Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

Influence of reservoir sedimentation on power generation
H. Okumura
Civil Engineering Office, Civil and Architectural Engineering Department, Electric Power
Development Company, Japan

T. SumiWater Resource Research Center, Disaster Prevention Research Institute, Kyoto University, Japan

ABSTRACT: Reservoir sedimentation is one of the most important problems for securing sustainablehydropower plant operation in the future. We have already classified the sedimentation problem regarding
the reservoir types and proposed some effective sediment control measures for regulating reservoirs. On
the other hand, sedimentation problems in storage reservoirs need to be more studied. In this paper, wehave evaluated the influence of sedimentation progress in storage reservoir on power generation by analyzing long-term operation record, and predicted the future influence of storage reservoir sedimentation
assuming active storage capacity decreasing.

1INTRODUCTION

Reservoir sedimentation is one of the most important problems for securing sustainable hydropower
plant operation. Figure 1 shows location andposition of hydropower plants in a river. Usuallycouples of hydropower plants are installed consequently in a river to use river water efficiently. In
upstream of a river, a storage reservoir with large
capacity is located to store inflow water by seasonalrain with typhoon and melting snow. And regulating reservoirs with small capacity are located indownstream of the river to regulate inflow water inscale of a day or a week for power generation.
We have already classified the sedimentation problems regarding the reservoir types andFigure 1.

concluded that there is present severe flood riskin regulating reservoirs. Then we have proposed
effective sediment control measures for regulating
reservoirs, which are mainly conducted by drawdown operation, regarding their properties. On theother hand, sedimentation problems in storage reservoirs are not clear and need to be more studied
shown in Figure 2.
Japan electric power civil engineering
association (2005) and JCOLD (2010) researched
the sedimentation problems in Japanese hydropower reservoirs and concluded that sedimentation influence on power generation was notsevere in present. Although sedimentation ratioin active water capacity of storage reservoirs isaveraged 6.5%.

Location and position of hydropower plants.

1157

Table 1. Reservoir turnover ratio (active water capacity)
original and as of 2011.

Figure 2.
reservoir.

Reservoir
turnover ratio(active cap.)

Reservoir(plant)

Original
active cap.
(1,000 m3)

Start ofoperation

Original

2011

AB
C
D
E
F
G

458,000
330,000
68,00072,50050,00089,000205,444

1960
1961
1962
1970
1964
1960
1956

3.14
4.14
4.42
5.33
7.56
12.55
24.98

3.19
4.21
4.47
5.65
7.66
13.63
31.97

Figure 3.

Location of reservoirs (A^G).

Sedimentation management in hydropower

In this paper, we studied influence of sedimentation in storage reservoir on power generation,
predicted the influence in the future, and discussed
countermeasures.

2SECULAR CHANGE OF POWER
GENERATION

Figure 1 shows roles of storage reservoir, whichis to store inflow water by seasonal water inflow,
with typhoon and melting snow. It means that it
is important for storage reservoir to have a large
active water capacity. Planning storage type hydropower plant, it is designed that active water capacity has more than 20% of yearly total water inflow,
this means storage reservoir should have less than
5 reservoir turnover ratio in active water volume.
Table 1 shows secular change of reservoir turnover ratio in active water capacity which J-POWER
owns. Figure 3 shows the area where 7 reservoirs
are located. They show all of 7 reservoirs havehigher turnover ratio than original ratio because ofcapacity loss by sedimentation. However, problems
of generation loss caused by capacity loss have notbeen issued in all of seven hydropower plants.
To evaluate the influence of sedimentation instorage reservoir on power generation, water use
efficiency of each plant is calculated by Equation (1)per annum.
Water use efficiency
Yearly total turbine disch ge volume
Yearly total fllow
l volume(1)

Water use efficiency is ratio of yearly turbine
discharge volume to yearly inflow volume. When it
rains much in reservoir catchment area and a reservoir discharges much water from spillway gate,
then water use efficiency of the year is calculated
low.
Table 2 shows summary of water use efficiency
calculation result with from 1988 to 2011 recordof 7 hydropower plants. Six reservoirs have sedimentation progressed and five plants have decreasing water use efficiency through the period of
the record. The summary indicates that reservoir
sedimentation in storage reservoir is gradually
increasing and water use efficiency is gradually
decreasing.
Figures 4 and 5 show secular changes of water
use efficiency in hydropower plant E and D. On
figures, regression line, inclination, yearly total

1158

Table 2.

Secular change of water use efficiency.
Sedimentation ratio

heavy seasonal discharge induced by typhoon andweather front and has many occasions of dam
spillway gate discharge every year. Then water
use efficiency of hydropower D plant is lower
around 90%.

