DAMS 2011

RESERVOIR SEDIMENTATION MANAGEMENT IN HYDROPOWERSTATION CONSIDERING PROPERTIES OF SEDIMENTATION AND
FACILITY CONDITION
Hirofumi OKUMURA1, Tetsuya SUMI2, and Sameh A. KANTOUSH2
1, Electric power development company, 15-1 Ginza 6-chome Chuo-ku Tokyo,
hirofumi_okumura@jpower.co.jp
2, Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University ,sumi.tetsuya.2s@kyoto-u.ac.jp, kantoush@ecohyd.dpri.kyoto-u.ac.jp
Abstract: Reservoir sedimentation is one of the most important problems which influence on sustainabilityof hydropower operation. There are two major kinds of hydropower stations, which are storage andregulating reservoir power stations. These two power stations are very different not only in reservoir capacity
and reservoir operation rules, but also surrounding infrastructures and sedimentation condition in reservoirs.
This paper analyses the current status of sedimentation problems in storage and regulating reservoirs. The
results showed some useful properties, one is the difference of sedimentation impact on surrounding
infrastructures, and the other is the relationship of reservoir operation and sedimentation condition.
Considering these properties, effective and economical reservoir sedimentation management measures were
proposed, which were mainly conducted by draw-down operation.
Key words:

1hydropower station, reservoir sedimentation, regulating reservoir, draw down operation

Introduction

Sustainability of hydropower stations are endangered by reservoir sedimentation. Practically many
reservoirs of hydropower stations are under sedimentation control or under planning. In 2005,
Japan Electric Power Civil Engineering Association researched 354 reservoirs of hydropower
stations, which have over 15m height dam or over 1 million m3 water capacity, in order to analyze
the impacts of increasing sedimentation [1]. Based on this analysis, 95 reservoirs were influenced byincreasing sedimentation, and the major influence was flood water level rise at upstream river s.
There are 2 major kinds of reservoirs of hydropower stations, one is storage reservoir, and theother is regulating reservoir. They are very different not only in reservoir operation rules but alsosurrounding infrastructures and sedimentation condition. However, few studies have analyzed thesedifferences. Therefore, this paper focused on the properties of reservoir sedimentation in order toplan effective and strategic sedimentation management measures.
In this paper, we analyze reservoir sedimentation of hydropower stations considering the kindsof reservoirs. Based on these analysis, we propose new classification of reservoirs and discussappropriate sedimentation control measures for each types by showing these advantages withnumerical and economical analysis.

DAMS 2011

Storage reservoir
Generator

Regulating reservoir

Sea area

Figure 1. Location and position of hydropower stations
2

Reservoir sedimentation in hydropower station

There are 1,887 hydropower stations in Japan, and 737 among them have reservoirs. About 230 are
operated as storage reservoirs, and 464 and 43 are operated as regulating and pumped storage ones,
respectively [1]. This chapter investigates functions of storage and regulating reservoirs, andanalyzes sedimentation properties considering operation systems.
2.1 Storage and Regulating reservoirs
A cascade hydropower stations are usually placed in one river as shown in Figure 1 where the large
generator with the large storage reservoir for power peak are on the upper stream and smaller
generators with the smaller reservoirs for level power need are on the downstream of the river.
Storage reservoir is working as a catchment of seasonal rainfall such as typhoons in rainy season
and snow melting floods. Regulating reservoir is made for adjustment between water supply andpower demands in the short term such as per a day or a week.
Storage and regulating reservoirs are very different in two points of views. Figure 2 shows therelation of storage and regulating reservoir capacities that are located in the same river. By
comparing two kinds of reservoirs from total capacity point of view, the storage reservoirs are

Regulating resevoir capacity （1,000m3）

1,000,000
Ratio of regulating over storage(total capacity)
Ratio of regulating over storage(effective capacity)

100,000

10,000
1:1
1 : 101,000

1 : 100
1 : 1,000
100
100

1,000

10,000100,000

1,000,000

Storage resevoir capacity （1,000m3）

Figure 2. Comparison of storage and regulating reservoir capacities located along the same river

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Regulating reservoir B available
draw down depth（0～5m）
Storage reservoir A available
draw down depth（0～30m）

