Reservoir Sedimentation Management in Hydropower Plant
Regarding Flood Risk and Loss of Power Generation
Hirofumi Okumura
Manager, Civil & Electrical Engineering Department, Electric Power Development Company, Japanhirofumi_okumura@jpower.co.jp

Tetsuya Sumi
Professor, Water Resource Research Center, Disaster Prevention Research Institute, Kyoto University, Japan

ABSTRACT:
Reservoir sedimentation is one of the most important problems for securing long term hydropower plant operation in the future.
Reservoir sedimentation causes various problems. There are two severe problems, one is increasing flood risk of infrastructures aroundreservoirs, and the other is decrease of the active water capacity of reservoir leading to loss of power generation.
The first problem is well known. In this paper considering the properties of reservoir sedimentation, facility condition andinfrastructures around reservoir, several measures have been proposed and verified regarding their feasibilities. The second problem is
not known and needs more study, therefore this paper has evaluated influence of reservoir sedimentation on loss of power generation,and has discussed measures against the problem.
Keywords: Reservoir sedimentation, Hydropower plant, Increasing flood risk, Loss of power generation

1. INTRODUCTION
Sustainability of hydropower stations is endangered byreservoir sedimentation. Practically many reservoirs ofhydropower stations are under sedimentation control orunder planning. In 2005, Japan electric power civilengineering association researched 354 reservoirs ofhydropower plants, which had over 15m height dam orover 1 million m3 water capacity in order to analyse theimpacts of increasing sedimentation. Table 1 shows theresult of this analysis where 95 reservoirs are influenced
by increasing sedimentation and the major influence isincreasing flood risk. Loss of power generation is not the
major problem at this analysis.
In order to solve the problem of increasing flood risk, thispaper discusses the current sedimentation problems by
Table 1. Influence by sedimentation on hydropower reservoirs
Items
Numbers
Total number of investigated reservoirs

354

Reservoirs influenced by sedimentation

95

Specific influences
Increasing flood risk

91

Environmental problem

5

Loss of power generation
6
Analysis on reservoirs that have over 15m height dam or over 1
million m3 storage capacity

considering the differences of two major types ofhydropower plants, which are storage type and regulating
type. Using the result of the analysis, effective andeconomical reservoir sedimentation management
measures which are mainly conducted by reservoir
draw-down operation are proposed.
Also this paper has focused on loss of power generation
by sedimentation in hydropower plant, which is not the
big problem now. By the analysis of relationship between
water use efficiency and sedimentation in active watercapacity, the influence of reservoir sedimentation onhydropower plant operation has been indicated in somereservoirs. And we have found that some reservoirs needsedimentation control against loss of hydropower plant
operation.

2. SEDIMENT MANAGEMENT MEASURES FOR
INCREASING FLOOD RISK
2.1. Storage Reservoir and Regulating Reservoir
Cascade hydropower plants are usually placed in a river
basin as shown in Fig. 1 where the large generator withthe large storage reservoir for power peak is placed on
the upstream and smaller generators with smaller
reservoirs for level power demand are placed on thedownstream of the river. Storage reservoir is working as

0Storage reservoir

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

Generator

Regulating reservoir

Sea area

Depth from high water level （m）

10

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

20
30
40
50

Storage reservior A

60

Regulating reservoir B
7080

Figure 1. Location and position of hydropower stations

a catchment of flood flow caused by typhoon, seasonalrain and melting snow. Regulating reservoir is set foradjustment between water supply and power demand in
the short term per a day or a week.
Storage and regulating reservoirs are very different in
two points of views. Fig. 2 shows the relation of storage
and regulating reservoir capacity. By comparing two
types in terms of total capacity, the storage types areabout 3 to 100 times larger than the regulating ones.
Regarding the active water capacity that can be used for
power generation, the storage types are about 10 to 200
times larger than the regulating ones. These differences
are caused by the difference of available draw-down
depth in active water capacity. Average draw-down
depth in storage type reservoirs is more than 25 meters,
and that in regulating ones 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 reservoir conditions, characteristics
of sedimentation (position, sediment inflow rate) anddam facility conditions were measured by J-PowerElectric Company. In total, 14 storage and 21 regulatingtype reservoirs are operated by J-Power. The average

