D-18

Fourth International Conference on Scour and Erosion 2008

Evaluation of Efficiency of Reservoir Sediment Flushingin Kurobe River
Tetsuya SUMI1
1 Member of JSCE, Associate Professor, Dept. of Civil and Earth Resources Eng., Kyoto University(Katsura, Nishikyo-ku, Kyoto, 615-8540, Japan)
E-mail: sumi@mbox.kudpc.kyoto-u.ac.jp

From viewpoint of the comprehensive sediment management in a sediment routing system, it isimportant to select appropriate sediment management strategy in each storage reservoirs. In the Kurobe
River, coordinated sediment flushing operations of Dashidaira and Unazuki dams have been executed since2001. These flushing operations have advantages on effective use of river water to discharge sedimentusing sediment erosion and transport phenomena by temporal reservoir drawdown operation.
If examining reservoirs satisfy suitable conditions, this sediment flushing operation is one of the most
attractive methods from the point of view of necessary costs and contribution for sediment supply todownstream. Possibility to apply this sediment flushing operation to reservoirs is largely dependent on thesediment flushing efficiency (Fe =S/W) that is defined by the sediment volume (S) and the water
consumption (W). The sediment flushing efficiency changes widely by various factors such asconfiguration of reservoir, elevation of sediment scouring gates, volume and grain size of deposited
sediment, discharge rate during sediment flushing, duration time from the start of draw down flushing andso on. In order to forecast the sediment flushing efficiency, it is important to analyze erosion process of
sediment in reservoirs.
In this paper, the coordinated sediment flushing of Dashidaira and Unazuki dams in the Kurobe river thatis the representative case of sediment flushing in Japan is discussed, and the effective operation and thesediment flushing efficiency are studied through by monitoring of erosion process of deposited sediment inDashidaira and Unazuki reservoir.
Key Words : Reservoir sediment management, sediment flushing, flushing efficiency, Kurobe river

1. APPROPRIATE SELECTION OF
RESERVOIR SEDIMENTMANAGEMENT
Sediment management in reservoirs is largely
classified into three approaches: 1) to reduce
sediment inflow to reservoirs, 2) to route sedimentinflow so as not to accumulate in reservoirs, and 3)
to remove sediment accumulated in reservoirs.
Fig.1 shows Japanese large dams, higher than 15m
and larger than one million m3 storage volume, thatare plotted by the parameter of the turnover rate ofwater (CAP/MAR=Total capacity/Mean annual
runoff)andsediment(CAP/MAS=Total
capacity/Mean annual inflow sediment).
In Fig.1, existing practice of sedimentmanagement in reservoirs can be specified referringto these parameters. It is understood that selected
measures have changed in order of the sedimentflushing, the sediment bypass, sediment check dam
and excavating, and dredging as CAP/MAR
increases (decrease in the turnover rate) roughly.

This is because the sediment management measures
are greatly dependent on the volume of water thatcan be used for the sediment transport. Here, thequality of sediment (size etc.) and the riverenvironmental conditions which may restrict the
sediment discharge are not considered.
It is important to clarify the range that the
sediment flushing or the sediment bypass that uses
the tractive force for the sediment discharge can beapplied for selecting the sediment management
strategy. Especially, it is a trade-off in the sedimentflushing between maximizing sediment discharge
and minimizing environmental impacts on thedownstream river.

2. THE COORDINATED SEDIMENT
FLUSHING IN KUROBE RIVER(1) Project outline
Unazuki dam, completed in 2001 by the
Ministry of Land, Infrastructure and Transport, andDashidaira dam, completed in 1985 by the Kansai

1
608

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

Capacity-inflow ratio and Reservoir Life
100000No measures (from inner factors)

Check damReservoir Life=CAP/MAS

Flushing

10000

Scoring gate
Scoring pipe
Bypassing

1000
⑱
⑥

④

100
⑫ ⑤ ③

②
⑧

⑮
⑪

⑭
⑨

①
⑬
⑲

10
0.001

Excavating⑦⑯ Sediment check dam,

Sediment replenishment⑰⑳

Dredging⑩

CD ： Check dam
F ： Flushing (Draw down)
PF ： Flushing (Partial draw down)
SG ： Scoring gate
SP ： Scoring pipe
B ： Bypassing
E ： Excavating
D ： Dredging

Sediment bypass, Sluicing
Sediment flushing, Sediment scoring gate

0.01
0.1
1
Capacity-inflow ratio=CAP/MAR

10

Fig. 1 Representative sediment control examples in Japan (Relationship between capacity-inflow ratio and reservoir life)m
1,600-

Kurobe dam
1,448.000

1,400-

1,200-

Altitute

1,000-

Sen-nin dam
851.000
800-

Koyadaira dam
529.400

600-

400-

200-

Unazuki dam
245.000

Dashidaira dam
343.000

Shimokurobe
-

-

-

-

10

-

-

Aimoto
0

20

30

40

50

60 km

The distance from the sea.