20
30
40
50

Storage reservior A

60

Regulating reservoir B

Depth from high water level (m)Depth from high water level （m）

10

Regulating reservoir D available
draw down depth（0～3m）

10

70

20

Storage reservoir C available
draw down depth（0～35m）

30
40
50
60
70

Storage reservoir C

80

Regulating reservoir D

90
100

80
0%

10%

20%

30%

40%

0%

50%

10%

20%

30%

40%

50%

Rate of sedimentation distribution

Rate of sedimentation distribution

Figure 3. Rate of sedimentation distribution
about 3 to 100 times larger than the regulating ones. Regarding to the effective capacity that can beused for power generation, the storage reservoirs are about 10 to 200 times larger than theregulating ones. These differences in two points of views are caused by the difference of effective
reservoir water depth. Averaged effective water depth in storage reservoirs is more than 25 meters,
and that in regulating reservoir is less than 5 meters.
2.2 Characteristics of reservoir sedimentation and dam facility conditions
In order to select an appropriate sediment management strategy according to conditions ofreservoirs, characteristics of sedimentation (position, sediment inflow rate) and dam facility
conditions were measured by J-Power Electric Company. In total, 14 storage and 21 regulating
reservoirs are operated and monitored by J-Power. The average ages after dam completion are 47
years old in storage reservoirs and 43 years old in regulating ones.
Figure 3 shows rate of vertical sedimentation profiles in reservoirs. Storage reservoir A andregulating reservoir B are located in the same river. Reservoir C and D are in the same relationship.
10,000Reservoir life span （year）

Area of storage reservoir

1,000

100

10

Area of regulating reservoir

Storage reservoir
Regulating reservoir

reservoirs under sedimentation control
1
1

10

100

1,000

Inflow rate of sedimentation （1,000m3／year）

10,000Figure 4. Relation of inflow rate of sediment and reservoir life span

DAMS 2011

Table 1. Impacts of reservoir sedimentation on infrastructures (average)
Items

Storage reservoir (14 dams) Regulating reservoir (21 dams)

Total capacity(average)

195,500,000m3

11,900,000m3

Effective capacity(average)
Rate of sedimentation to total capacity

149,800,000m3
11.0%

3,700,000m3
22.3%

6.5%
57.1％

4.31%
71.4%

4,100m2／a reservoir
0m／a reservoir

22,700 m2／a reservoir
403m／a reservoir

Rate of sedimentation to effective capacity
Rate of reservoir that has inundation area
Area of inundation
Road length of inundation

Bridge affected by flood water level
0.1 bridge／a reservoir
1.5 bridge／a reservoir
Note) These data are averaged figures of dams which J-Power owns.

It was found that there is high rate of sedimentation at high water level in regulating reservoirs,
which causes high flood water level. Available effective water depth and vertical sedimentation
profiles are correlated, which indicates that control of reservoir water level can be effective forsedimentation control.
Figure 4 shows relation of inflow rate of sediment and reservoir life span. Life span can be
defined as the ratio of the reservoir total capacity over annual inflow sediment volume. Plot areasof storage reservoirs and regulating reservoirs are separated. Highlighted reservoirs by dotted circle
are ones under sedimentation control. Many regulating reservoirs are highlighted because of highsedimentation near high water level. Moreover, it was noticed that the necessity of sedimentation
control did not depend on sediment inflow rate, but on the reduction of reservoir life spans.
Table 1 shows impacts of sedimentation in regulating and storage reservoirs on surrounding
infrastructures (bridge and road). Flood water level is the maximum flood water level under design
flood discharge. It can be noticed that regulating reservoir has a severe sedimentation problem
more than storage reservoir. Because the regulating reservoir has about 5 times inundation area, 15
times number of affected bridges compared with storage reservoir. Moreover, no affected road wasfound in storage reservoirs. There are two reasons that make these differences; the first reason is
location of reservoirs, and the second one is sedimentation condition determined by reservoir
operation. Compared to storage reservoir sites, regulating reservoirs are located relatively
downstream river near large cities that have more infrastructures like roads, bridges, farms,
factories and houses around reservoirs. Regulating reservoir usually has very small available
effective water depth so the coming sediment will be deposited in almost higher water level.
It can be concluded that it is more important to implement sedimentation control in regulating
reservoirs than in storage reservoirs. .
3

Effective sediment management measures for regulating reservoirs

Since we already know that sedimentation management for regulating reservoirs is very important,effective management measures considering properties of sedimentation and reservoir operation
should be planned. Regulating reservoirs can be classified into three types by shape of flood waterlevel and condition of sedimentation. Table 2 summarizes characteristics of these types, which areriver, lake and intermediate types.

DAMS 2011

Table 2. Effective measures of sedimentation management for three regulating reservoir types
Type

Shape of flood water level
Condition of sedimentation

Heightofdam*

Ratio of
Designspillway gate Number
floodheight to dam of dams
volume* height (%)*

2,700m3/s

Draw down operation during
flood and guide sedimentation to
13% 5（26%） lower dead space. Excavatesome sedimentation for total
management.

