lit de la rivière et la chasse de sédiment

2007

2005

2003

2001

1999

1997

1995

1993

1991

1989

Ave. 3.2-6.2k
Ave. 11.4-13.2k
Ave. 15.8-20.4k
1987

1985

1981

-2.50

1983

Ave. 0-3.0k
Ave. 6.4-11.2k
Ave. 13.4-15.6k
Ave. of all section

-2.00

1979

Variation from 1980 (m)Average river bed level

2009

2007

2005

2003

2001

1999

1997

1995

1993

1991

1989

1987

1985

1983

1981

1979

1977

Q. 89 – R. 6

10
Unazuki dam
completion 2000

Dashidaira dam
completion 1985

D60(mm)Start of Dashidaira
dam flushing 1991
100

0－3k
7-11k
14-15k
Ave. of all section

1000

4-6k
12-13k
16-20k

Fig. 13

Relationship between representative grain size (D60) and sediment flushing for different
sections

Relation entre les grosseurs représentatives (D60) et la chasse de sédiment pourdifférentes sections

Fig. 13 shows these changes in terms of the riverbed materials. This figure
indicates that for a representative grain size of D60(mm), the riverbed materials
had generally been getting larger after the completion of the Dashidaira Dam by“armoring”; however, after the start of sediment flushing, they began to get smaller
in all sections. This is considered to be due to the supply of the sand component.

3.3.

QUALITATIVE CHANGES OF SEDIMENT IN THE RESERVOIRS

We conducted surveys of quality of bottom sediments in the Dashidaira
Dam reservoir. These surveys were conducted at five locations before thesediment flushing season (May), immediately after the sediment flushing season(July) and after the sediment flushing season (September). Reservoir bottom
sediments were analyzed for appearance, smell, pH, COD (Chemical Oxygen
Demand), IL (Ignition Loss), TOC (Total Carbon), T-N (Total Nitrate), T-P (Total
Phosphorus), ORP (Oxidation-reduction Potential), sulfide and grain size
distribution.
The analysis of reservoir bottom sediments before and after sedimentflushing in 1998 and later generally reveal the following characteristics: Organic
material indices (COD, IL and TOC), T-N and T-P dropped after sediment flushing,
and then rose in September to about the same level as before sediment flushing.
Average sediment size (D50) increased after sediment flushing, and decreased in
September to about the same size as before sediment flushing. Although
Reduction Index (ORP) showed an oxidizing tendency immediately after sedimentflushing, it showed about the same level of reduction tendency in September as
before sediment flushing.

Q. 89 – R. 6
Based on the above analysis, we can say that the bottom sediments of theDashidaira Dam reservoir follow a cycle associated with sediment flushing.
Sediment flushing flushes out fine particles, and then reduces the organic
materials indices and the eutrophication index. As new sediment, which includes
fine particles, is accumulated, the organic materials indices and eutrophicationindex are increased back to their original levels. In other words, sediment flushinghas the effect of replacing the surface layer of the reservoir bottom sediments sothat fresh sediment only accumulates all time.

4.

TECHNOLOGICAL DEVELOPMENT OF SEDIMENT
FLUSHING OPERATIONS

Among the initiatives employed to make the sediment flushing operations
more advanced at the Kurobe River are the following: 1) forecasting of rainfall andrunoff, 2) monitoring of sediment transport in the reservoirs, and 3) monitoring ofhighly turbid water and amount of sediment transport rate.

4.1.

FORECASTING OF RAINFALL AND RUNOFF [4]In order to minimize the impacts of sediment flushing on the environment at
the Kurobe River, sediment flushing is performed to coincide with natural flood
events. In order that, it is essential to forecast the amount of rainfall and runoff asaccurately as possible. To manage the sediment flushing operations in a moresophisticated manner, we have developed a rainfall-runoff forecast model whichtakes into account the effect of geographical features of the surrounding
mountains. These models have been applied to the Dashidaira dam to forecast
real-time inflow discharge.
The rainfall model employs kinematic methods based on radar data, andphysical methods based on local weather models. This system forecasts theamount of rainfall in a 2.5 km x 2.5 km grid placed to cover the Sen’nindani Dam,
Koyadaira Dam, and Dashidaira Dam every ten minutes, up to six hours inadvance. The runoff model forecasts the amount of runoff discharge up to six
hours in advance, using the forecasted rainfall data, the slopes of the mountains,
and the water discharge in the river channels. The steep and complicatedgeographic features of the Kurobe River were taken into consideration, and we
employ Kalman filtering to the water flow calculation models to improve theaccuracy of the forecast.