Fig.2

Kurobe river basin

Fig.3 Unazuki dam and Dashidaira dam

2609

Table 1 Sediment flushing dams in the WorldName of Dam

Dam
Dam
Heightcompleted(m)Country

InitialMean Annual １／(Mean Reservoi Average
Storage
Flushing Flushing
Sediment
Annualr LifeFlushing
Capacity
Duration Frequency
Inflow (MAS)
Runoff)(=CAP/
Discharge
(CAP)
（hrs）
(1/ yr)(m3/s）(million m3)１） （=CAP/MAR) MAS)(million m3)

76.7
9.01
97
24.7
Gebidem
Switzerland
1968
113
9
VerboisSwitzerland
1943
32
15
Barenburg
Switzerland
1960
64
1.7
Innerferrera Switzerland
1961
28
0.23
Genissiat
France
1948
104
53
Baira
India
1981
51
9.6
Gmund
Austria
1945
37
0.93
China
1966
65
13.3
Hengshan２）
Santo Domingo Venezuela
1974
47
3
Taiwan1938
30
7
Jen-shan-pei２）
Guanting
China
1953
43
2270
Guernsey
USA
1927
28.6
91
Heisonglin
China
1959
30
8.6
Ichari
India
1975
36.8
11.6
２）
1961
35
56
Ouchi-Kurgan Former USSR
China
1960
45
9640
Sanmenxia２）
Iran
1962
82
1760
Sefid-Rud２）
Shuicaozi
China
1958
28
9.6
１）Average after dam completion， ２）Sluicing dams
Dashidaira

Japan

1985

Unazuki

Japan

2001

0.62
0.96
0.5
0.33
0.02
0.008
0.73
0.3
0.07
1.18
0.08
0.23
60
1.7
0.7
5.7
13
1600
50
0.63

0.00674
0.014
0.021
0.001440.000473
0.000180.00467
0.004890.00465
0.842
0.00667
N.A.
1.5
0.04330.6
0.002180.00376
0.224
0.352
0.0186

14.5
25.7
18.0
45.5
85.0
28.8
72.6
32.0
13.3
11.3
37.5
30.4
37.8
53.5
12.3
2.0
4.3
6.0
35.2
15.2

200
300
15
600
90
80
600
90
6
2
5
12.2
80
125
0.8
2.16
1000
2000
100
50

12
12
70
30
20
12
36
40
168
672
72
1272
120
120
72
24
2400
2900
2900
36

1
1
1
3
5
5
3
1
N.A.
2～3
N.A.
1
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.

④ 1720×103 m3

8000⑤ 800×103 m3

3

3

Sedimentation Amount （×10 m ）Gross storage: 9×106m3

7000⑫ 280×103 m3

The biggest flood in 1995

6000

⑧ 700×103 m3（3450×103m3）⑩ 60×103 m3

5000② 80×103 m3

4000
3000

⑨ 590×103 m3

2000

3

⑦ 340×103 m3

3③ 20×10 m

1000

3⑥ 460×103 m3⑪ 90×103 m33① 460×10 m

Total Flushed Sediment Volume（S）（103m3）

10000
9000

Dashidaira
VerboisGebidem
Baira

1000

'03

Fe=0.02

99

96

01
98

97

Fe=0.01
04
03

02

1.0

‘04

'02

'01

'00

'99

'98

'97

'96

'95.7

'95.11

'94

'95.6

'93

'92

'91

'90

'89

'88

'87

'86

'85

Fig.4

Fe=0.0595100

0Fe=0.1

10.0

100.0

Total Water Volume（W）（106m3）

Sedimentation volume change in Dashidaira dam

electric power co. are both located in thedownstream of Kurobe river basin (Fig.2). They
have been constructed as the first dam with fullscale sediment flushing gates in Japan in place of
securing 100 years’ sedimentation volume since
extremely a lot of sediment volume compare tothese possible reservoir capacities was expected(Fig.3). Sediment flushing also has the following
purposes; 1) sustaining these original functions suchas flood control and hydro power, 2) maintaining
sediment routing system in the basin. Actually,
beach erosion progresses in the downstream coastal
areas and the necessity of the sediment supply is
extremely high.
In Dashidaira dam, the sediment flushing of 18
times in total have been carried out by July, 2007,
and the sediment of 6 million m3 or more that
equaled to 2/3 or more of the total reservoir
capacities in total was discharged as in Fig.4. In