42m

4,000m3/s

Draw down operation during
flood and guide sedimentation tolower dead space or pass
29% 9（48%）
through the gate. Excavatesome sedimentation for total
management.

27m

8,000m3/s

Draw down operation during
54% 5（26%） flood, and pass sedimentation
through the gate.

Lake when it floods
Lake

58m
Partially sedimentation
Lake and river when it floods

IntermediateEffective sedimentation
management measures

Sedimentation all overRiver when it floods
River
Sedimentation all over

*These data are averaged figures of dams which J-Power owns.

Also table 2 shows effective sedimentation management measures for each type of reservoirs.
According to the condition of each type, shear velocity, deposition and erosion location during
flood event, appropriate sediment measures can be proposed as shown in Table 2.
Lake type reservoir is relatively large reservoir which looks like lake even in the design floodand has sedimentation partially in the upstream of reservoir. Lake type reservoir has large dam
whose average height is 58m, and has small design flood volume whose average is 2,700m3/sec,
and average ratio of spillway gate height to dam height is 13%.
River type reservoir is relatively small one that looks like river and has sedimentation all over
the reservoir area. River type reservoir has relatively small dam whose height is 27m, and design
flood discharge is large about 8,000m3/sec. And the ratio of spillway gate height to dam height is54%.
Intermediate type reservoir, looks like between lake and river types even in the design flood andhas sedimentation all over the reservoir. Intermediate type reservoir has dam whose height is 42m,
and design flood discharge and the ratio of spillway gate height to dam height are around
4,000m3/sec and 29% respectively.
Sediment management measures by draw down operation for each type of reservoir are proposed.
Effectiveness of draw down operation depends on the type of reservoirs. In the river type reservoir,
bed load material is easily passed through the spillway gate, while hardly possible in the lake type.
In lake and intermediate types of reservoir, there will be remained sedimentation located at
upstream reservoir area even after draw down operation, and combination with excavation will beneeded for total sediment management.
3.1 River type reservoir
3.1.1 Summary of verification on river type reservoir

DAMS 2011

To check the physical feasibility of the measure proposed, we conducted numerical analysis for one
reservoir. The selected reservoir has 9,930,000m3 storage capacity, 1,692 km2 catchment area, andhas been under operation since 1958. It has some problems by sedimentation which is one of themost important ones is rising flood water level in the middle reservoir area. Recently, we have been
excavating sedimentation which is inconvenient for reservoir management.
Table 3 shows the results of numerical analysis for river type. No action for sedimentmanagement for CASE-A1 is needed in next 28 years. While, Draw down operation during flood isselected for CASE -A2. Finally, annual excavation strategy of 30,000 m3 is selected for CASE -A3.
We used the 1D numerical model and fit the model to actual phenomenon by using surveyed datain 28 years. In the model, the amount of bed material load and suspended load is calculated by
Ashida-Michiue formula [2].
3.1.2 Result of verification on river type reservoir
Figure 5 shows the location of sediment deposition in the reservoir for three cases. In CASE-A1,
sedimentation was increasing equally all over the reservoir. This made flood water level higher,
and infrastructures around the reservoir was more affected. In CASE-A2 and A3, sedimentation
was increasing near the dam, and decreasing in the middle of reservoir. This made flood water level
low in middle area, then it is effective to conduct these 2 cases for total sediment management.
These 2 cases are very different from sedimentation control point of view but have very similareffectiveness.
Figure 6 shows annual change of total sedimentation in the reservoirs within 28 years. In
CASE-A1, total sedimentation was generally increased during 28 years, but in CASE-A2 and A3,
total sedimentation remained constant without change. That means it is sustainable to implement
these 2 strategies for sedimentation control.
Table 3. Cases of numerical analysis for river type reservoir

Total sedimentation in 28 years (m3)Case

Scenarios for sediment management

CASE-A1･No sedimentation control in next 28 years.

CASE-A2･Sedimentation control in next 28 years.
･The control means draw down operation during flood

CASE-A3･Sedimentation control in next 28 years.
･The control means excavation of sediment in reservoir which is amount of 30,000m3
a year.