Water use efficiency

Res.
1988^2011 1988^2011 1988^2011 1988^2011(plant) averagechange*averagechange*
AB
CDE
F
G

1.68%
1.59%
1.10%
5.61%
1.36%
7.94%
21.86%

0.058%/yr. 99.67%
0.045%/yr. 99.11%
0.066%/yr. 78.73%
0.105%/yr. 91.22%
0.133%/yr. 97.89%
0.058%/yr. 78.84%
0.027%/yr. 89.82%

0.059%/yr.
0.035%/yr.
0.485%/yr.
0.158%/yr.
0.183%/yr.
0.104%/yr.
0.125%/yr.

*Change means inclination of regression line.

33.1

FACTORS OF INFLUENCE ON POWER
GENERATION
Active water capacity

Water use efficiency does not depend only on inflow
regime, but on active water capacity. Figure 6 shows
the typical inflow regime of reservoirs on Japansea side and ones on the Pacific Ocean side. Inflow
regime is presented by total water inflow volume
more than ninety-five-day discharge. Ninety-fiveday discharge is almost equal to discharge for firm
power. When inflow is more than ninety-five-day
discharge, it tends to discharge water through dam
spillway gate not through the turbines.
Figure 7 shows relation among water use efficiency, inflow regime and active water capacity, ofhydropower plant E and D. Figure 7 indicates that

Figure 4. Secular change of water use efficiency inhydropower plant E (1988–2011).

Figure 6.

Inflow regime of reservoir E and C.

Figure 5. Secular change of water use efficiency inhydropower plant D (1988–2011).

inflow volume and turbine discharge are placed.
Regression lines in both figures are downward toright. Two hydropower plants have different waysof water use. Hydropower plant E is on Japan Seaside, water discharge is tranquil through seasonscompared to on the Pacific Ocean side. Reservoir
E is expected to be averaged inflow including snow
melting discharge and has less dam discharge than
Reservoir D, and water use efficiency is high every
year almost 100%. Hydropower reservoir D has

Figure 7. Relation between water use efficiency andtotal inflow volume more than 95-day discharge.

1159

Figure 8.

Comparison of dam gate discharge condition between record and simulation result at reservoir D.

Table 3. Summary of comparison between record andsimulation result at reservoir D.
Sedimentation
ratio in active
water capacity

Maximumof damgate
discharge

Date ofdam
spillwaygate open

6.4% (2005)

601 m3/sec

20%

670 m3/sec

50%

700 m3/sec

2:00,6th Sep.
14:00,5th Sep.
3:00,5th Sep.

Total volumeof dam gate
discharge
29,458,000 m3
34,575,000 m3
45,800,000 m3

Average yearly total inflow volume: 384,572,000 m3.
Figure 9. Relation between water use efficiency andmaximum turbine discharge.

three index are correlated clearly. If total water
inflow volume which is more than ninety-five-day
discharge is high, water use efficiency is low. And if
active water capacity is small, water use efficiency
is low, too. These things mean that progress of reservoir sedimentation makes active water capacity
small, clear influence on power generation results.
On the other hand, keeping active water capacity
large preventing from sedimentation is very important for sustainable use of hydropower plant.
Figure 8 shows time-discharge curves of reservoir D during the flood time. When flood occurred
sedimentation ratio in active water capacity was
6.4% in fact. Simulation results in case of sedimentation ratio 20% and 50% are shown on Figure 8.
Table 3 shows comparison of dam gate discharge
condition between the record and simulation
results. Sedimentation progress in active watercapacity makes dam gate discharge volume larger,
and start of dam gate open earlier.
3.2

Maximum turbine discharge

Water use efficiency also depends on the maximum
turbine discharge. Figure 9 shows relation between

water use efficiency and the maximum turbine discharge. It is clear that hydropower plant has thelarger maximum turbine discharge, the plant has
the higher water use efficiency. This is opposite
way from way of planning the hydropower plant.
44.1

WATER USE EFFICIENCYEVALUATION MODEL
Conceptual diagram

Figure 10 is the conceptual diagram of water use
efficiency and factors influencing on the efficiency.
The conceptual model is based on the result ofchapter 3. Power generation (Water use efficiency)
is influenced by relation between active watercapacity and inflow regime and relation between
the maximum turbine discharge and inflow regime.
When hydropower plant is under planning, these
relation, power demand, and economic efficiency
are considered to fix the design of the plant. Now,
50 years after installation, sedimentation in active1160

Figure 10. Conceptual diagram of water use efficiency
and factors.

Table 4.