Q. 89 – R. 6
4.2.

MONITORING OF SEDIMENT TRANSPORT IN THE RESERVOIRS [5] [6] [7]

For a successful sediment flushing operation, it is important to increase thesediment flushing efficiency (ratio of flushed sediment to the quantity of waterused) and the sediment flushing effect (relationship between the flushed sediment
to the accumulated sediment before a sediment flushing operation). It is also
important to forecast how the sediment erosion channel is formed due to the waterlevel change and how highly concentrated SS occurs during the sediment flushing
operation.
In order to monitor them, we conducted field measurements during acoordinated sediment flushing operation at the Unazuki Dam reservoir in 2006.
These measurements included a flow velocity measurement using PIV (Particle
Image Velocimetry) performed on images from CCTV cameras, in combination
with geographic feature measurement with a 3D laser scanner located within thereservoir. We analyzed the flow velocity and the discharge of fine sediments
during the water level draw down associated with a sediment flushing operation.
A highly concentrated SS downstream of the dam was observed
immediately before the start of the free flow phase and its maximum value isrelated to the speed of drawing down the reservoir water level. To protect the riverenvironment, it is necessary to develop an optimal operation scheme for the
flushing gate, taking into account the flow of water within the reservoir during thedrawing down operation as well as the sediment transport characteristics.

4.3.

MONITORING OF HIGHLY TURBID WATER AND AMOUNT OF SEDIMENTTRANSPORT RATE

[8]

In order to evaluate environmental impacts of sediment flushing by
discharging highly turbid water downstream and to assess sediment transport
during the operation, various types of sediment monitoring techniques should be
developed. We are applying several new monitoring instruments such as SMDP
(Suspended Sediment Concentration Measuring System with Differential
Pressure Transmitter), turbidimeter designed for high sediment concentrations
using glass fiber probe, and a multiple pipe sediment sampler to collect bed loadand suspended load at one time.
The SMDP makes it possible to monitor suspended sediment concentration
by measuring density of water directly with differential pressure transmitter. Twopressure detectors are placed vertically with some interval, e.g. H=1,000mm, andconnected by small pipes filled up with water to the differential pressuretransmitter mounted at the center of the system. It has an advantage in highturbidity measurement without any errors caused by properties of sediments suchas grain size, color and so on.

Q. 89 – R. 6
5.

DEVELOPMENT OF TECHNOLOGIES TO MITIGATE ENVIRONMENTAL
IMPACT OF SEDIMENT FLUSHINGAt the Kurobe River, technologies have been developed and implemented tomitigate the environmental impacts of sediment flushing on the river channel
downstream. These include “rinsing discharge” and “evacuation channels for fish
and other creatures”.

5.1.

CHANGES IN THE SEDIMENTARY FINE PARTICLES IN THE RIVER BED DUE TO
THE RINSING DISCHARGE

The rinsing discharge has been practiced at the Kurobe River in order towash away locally remaining fine sediment in the riverbed downstream using
relatively clear water after the free flow phase, for the purpose of maintaining thehabitat for fish and other creatures in spite of the sediment flushing. In 2007,
rinsing discharge was performed for three hours with a flow rate of about 300 m3/s.
In order to understand the effects of the rinsing discharge, we conducted surveys
on the fine sediments accumulated on the surface layers of the riverbed on two
occasions; one immediately after the sediment flushing operation and the otherimmediately after the rinsing discharge operation. The location of the survey wasin a section 4-5km from the river mouth where the river bed slope becomes very
gradual and fine sediments may accumulate on the river bed. The visualclassification was according to the simplified classification, and resulted in a
distribution diagram describing four levels of surface coverage by both sand andmud as shown in Table 2. The survey revealed that the change in the finesediment distribution due to the rinsing discharge was such that the mud
component was reduced from 7% to 3% overall. The grain size distribution
showed that a change was observed from mud to sand in two locations, and thatthe grain size became larger at four locations as shown in Fig. 14. These
measurements confirmed the effect of the rinsing discharge.
Table 2
Coverage classification of fine sediment particles