Fig.5

Water volume and flushed sediment volume insediment flushing dams

addition, after completion of Unazuki dam, acoordinated sediment flushing of these dams havestarted and the sediment flushing of 7 times andsediment sluicing of 7 times have been carried out
up to July, 2007. The sediment flushing is executedat the first major flood event every year and thesediment sluicing is done at the successive bigger
ones by the same reservoir operation to prevent
additional sediment deposit in the reservoir.
At present, sediment flushing operation in the
Kurobe river is aiming at maintaining a constant
bed form without storing sediment in the reservoir
as much as possible by executing at the natural
flood events between June and August. As a result,
the water quality deterioration such as at the
sediment flushing in 1991 doesn’t occur recently
and it contributes to maintain the reservoir capacity
greatly. The present rule that executes the sedimentflushing according to the natural flood in a constant3
610

frequency so that the interval should not
become longer agrees well with thefindings in Switzerland and France thathave longtime results for the sedimentflushing. It becomes a good reference topromote the reservoir sedimentation
management in the future very much.

Shui ca ozi
Sef i d-R ud
J en-s ha n-pei
Santo Dom i ng o
F erreraB a renburg
B a i raV erboi s
Gebi dem
Da s hi da i ra(2) Sediment flushing efficiency
0
0.05
0.1
0.15
0.2
Furthermore, it is a very much interest
Flushing efficiency Fe
how much efficiencies have been obtained
Fig. 6 Sediment flushing dams and flushing efficiency
by these sediment flushing operations.
Table 1 shows sediment flushing dams influshing needs more volume of water.
the world. Among them, the relation between the
The sediment flushing efficiency of other dams
water consumption and the amount of the sedimentisshown
in Fig.6. The sediment flushing efficiency
flushing was shown in Fig.5 about four dams,
is about Fe =0.01-0.15, and it thought to be
Dashidaira dam in Japan, Gebidem and Verbois
decreased to about Fe =0.05 or less whendams in Switzerland and the Baira dam in India,
consideration to the river environment is especiallywhere each flushing result was recorded. Here, thenecessary.
water consumption for the sediment flushing is only
calculated during the fully draw down period(3)Feasibility evaluation of the sediment flushing
though the fine sediment discharge actually starts
According to the research on the feasibility
from the reservoir drawdown period and this water
evaluation of the sediment flushing, a possible
volume should be included in the water
range of the sediment flushing can be obtained byconsumption.
the following equation by the parameters shown in
Figure shows sediment flushing efficiency (FeFig.1. Here, the sediment flushing efficiency and=S/W) calculated by the sediment volume (S) andthe proportion of the water consumption by thethe water consumption (W). Among them, sedimentsediment flushing to the mean annual runoff volume
flushing efficiency in Gebidem dam is(MAR) are defined Fe and β respectively.
comparatively high since the sediment flushing is
CAP
executed with a low flow discharge for a long time.
CAP
Moreover, in Baira dam, flushing efficiency is also
MAR
(1)
＞
comparatively high though there are some
CAP ⎞
MAS
⎛
Fe ⎜ β −⎟fluctuations. On the other hand, since Verbois dam
MAR⎠⎝
is located at the mainstream of the Rhône River and
In Fig.7 (a) and (b), possible range of thethe sediment flushing is executed with a large
sediment flushing in the case where Fe changes toamount of water from Lac Léman, the sediment
0.01, 0.02 and 0.05 under the fixed β=0.1, and in the
flushing efficiency is not large. In Dashidaira dam,case where β changes to 0.05, 0.1, 0.2 under thethough there are great fluctuations in the amount of
fixed
Fe =0.2 are shown respectively. Sediment
the flushing sediment and the water consumption,
flushing
is feasible in the left side of each line.
the flushing efficiency is not so high similarly to
According to them, the change in Fe mainly
Verbois dam except one after the big flood in 1997.
influences within the small range of CAP/MAS andIn these four dams, the sediment flushing iseven a small turnover rate of the reservoir, e.g. large
strictly managed in Dashidaira and Verbois dams so
CAP/MAR, becomes a possible increase of Fe under
as not to cause a remarkable water quality change toconstant
β. If the river environmental is considered,the downstream river by maintaining considerably a
possible
range of the sediment flushing becomes
lot of water discharge compare to the amount of
narrower because Fe should be estimated low. On
sediment. As a result, in consideration of thethe other hand, if β can be increased, the sedimentdownstream river environment, enough volume offlushing possibility will be increased under the samewater is required so that the flushing efficiency may
Fe since the water volume ratio for the sediment
be decreased. In the sediment flushing of Kurobe
flushing increases. However, β and the original
River, the sediment flushing is executed by securingstorage purposes of the reservoir are in the relation
enough river discharge just after the natural floodsof the trade-off and too large β can not be assumed.
and also the additional discharge is recently
Even in Dashidaira dam, β are ranging between 0.01
examined to wash out the fine sediment silted in theand 0.07 from 2001 to 2007 actually.
downstream river channel and thus the sediment4611