Multi regression analysis result.
Water use efficiency

Plant
reservoir
ABCDEFG

Coefficient

RecordCalculation

a1  0.03509
a2  0.06467a3  0.1183
b  1.065

99.67%
99.11%
97.89%
89.82%
78.73%
91.22%
78.34%

101.79%97.50%
92.84%
91.30%
84.35%
90.91%
76.08%SEDIMENTATION MANAGEMENTFOR STORAGE RESERVOIR

5.1

Sedimentation influence in the future

Water use efficiency in the future of 7 hydropower plants A^G are evaluated by Equation (2).
Figure 11 shows evaluation results. This indicatesthat progress of sedimentation in storage reservoir
makes water use efficiency of plants low, and influence of sedimentation are varied. Efficiency ofplant C, D, F which have heavy rainy season areinfluenced much by sedimentation. On the other
hand, efficiency of plant A, B, E, G are not somuch. This is the reason why reservoir which has
heavy rainy season needs active water capacity forstorage of flood inflow water more than the otherreservoirs.
Average of sedimentation ratio of hydropower
reservoir is about 10%. According to estimation
result, water use efficiency is not much influenced
in case of 10% sedimentation. Research result ofJapan electric power civil engineering association(2005) and JCOLD (2010) are appropriate.
5.2

Correlation coefficient 0.926.

water capacity has been progressed over estimation, and water use efficiency has been gradually
decreasing.
4.2

5

Evaluation model

For prediction of the future influence on power
generation by progressed sedimentation in storage reservoir, evaluation model of water use efficiency is needed. Model structure is shown inEquation (2), coefficients a1, a2, a3 are determinedby multi regression analysis.

Sedimentation management

Hydropower plant is expected to generate
sustainably. Influence on generation by sedimentation in the future, especially in plant which has
heavy rainy season, is elucidated in chapter 5.1.
Sedimentation management is needed from thetwo points of view bellow. The first point is thatrack of storage function of reservoir which isinstalled upstream of the river damaged largely
to hydropower system in the river, including regulating type hydropower plant downstream of theriver. The second point is that storage reservoir is
too large to excavate sedimentation in reservoir. It
is better to take away before sediment flows into
the reservoir.
Table 5 shows simulation cases of sedimentation
management in storage reservoir F. Reservoir F
is needed to treat averaged 180,000 m3 sediment

Water use efficiency
a1 r total infloiw volumea2
a total volume more than 95-day discharge
Active water capacityt

a3 r total water infllow
o volume more than 95-day dischargeb(2)Maximum turbine discharge for 95
9 daysResult of multi regression analysis is summarized on Table 4. Correlation coefficient is 0.926.
Record and results of water use efficiency are similar, and evaluation model has sufficient reliability
for future prediction calculation.

every year, and has a regulating reservoir hydropower plant downstream. Case A is simulation of
no sedimentation management and loss of power
generation is recorded as an expense. In Case B^D,
loss of power generation is also recorded as an1161

Table 5. Simulation cases of sedimentation management in storage reservoir F.
No. Measure

Unit cost

Start period

A

Nothing––

B

Dredging¥100,000,000/port¥5,000/m3

C

Check dam & ¥100,000,000/dam
excavation¥2,500/m3

1st year
25th year
50th year
100th year
150th year

DSediment
bypass¥13,200,000,000/pass¥121,000,000/year

Figure 11.

Prediction of water use efficiency.

Figure 12.

Sedimentation management cost simulation result in storage reservoir F.

Subject Amount of Sediment (yearly): 180,000 m3.

expense. Unit price used in calculation of generation loss is ¥8.4/kWh, which was average of JEPX
(Japan Electric Power Exchange) from 2005 to2010.
Figure 12 shows simulation result of 300 years.
From the view point of sustainable use ofhydropower plant, it is economically feasibleto install a sediment bypass, which is appropriate way to restore sediment to river for environment including the coastal area. The result
indicates the period of starting management. In
situation sediment bypass could be installed, itwould be better to be done earlier for cost andenvironment.

6CONCLUSION

This paper presents the reservoir sedimentationinfluence on power generation, analyzing longterm operation record. Also this paper predicts
the future sedimentation influence using the model
which is based on the analysis. Then this paper discussed the countermeasures for sedimentation instorage reservoir considering properties and purpose of storage reservoir. Result includes some
useful findings as follows.
1. Reservoir sedimentation in storage reservoir is
gradually increasing and water use efficiency is
gradually decreasing.

1162

2. In storage reservoir type hydropower plant, water
use efficiency is influenced by sedimentation.
3. Evaluation model of water use efficiency isformulated.
4. Water use efficiency in hydropower plant which
has heavy rainy season will be much influenced
by sedimentation in the future.
5. Cost simulation of sedimentation management
in storage reservoir results that it is the best way
to install sediment bypass because of cost andenvironment.

Okumura, H., Sumi, T., 2012, Reservoir Sedimentation
Management in Hydropower Plant Regarding Flood
Risk and Loss of Power Generation, International
Symposium on Dams For A Changing World ICOLD
2012, Kyoto, Japan.

REFERENCES
Okumura, H., Sumi, T., Kantoush, S.A., 2011, Reservoir
Sedimentation Management in Hydropower Station
Considering Property of sedimentation and Facility
Condition, Proc. of International Symposium on
Modern technologies and Long-Term Behavior ofDams, Zhengzhou, China. pp 100upp 200.

1163