0%

10%

20%

30%

40%

50%

Rate of sedimentation distribution

Figure 3. Rate of sedimentation distribution

ages after dam completion are 47 years old in storage
reservoir and 43 years in regulating type.
Fig. 3 shows rate of vertical sedimentation profiles inreservoirs. Storage reservoir A and regulating reservoir B
are located in the same river. It is found that there is highrate of sedimentation at high water level in regulating
reservoirs which causes high flood water level. Available
draw-down depth and vertical sedimentation profiles arecorrelated which indicates that water level control can beeffective for sedimentation control.
Fig.4 shows relation of sedimentation inflow rate and life
span of reservoirs. Life span can be defined as the ratioof the reservoir capacity over annual inflow sedimentvolume. Plot areas of storage reservoirs and regulating
reservoirs are separated. Highlighted reservoirs by dotted
circle are under sedimentation control. Many regulating
reservoirs are highlighted because of their huge
sedimentation near high water level. Moreover, it isnoticed that the necessity of sedimentation control does
not depend on sediment inflow rate, but on the reductionof the reservoir life span.
10,000Area of storage reservoir
Ratio of regulating over storage(Total water capacity)

3

Ratio of regulating over storage(Active water capacity)

100,000

Reservoir life span （year）

Regulating resevoir capacity（1,000m ）1,000,000

10,000
1:1
1 : 10

1,000

1,000

100

10

Area of regulating reservoir

Storage reservoir
Regulating reservoir

1 : 100
1 : 1,000

reservoirs under sedimentation controll

100

1

100

1,000

10,000100,000

1,000,000

Storage resevoir capacity （1,000m3）

Figure 2. Comparison of storage and regulating reservoir
capacity

1

10

100

1,000

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

10,000Figure 4. Relation of sedimentation discharge and life span ofreservoirs

Table 2. Impacts of reservoir sedimentation on infrastructures

Storage reservoir（ 14）

Items

Regulating reservoir（ 21）3

Total water capacity(average)
Active water capacity(average)
Rate of sedimentation to total water capacity
Rate of sedimentation to active water capacity
Rate of reservoir that has inundation area
Area of inundation
Road length of inundation
Bridge affected by flood water level

195,500,000m
149,800,000m3
11.0%
6.5%
57.1％
4,100m2／reservoir
0m／reservoir
0.1 bridge／reservoir

Table 2 shows impacts of sedimentation in regulating
and storage reservoirs on surrounding infrastructures(bridge and road). Flood water level is the maximum one
under the design flood discharge. It can be noticed that
regulating type has a severe sedimentation problem morethan storage type reservoirs. For example, the regulating
reservoir has about 5 times inundation area, 15 times
number of affected bridge compared with the storage
reservoir. Moreover, no affected road is found in storage
reservoirs. There are 2 reasons that make thesedifferences; the first is location of reservoirs and thesecond is sedimentation condition determined byreservoir operation. Compared to storage reservoir sites,
regulating reservoirs are located relatively in downstream
river near large cities that have more infrastructures likeroads, bridges, farms, factories and houses aroundreservoirs. Since a regulating reservoir usually has very
little available draw-down depth, the inflow sedimentwill be deposited in almost higher water level.

Type Reservoir Sedimentation

It can be concluded that it is more important toimplement sedimentation control in regulating reservoirs
than in storage ones.