Classification de la couverture par les particules fines de sédiment

Coverage of river bed surface

0-25%

25-50%

50-75%

75-100%Sand ( 0.125-4mm )Sand 1

Sand 2

Sand 3

Sand 4

Silt and Mud ( <0.125mm )

Mud 1

Mud 2

Mud 3

Mud 4

Q. 89 – R. 6

Percentage
of sluiced weight
通過質量百分率100%
80%

Before Rinsing
Discharge

60%
40%

After Rinsing
Discharge

Rinsing
Discharge
300m3/s
3hrs

20%
0%
0.001

0.010
Mud and Silt

0.100

1.000

d
10.000 粒径

Sand (Fine and Coarse) Gravel(mm)

Fig. 14

Changes in grain size of river bed material on surface layer at 4.9 km from river mouth

Modification de la granulométrie du matériau de la couche de surface de la rivière, à 4,9
km de son embouchure

5.2.

IMPLEMENTATION AND EFFECT OF THE EVACUATION CHANNELS FOR FISH AND
OTHER CREATURES

Evacuation channels for fish and other creatures are implemented at the
Kurobe River in order to protect them from an increase in the SS concentration in
the river associated with sediment flushing. These channels take water fromagricultural water and spring water, and maintain a lower SS concentration thanthe main river channel during a sediment flushing operation. There are currently
nine channels at both sides of the river and we conducted surveys between 2003
and 2007 in order to verify their effectiveness.
The surveys were conducted at eighteen locations in the river including theevacuation channels in three locations by quantitative collections using a cast net
during the flooding period (draw down period of the water level) and immediately
after sediment flushing operation (recovery period of the water level). There weredifferences in the number of fish between the flooding period and immediately
after sediment flushing operation at each location. In particular, the largest numberof fish was collected immediately after a sediment flushing operation at one
evacuation channel.
We think that this is because this channel maintained a lower SS
concentration and a higher water temperature than the main river. Therefore, weneed to consider the use of infiltrated water and the possibility of leading watercollected from porous pipes buried underground to secure clear water in theevacuation channels as shown in Fig. 15.

Q. 89 – R. 6

①

②

⑧
③
⑦

④
⑤

⑥
Fig. 15

Conceptual schematics of evacuation facilities

Schéma conceptuel des installations d’évacuation①Shallow area appropriate for hatchingand still-water fish
②Complex features of the shoreline toaccommodate varied environmental
conditions③Immersed plant growth④ Depth and body of water to securesinking sand and subsoil water① Eaux peu profondes pour éclosion et⑤ Slope of moving water (undergroundwater)⑥Slow and long sloped opening to beable to bring in fish going up and down
the stream⑤ Pente de l’eau en mouvement (eau

⑦ Movement of fish up and down thestream
⑧Slightly angled in order to reduce thegeneration of separating current at theentrance

poissons d’eau calme②Caractéristiques complexes de la berge

pour créer des conditions variées del’environnement③Croissance de plantes submergées④Eau profonde afin de garantir un fondsablonneux et la recharge du sous-sol.
souterraine)⑥ Ouverture

longue, progressive et
inclinée afin d’amener le poisson se
déplaçant en aval et en amont du coursd’eau⑦Mouvement des poissons se déplaçanten amont et en aval
⑧Légèrement à angle afin de réduire la
production d’un courant séparateur àl’entréeQ. 89 – R. 6
CONCLUSIONS