100,000

(a)

1,000

β=0.1
100

10

1
0.001

β=0.2
β=0.1
β=0.05

10,000CAP/MAS

CAP/MAS

10,000100,000

Fe=0.01
Fe=0.02
Fe=0.05(b)

1,000
Fe=0.02
100

10

0.01

0.1
CAP/MAR

1
0.001

1

0.01
0.1
CAP/MAR1

Fig. 7 Possible range of the sediment flushing((a) Proportion of the water consumption to the mean annual runoff volume β is fixed; (b) Sediment flushing efficiency Fe is fixed)

3. EVALUATION OF THE SEDIMENT
FLUSHINGEFFICIENCY
BY
MONITORINGOF
SEDIMENT
FLUSHING PROCESSAs we can find in Figs.5 and 6, the sedimentflushing efficiency changes widely by various
factors such as configuration of reservoir, elevationof sediment flushing gates, volume and grain size ofdeposited sediment, discharge rate during sedimentflushing, duration time from the start of draw down
flushing and so on. In order to forecast the sedimentflushing efficiency, it is important to understand theerosion process of sediment in reservoirs. Photo 1
shows erosion process of deposited sediment inDashidaira reservoir in 2003. We can find that
longitudinal and lateral erosions created by river
bed degradation and side bank erosion.
Recently,
wehaveconducted
fieldmeasurements of sediment erosion process with 3D
laser scanning technology in Unazuki reservoir(Sumi, Murasaki, Fujinaga, Nagura and Tamaki,
2004, Sumi, Murasaki, Nagura, Tamaki and Imaki,2005). The 3D laser scanner used for themeasurement is shown in Photo 2. Side bankerosion process of sand bars formed in the Unazuki
reservoir and water surface profiles near by banks
have been measured as shown in Fig.8. By these
data, we could estimate that side bank of the heightof 1.0-1.2m was eroded with the speed of 7.5m/hr.
Slope of water surface is also estimated as 1/75 andwater waves generated by the anti-dune are also
observed. Since the bed morphological change
where water and sandbars exist together
complicatedly can be also measured even in a nighttime, this technique can be used to understandsediment transport in a reservoir and to estimate thesediment volume flushed out by the operation.
Time and spatial variations of reservoir surface
velocities during drawdown and flushing period

Photo 1 Sediment Flushing in Dashidaira Reservoir

were also measured by PIV (Particle Image
Velocimetry) using CCTV camera and image dataprocessor (Sumi, Murasaki, Taira, Shinbo, Nagura,and Tamaki, 2007). These data can be converted toactual velocities by the 3D laser scanner data. Bothreservoir water level and flow velocity changing
during the drawdown and flushing period are verymuch helpful for us to understand sediment erosionand discharge processes from the reservoir.
From the viewpoint of the comprehensive
sediment management in a sediment routing system,
monitoring of quantity and quality of sedimenttransport during these flushing events in rivers andreservoirs is also very important. Field observation
was carried out to measure suspended sedimentconcentration (SS) both by the manual sampling and
SMDP (Suspended Sediment Concentration
Measuring System with Differential Pressure
Transmitter), turbidity by a turbidimeter for highsediment concentrations and grain size distribution(Sumi, Baiyinbaoligao and Morita, 2007). Based onthese data, movement of the suspended sedimentload discharged from Unazuki dam during flood,
flushing and sluicing periods were clarified.