2.3. Effective Management Measures for Regulating

11,900,000m3
3,700,000m3
22.3%
4.31%
71.4％
2
22,700 m ／reservoir
403m／reservoir
1.5 bridge／reservoir

We already know that sedimentation management ofregulating reservoir is very important, and we have toplan effective management measures considering
properties of sedimentation and reservoir operation.
Regulating reservoirs can be classified to 3 types byshape of flood water level and condition of sedimentation.
Table 3 summarizes characteristics of these types, which
are river, lake and intermediate types.
Also table 3 shows effective sediment management
measures for each type of reservoirs. According to thecondition of each type, shear velocity, deposition anderosion location during flood event, appropriate sedimentmanagement measures are proposed as shown in the
Table 3. It is verified by numerical analysis that thesemanagement measures were all physically effective.
Lake type reservoir is relatively large which is similar tolake even at the time of flood and has sedimentation
partially in the upstream of reservoir. Lake type reservoir
has generally large dam whose average height is 58m. In
this case, design flood discharge is small about
2,700m3/sec and average ratio of spillway gate height todam height is about 13%.

Table 3. Effective sediment management measures for 3 regulating reservoir types
Ratio of
Shape of flood water level Height
Design flood spillway gate Number
Effective sedimentation
Condition ofofTypemanagement measures
sedimentation
dam* discharge* height to dam of dams
height (%)*
Keeping lake style during floods

Lake

13%

Draw-down operation during floodand guide sedimentation to lower
5
dead space. Addition of facility like(26%)
sediment bypass or sediment
trapping dam

42m 4,000 m3/s

29%

Draw-down operation during floodand guide sedimentation to lower
9
dead space or pass through the gate.
(48%)
Excavate some sedimentation for
total management.

27m 8,000 m3/s

54%

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

58m 2,700m3/s
Partially sedimentation
Usually lake style and change to riverstyle during floods

IntermediateSedimentation all area
Usually river typeRiver
Sedimentation all area

*average figures of dams J-Power owns

Type

Lake

Table 4. Cost comparison between draw-down operation and excavation for 3 regulating reservoir types
Case No.
Contents of measure
Cost/30 years
Model reservoir
Draw-down operation
CASE-L01¥ 9,780million
Sediment bypass
Reservoir capacity : 43,000,000m3
Sediment bypass
Catchment area : 801km2
CASE-L02¥ 20,310million
Excavation (60,000m3)
Annual sedimentation : 250,000m3/a year
Remark : large sediment discharge from aDraw-down operation
CASE-L03¥ 20,334million
branch into reservoir
Excavation (100,000m3)¥ 31,524million

CASE-I02
CASE-R01

Excavation (160,000m3)
Draw-down operation
Excavation (15,000m3)
Excavation (30,000m3)
Draw-down operation

CASE-R02

Excavation (30,000m3)¥ 4,434million

CASE-L04
IntermediateRiver

CASE-I01

River type reservoir is relatively small that looks likeriver and has sedimentation all over area. It has relatively
small dam whose average height is 27m. In this case,design flood discharge is large about 8,000m3/s and theratio of spillway gate height to dam height is about 54%.
Intermediate type reservoir looks like between lake andriver types during the flood and has sedimentation allover area. It has dam whose average height is 42m. In
this case, design flood discharge is around 4,000m3/s andthe ratio of spillway gate height to dam height is about29%.

¥ 4,361million¥ 5,236million¥ 1,066million

Reservoir capacity : 4,420,000m3
Catchment area : 217km2
Annual sedimentation : 30,000m3/a year
Reservoir capacity : 9,930,000m3
Catchment area : 1,692km2
Annual sedimentation : 30,000m3/a year

excavation is not economical at the situation where large
volume of reservoir has much sedimentation volume.
Intermediate type needs excavation whether draw-down
operation is conducted or not. But it is more economical
to implement draw-down operation than no draw-down
operation.
Sediment management in river type is completed bydraw-down operation, and it is much more economical
than excavation.
2.5. Sediment Management by Draw-Down Operation