The major conclusions of this paper are as follows.
1) At the Kurobe River, both sediment flushing and sluicing are performed
approximately once a year without major environmental impacts by restricting
sediment flushing operations to appropriate sediment flushing seasons (June toAugust), and times when natural water flow rate exceeds a certain level, 300 (250
in some special case) m3/s or more for sediment flushing, and 480 m3/s or more
for sediment sluicing.
2) The sediment flushing efficiency at the Dashidaira Dam, which is
calculated by the quantity of flushed sediment and the quantity of water usedduring the free flow phase, was about 0.01 to 0.03. Drawing sediment by means oflower water level operation before to the sediment flushing is effective in
improving flushing efficiency.
3) Discharged SS at the Dashidaira Dam during a sediment flushing
operation was 8,000mg/l on the average (40,000mg/l peak), averaged over 14
sediment flushing operations. The average SS values are closely related to thequantity of flushed sediment as well as the quantity of water used during a flushing
operation, and the peak SS values are closely related to the speed of drawing
down the reservoir water level.
4) The Dashidaira Dam is currently at an equilibrium state in terms of its
sediment, and the quantity of passing through sediment is approximately one
million cubic meters yearly. In contrast, sediment is still being accumulated at the
Unazuki Dam. While the majority of sediments of grain size larger than 2 mm aretrapped at the reservoir, about 70% of the sediment, which is mostly of grain size
smaller than 2 mm, is sluiced.
5) Active shifting of sand bar and water path locations are frequently seen in
the river channels downstream, which is considered to be the positive effect by
the maintenance of sediment supply due to the coordinated sediment flushing of
two dams. In particular, mainly by the supply of sand materials, the lowering of theriver bed in some sections has been reversed and “armoring” in the riverbed
materials where the coarse sediment component increased after the completion of
the dams is being alleviated in all sections.
6) The sediment flushing operation ensures that the surface layer ofaccumulated sediment on the bottom of the reservoir is continually replaced withfresh sediments, decreasing the organic materials and the eutrophication indices.
7) To make the sediment flushing operation more sophisticated at the
Kurobe River, work is in progress to develop rainfall-runoff models using radarQ. 89 – R. 6
data, to monitor sediment transport using 3D laser scanners and PIV technology,
and to monitor highly turbid water and the sediment transport rate.
8) In order to prevent accumulation of residual fine particle sediment on thesand bars in the river channel after sediment flushing operations, the rinsing
discharge is practiced. This is particularly effective after a sediment flushing
operation when a large quantity of fine sediments, such as silt, has beendischarged, and has proved to be an effective means to reduce the environmental
impact.
9) The evacuation channels have been proven to be effective in reducing
the environmental impact since many species of fish are evacuating when theturbidity of the main river rises due to the sediment flushing. It is essential tosecure clear water sources to these channels.

REFERENCES

[1]

[2]
[3]

[4]

[5]

[6]

[7]

[8]

KOKUBO, T., ITAKURA M. and HARADA, M., “Prediction Methods and Actual
Results on Flushing of Accumulated Deposits from Dashidaira Reservoir”,
19th ICOLD Congress, Florence, Q. 74, R.47, 761-791, 1997.
LIU, J. et al, “Environmental impacts of coordinated sediment flushing”,
Jour. of Hydraulic Research, Vol.42, 461-472, 2004.
SUMI, T., and KANAZAWA, H., “Environmental Study on Sediment Flushingin the Kurobe River”, 22nd International Congress on Large Dams,
Barcelona, Q.85-R.16, 2006.
HASHIMOTO, N., FUJITA, A., SHIIBA, M., TACHIKAWA, Y. and ICHIKAWA, Y.,“Development of Dam Inflow Prediction System Based on Distributed
Rainfall-Runoff Model”, Annual Journal of Hydraulic Engineering, JSCE,
Vol.50, 289-294, 2006(In Japanese).
SUMI, T., MURASAKI, M., FUJINAGA, K., NAGURA, H. and TAMAKI, H., ” Studyon Measurement of Erosion-Deposition Process of Reservoir Sediment
Flushing Operation”, Annual Journal of Hydraulic Engineering, JSCE,
Vol.48, 1147-1152, 2004(In Japanese).
SUMI, T., MURASAKI, M., NAGURA, H. TAMAKI, H. and IMAKI, T., ”Study on
Measurement of Erosion-Deposition Process of Reservoir Sediment
Flushing Operation (Part 2)”, Annual Journal of Hydraulic Engineering,
JSCE, Vol.49, 1033-1038, 2005(In Japanese).
SUMI, T., MURASAKI, M., TAIRA, K., SHINBO, K., NAGURA, H. and TAMAKI,
H., ”Study on Measurement of Fine Sediment Discharge Process during
Sediment Flushing Operation using PIV”, Annual Journal of HydraulicEngineering, JSCE, Vol.51, 1075-1080, 2006(In Japanese).
Sumi, T. and Baiyinbaoligao, “Characteristics of Fine Sediment Dischargeduring Sediment Flushing of Unazuki dam”, 10th International Symposium
on River Sedimentation, 2007