100,000
80,000

Excavation area (CASE－A3, 30,000m3 /a year）

60,000CASE-A1：No sedimentation control
CASE-A2：Draw-down operation
CASE-A3：Excavation

40,00020,0000-20,000-40,000-60,000-80,000
0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Distance from the dam (m)Figure 5. Sedimentation position in the results of calculation (river type)

9,000

10,000DAMS 2011

Total sedimentation (m3 )

500,000
CASE-A1：No sedimentation control
CASE-A2：Draw-down operation
CASE-A3：Excavation

400,000
300,000
200,000
100,000
0-100,000-200,000
0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Year passed

Figure 6. Calculated annual sedimentation volumes for river type
Table 4. Cost comparison of sedimentation measures for river type in 30 years (million¥)
Case
Item
Cost/30 years
Generation loss by the operation
300
CASE-A2
Maintenance of facility
210
Generation loss by the maintenance
126
Draw down
Examination and Analysis needed
430
operation
Total
1,066
Excavation and disposal
3,300
CASE-A3
Generation loss by the excavation
540
Sediment
Fixture of disposal area
510
excavation
Examination and Analysis needed
80
30,000m3/yr
Total
4,430

Table 4 shows the cost comparison of two effective strategies for sedimentation management
measures. It is more economical to apply draw down operation (CASE-A2) compared to excavation
strategy (CASE-A3).
3.2 Intermediate type reservoir
3.2.1 Summary of verification on intermediate type reservoir
To check the physical feasibility of the sediment measure, we conducted numerical analysis for one
reservoir, which has 4,420,000m3 capacity and 217 km2 catchment area, and has been underoperation since 1960. Reservoir sedimentation caused several problems as flood water level risenear the bridge over the reservoir, which needs clearance between flood water level and the bridge.
Recently, sediment management strategies have been considered in these reservoirs.
Table 5 shows cases of the numerical analysis and scenarios for sediment management forintermediate type. CASE-B1 is simulated where no sediment measure was implemented in the next
22 years, CASE-B2 implemented draw down operation during flood events, CASE-B3 used
excavation strategy about 30,000m3 sediment volume every year. CASE-B4 is simulated as
combined method of draw down and excavation strategies, and the annual excavated sedimentvolume is 15,000m3.
We used the 1D numerical model and fit the model to actual phenomenon by using surveyed datain 22 years. In the model, the amount of bed material load and suspended load is calculated by
Ashida-Michiue formula [2].

DAMS 2011

Table 5. Cases of numerical analysis for intermediate type reservoir
Case

Scenarios for sediment management

CASE-B1･No sedimentation control in next 22 years.

CASE-B2･Sedimentation control in next 22 years.
･The control means draw down operation in flood

CASE-B3･Sedimentation control in next 22 years.
･The control means sediment excavation in reservoir which amount is 30,000m3 a year.

CASE-B4･Sedimentation control in next 22 years.
･The control includes sediment excavation and draw down operation, amount of
excavation is 15,000m3 a year.

3.2.2 Result of verification on intermediate type reservoir
Figure 7 shows the status of sedimentation in the reservoir for four cases. In every CASE
sedimentation was located near the dam, because there was a vacant zone of sedimentation. In
CASE-B1, sedimentation was increasing in the middle area and near the dam. This made flood
water level higher in the middle of the reservoir, and the clearance between water level and thebridge was decreasing. In CASE-B2, sedimentation was increasing in the middle area and near thedam as in CASE-B1. That can explain the same result as in CASE-B1. Therefore, the sedimentmanagement strategy for CASE-B2 is not worth as sedimentation control. In CASE-B3,
sedimentation was not increasing much all over the reservoir, and this did not make flood waterlevel higher in middle area. That scenario is worth as sedimentation control. In CASE-B4,
sedimentation was increasing near the dam, but decreasing in the middle reservoir area. This did
not make flood water level higher in middle area. This case is worth as sedimentation control.
It is effective to use two scenarios of CSAE-B3 and B4 in order to implement total sedimentmanagement control. These cases are very different from each other in the ways of sedimentation
control, but very similar in the effectiveness of the results.
Figure 8 shows annual change of total sedimentation volume in the reservoirs in 22 years. In
CASE-B1 and B2, total sedimentation was generally increasing in 22 years, but in CASE-B3 andB4, total sedimentation were not increasing frequently, but almost remain constant. This remaining
means that it is sustainable to conduct these two ways of sedimentation control.
Table 6 shows the cost comparison of three sedimentation management measures. It is not

Total sedimentation in 22 years (m3)enough only to make draw down operation, CASE-B2. CASE-B4 is the measure combined draw
down operation and excavation. It is still economical to consider CASE-B4 more than only
excavation, CASE-B3.
350,000

CASE-B1：No sedimentation control
CASE-B2：Draw-down operation
CASE-B3：Excavation
CASE-B4：Draw-down operation & Excavation

300,000
250,000
200,000

Excavation area (CASE－B4, 15,000m3/a year）

150,000

Excavation area (CASE－B3, 30,000m3/a year）

100,000
50,0000-50,000-100,000
0

500

1,000

1,500

2,000

2,500

3,000

Distance from the dam (m)Figure 7. Sedimentation position in the results of calculation (intermediate type)3,500