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Unazuki dam

Sand bar

Side bankLine 7

Ononumain Unazuki reservoir(Riegle LMS-Z210)
2-300m, 2.5cm
accuracy

Section 5

3D laser scannerPhoto 2 Sediment Flushing in Unazuki Reservoir and 3D laser scanner(m)Sand bar45

縦断面7_水面勾配

断面5_側岸変化図
Side bankslope; Section 5

40

35

30

Water surface: Line 7

-22.4
-22.6
-22.8
Water -23
surface -23.2
-23.4
-23.6
-23.8
-24
-24.2
-24.4
25
20 (m)

Side
bank

15:02
15:15
15:30
15:44
16:01
16:16
16:31
16:45
17:00

Waterwaves generated by
the antidune

-23.2
-23.4
-23.6
-23.8
-24
-24.2
-24.4
0

5

10

15

20

25

30

15:02
15:15
15:30
15:44
16:01
16:16
16:31
16:45
17:00

Fig.8 Process of the side bank erosion and water surface changes in the channel

reservoir and to evaluate the sediment flushingefficiency.

4. CONCLUSION
The conclusions of this paper are as follows.
1) From viewpoint of the comprehensive sedimentmanagement in a sediment routing system, it isimportant to select appropriate sedimentmanagement strategy in each reservoir. Existing
practices of sediment management in reservoirs
can be specified referring to the parameter of theturnover rate of water and sediment.
2) In Kurobe River, coordinated sediment flushing
operations of Dashidaira and Unazuki dams havebeen successfully executed since 2001 and theseflushing operations are contributing to sustain
these original functions such as flood control andhydro power and to maintain sediment routing
system in the basin.
3) Possibility to apply the sediment flushing
operation to reservoirs is largely dependent on thesediment flushing efficiency that is defined by the
sediment volume and the water consumption. The
sediment flushing efficiency changes widely by
various factors such as configuration of reservoir,
elevation of sediment scouring gates, volume andgrain size of deposited sediment, discharge rateduring sediment flushing, duration time from thestart of draw down flushing and so on.
4) In order to forecast the sediment flushingefficiency, it is important to understand theerosion process of sediment in reservoirs such as
longitudinal and lateral erosions created by river
bed degradation and side bank erosions.
5) 3D laser scanning technology and PIV are
useful to monitor sediment erosion process in a

REFERENCES
1) Atkinson, E.: The feasibility of flushing sediment fromreservoirs, Report OD137, HR Wallingford, 1996.
2) Morris, G.L. and Fan, J.: Reservoir sedimentation handbook,
McGraw Hill Publishes, New York, 1998.
3) Sumi, T., Murasaki, M., Fujinaga, K., Nagura, Y. andTamaki, H.: Study on Measurement of Erosion-Deposition
Process of Reservoir Sediment during Flushing Operation,
Annual Journal of Hydraulic Eng., JSCE, Vol.48, pp.11471152, 2004(Japanese).
4) Sumi, T., Murasaki, M., Nagura, Y., Tamaki, H. and Imaki,T.: Study on Measurement of Erosion-Deposition Process ofReservoir Sediment during Flushing Operation (Part 2),
Annual Journal of Hydraulic Eng., JSCE, Vol.49, pp.10331038, 2005(Japanese).
5) Sumi, T. and Iguchi M.: Technical Feasibility Study of
Sediment Flushing in Reservoirs using the RESCON Model,
Journal of Japan Society of Dam Engineers, 15(2), pp.92-105,
2005(Japanese).
6) Sumi, T., Takata, Y., Iguchi M. and Nakanishi, Y.:
Feasibility Study of Sediment Flushing Operation in
Reservoirs using the RESCON Model, Advances in River
Engineering, JSCE, Vol.11, pp.285-290, 2005 (Japanese).
7) Sumi, T. and Kanazawa, H.: Environmental Study on
Sediment Flushing in the Kurobe River, 22nd International
Congress on Large Dams, Barcelona, Q.85-R.16, pp.219-242,
2006.
8) Sumi, T., Baiyinbaoligao and Morita, S.: Characteristics ofFine Sediment Discharge during Sediment Flushing of
Unazuki dam, Journal of Hydroscience and HydraulicEngineering, JSCE, Vol.25, pp.99-106, 2007.
9) Sumi, T., Murasaki, M., Taira, K. Shinbo, K. Nagura, Y. andTamaki, H.: Study on Measurement of Fine Sediment
Discharge Process during Sediment Flushing using PIV,
Annual Journal of Hydraulic Eng., JSCE, Vol.51, pp.10751080, 2007(Japanese).
10) White, W. R., Attewill, L., Ackers, J. and Wingfield, R.:
Guidelines for the flushing of sediment from reservoirs,
Report SR 566, HR Wallingford, 2000.

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