Q. 89 – R. 6
SUMMARY

The Unazuki Dam (completed in 2001, height: 97 m, gross capacity ofreservoir: 24,700,000 m3), which is under the control of the Ministry of Land,
Infrastructure, Transport and Tourism, is located at the farthest downstream pointof the Kurobe River, which is one of the typical steep rivers in Japan. The
Dashidaira Dam (completed in 1985, height: 76.7 m, gross capacity of reservoir:
9,010,000 m3) is owned by Kansai Electric Power Co., Ltd. These two dams havean extremely large amount of sediment inflow compared to their gross capacity ofreservoir; therefore, they were the first in Japan to be built with full-scale sedimentflushing facilities. Sediment flushing has been conducted on the Dashidaira Damsince 1991. Since 2001, when the Unazuki Dam was completed, sedimentflushing and sluicing have been conducted at coordinated times for these twodams. This paper covers an analysis of sediment flushing operations to date, theeffect of sediment flushing on the river’s sediment balance and physicalenvironment, technological initiatives to make the sediment flushing operations
more sophisticated, and environmental protection initiatives in order to harmonize
with the downstream river environment.
In the Kurobe River, sediment flushing and sediment sluicing have eachbeen effectively conducted roughly once per year by restricting sediment flushingand sluicing operations to times when there is natural flooding above a certain
scale in the summer period from June to August. The sediment flushing operation
limits sediment accumulation volume and quality deterioration of accumulated
sediments in the reservoirs. Downstream sand bars and water courses arechanging frequently before and after a flood, which is considered to be the resultof maintaining the sediment supply from the dams by sediment flushing. In orderto make the sediment flushing operations more sophisticated, work is in progresson the development of rainfall-runoff models, as well as on monitoring sedimenttransport, highly turbid water and so on. The rinsing discharge and the evacuation
channels for fish and other creatures are utilized in order to mitigate theenvironmental impact on the downstream river channels, and they have proven tobe effective.

RÉSUMÉ

Le barrage Unazuki (achevé en 2001, hauteur : 97 m, capacité globale duréservoir : 24 700 000 m3), lequel est sous la juridiction du Ministère des terres,infrastructures, transport et tourisme, est situé au point le plus en aval de la rivière
Kurobe, laquelle est une des rivières abruptes typiques du Japon. Le barrage
Dashidaira (achevé en 1985, hauteur : 76,7 m, capacité globale du réservoir :9 010 000 m3), est la propriété de Kansai Electric Power Co., Ltd. Ces deuxbarrages ont un extrêmement grand débit entrant de sédiment comparativement àQ. 89 – R. 6
la capacité globale de leur réservoir ; conséquemment, ils furent les premiers au
Japon à être construits avec des installations de débit à pleine échelle pourchasse de sédiment. La chasse de sédiment est effectuée au barrage deDashidaira depuis 1991. Depuis 2001, lorsque le barrage d’Unazuki a été achevé,la chasse (flushing) et l’écoulement (sluicing) de sédiment ont été effectués à desmoments coordonnés pour ces deux barrages. Cet article couvre une analyse desopérations de chasse de sédiment réalisées à ce jour, les conséquences de lachasse de sédiment sur l’équilibre du sédiment de la rivière et de l’environnement,
les initiatives technologiques afin de rendre les opérations de chasse de sédimentplus sophistiquées, et les initiatives de protection de l’environnement afind’harmoniser la chasse de sédiment avec l’environnement en aval de la rivière.
Dans la rivière Kurobe, la chasse et l’écoulement de sédiment ont
effectivement été faits approximativement une fois par année, en limitant lesopérations de chasse et d’écoulement de sédiment à des moments où il y a desinondations naturelles dépassant une certaine échelle, durant la période d’été dejuin à août. L’opération de chasse de sédiment limite l’accumulation de volumes
importants de sédiment et la détérioration de la qualité des sédiments accumulés
dans les réservoirs. Les bancs de sable et cours d’eau en aval changent
fréquemment avant et après une inondation, ce qui est considéré comme étant lerésultat du maintien de la source de sédiment en provenance des barrages par
chasse de sédiment. Afin de rendre les opérations de chasse de sédiment plus
sophistiquées, des travaux sont en cours pour le développement de modèlesd’écoulement d’eau de pluie, ainsi que pour la surveillance du transport desédiment, de l’eau très turbide et ainsi de suite. Le rinçage de décharges et lescanaux d’évacuation pour les poissons et autres créatures sont utilisés afind’atténuer l’impact sur l’environnement des canaux des rivières en aval, et ils sesont avérés efficaces.