DAMS 2011

Total sedimentation （m3 ）

800,000
CASE-B1：No sedimentation control
CASE-B2：Draw-down operation
CASE-B3：Excavation
CASE-B4：Draw-down operation & Excavation

700,000
600,000
500,000
400,000
300,000
200,000
100,000
0-100,000
0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

Year passed

Figure 8. Calculated annual sedimentation volumes for intermediate type
Table 6. Cost comparison of sedimentation measures for intermediate type in 30 years (million¥)
Case

Item

Generation loss by the operation
CASE-B2
Maintenance of facility
Only
implement
one
Generation loss by the maintenancestrategy of
Examination and Analysis neededDraw-down operation
Total
Excavation under water
CASE-B3
Fixture of harbor
Only
implementone
Fixture of disposal area
strategy of
Examination and Analysis needed
Excavation 30,000m3/yr
Total
Generation loss by the operation
Maintenance of facility
CASE-B4
Generation loss by the maintenance
Combinationof
two
Excavation under water
strategies
Fixture of harbor
Draw down operation
Fixture of disposal area
3
Excavation 15,000m /yr
Examination and Analysis needed
Total

Cost/30 years
300
210
105
430
1,045
4,500
140
510
80
5,230
300
210
105
2,700
140
390
510
4,355

3.3 Result of the verifications
We verified the effectiveness of draw down operation for sedimentation control in regulating
reservoirs. Regulating reservoir is usually under operation of smaller available effective water
depth, and it causes much deposition in the same area of reservoir. On the other hand, it is very
effective to make regulating reservoir under unusual operation, which is to make the dam waterlevel to below usual available effective water depth. Draw down operation reduces the head ofpower generation, but still more economical solution than excavation.
4River environment and effective utilization

It is not enough only to take care of infrastructure around reservoir and life span of reservoir, butalso we have to think about downstream river environment below the dam, and effective utilizationof sediment.
Dam interrupts continuous sedimentation transport along river. No matter how long the reservoir
as hydropower station facility lasts, it will not worth as the river environment get worse [3] [4].

DAMS 2011

Keeping downstream river environment healthy is the important issue from the sustainability ofhydropower operation point of view.
In Japan, we do not have much sand to construct the infrastructures, and we have destroyed
mountains to get sand. Sand in rivers is usually under strict regulation to protect rivers, especiallyfor construction, and the regulation has covered the reservoir sedimentation [5]. Thus there is a
miss-match between much sand in reservoirs and need for construction. We have to explain to rivermanagement office and river basin stakeholder that there is a way to take out sand from reservoir
without damaging the river.
5

Conclusion

The paper presents the reservoir sedimentation in hydropower stations, considering operation rulesof storage of regulating reservoirs. Based on useful sedimentation characteristics and dam facility
condition, we can propose suitable measures against reservoir sedimentation by classifying theregulating reservoir to three types. The following points can be concluded:1) Since sedimentation in regulating reservoir is dominant to surrounding infrastructures morethan the storage one, it is more important to manage sedimentation in regulating reservoir thanstorage one.
2) It is effective to classify regulating reservoir into three types; lake, river, and intermediate
type, and we can propose the measures for each type. The measures for river type andintermediate type which are mainly conducted by draw down operation which is physicallyeffective and economically feasible comparing to other alternative measures.
3) It is important to consider the downstream river environment below dams and to consider
effective utilization of sediment for sustainable hydropower operation.
References[1] Japan Electric Power Civil Engineering Association, Research and enlightment on reservoir
sedimentation in hydropower station, 2006.(in Japanese)[2] Kazuo Ashida, Michiue Masanori, Study on Hydraulic Resistance and Bed-Road Transport
Rate Alluvial Stream, Jurnal of Japan Society of Civil Engneers, vol.206 pp56-69, 1972. (inJapanese)[3] S.A. Kantoush, Tetsuya Sumi, Akira Kubota, Geomorphic response of rivers below dams by
sediment replenishment technique, River Flow 2010-Dittrich, Koll, Aberle & Geisenhainer(eds), 2010.
[4] Tetsuya Sumi, S.A. Kantoush, Integrated Management of Reservoir Sediment Routing by
Flushing, Replenishing, and Bypassing Sediments in Japanese River Basin, 8th International
Symposium of Ecohydraulics, 2010.
[5] Masaru Banda, Tetsuya Sumi, Recycling Business Making of Reservoir Sedimentation from
Viewpoint of Sediment Resource Management, 15th Proceedings of River Engineering JSCE,
2009. (in Japanese)