Effectiveness of draw-down operation depends on thetype of reservoirs. In the river type reservoir, bed loadmaterial is easily passed through the spillway, while it isalmost sedimented in the lake type. In lake andintermediate types of reservoir there will be remained
sedimentation located at upstream area of reservoir evenafter draw-down operation, and combination withexcavation will be needed for total sedimentmanagement.
2.4. Economic Superiority of Sediment Management
Measures Mainly Conducted by Draw-Down
Operation
There are several measures of sediment management inreservoir. One of the most usual ways is excavation.
Table 4 shows the cost comparison between proposed
measures and the usual one which is mainly conductedby excavation. By numerical analysis, these different
measures are the almost same on the viewpoint of floodrisk mitigation. Cost includes constructing andmaintenance of bypass system, excavation, fixture of
disposal area and transportation. In case of the lake typeenvironmental protection measure and loss of power
production are added.
Lake type needs sediment management if it has large
sediment discharge from a branch into the reservoir.
Sediment bypass is very effective if it is very expensive
to construct sediment bypass. From a viewpoint of longterm use of power plant, e.g. 30 years, 50 years, it would
be reasonable to pay the initial cost. On the other hand,

We verified the effectiveness of draw-down operation for
sedimentation control in regulating reservoirs.
Regulating reservoir is usually under operation of smaller
active water depth, and it causes much deposition in thesame area of reservoir. On the other hand, it is very
effective to make regulating reservoir under suchoperation as to lower the dam reservoir water level below
usual active water depth. Draw-down operation reduces
the head of power generation and loses some power
generation, but it is still more economical than
excavation.

3. EVALUATION OF THE SEDIMENTINFLUENCE ON POWER GENERATION
3.1. Introduction
Fig. 5 shows schematic drawing of sedimentation andactive water capacity in reservoirs. Fig. 6 shows that
sedimentation is increasing in the reservoir and thus
active water capacity is decreasing in some hydropower
plants. But Table 1 shows only 6 of 354 reservoirs are
influenced by sedimentation on their hydropower plant
operation. There are 2 major reasons of it. One is that
small loss of active water capacity could be almost
covered by plant operation abilities, and the other is that
we hardly realize the loss of power generation because of
its smallness and difficulty to find the actual loss in theannual big fluctuation of river flow discharge.

A res. sedimentation ratio
C res. sedimentation ratio
A res. active water capacity left
C res. active water capacity left

20%

80%

15%

75%

10%

70%

5%

65%

0%

60%
2010

year

Active water capacity left

85%

2005

25%

2000

90%

1995

30%

1990

In this chapter, we picked up some hydropower plants toanalyse the relation between sedimentation progress andloss of power generation. And countermeasures havebeen discussed based on the result of the analysis.

95%

1985

Figure 5. Schematic drawing of sedimentation and active
water capacity in a reservoir

35%

1980

Dam

100%

1975

Total watercapacity

To power generator

B res. sedimentation ratio
D res. sedimentation ratio
B res. active water capacity left
D res. active water capacity left

40%

1960

Low water level

IntakeSedimentation ratio to total water capacity

Active water capacity

1970

High water level

1965

Sedimentation

Figure 6. Relation between sedimentation and active water

3.2. Relation between Sedimentation and Power
Generation

7 reservoirs shown in Table 5 were picked up to analyse
the history of the relation between sedimentation in
active water capacity and water use efficiency which is
described in Fig. 8. Annual turnover rate of reservoir
water in Table 5 is defined as the ratio of the reservoir
total capacity over annual inflow water volume. In Fig. 8
active water capacity and water use efficiency are
averaged every ten years (60’~70’, 71’~80’, 81’~90’,
91~00’, 01’~10’) and the relations are different on dams.
Reservoir C, shown in Fig. 7, is in tendency that
sedimentation is increasing and water use efficiency is
Table 5. Characteristics of reservoirs for analysis of therelation between sedimentation in active water capacity andwater use efficiency
Active water Annual turnover rate
Reservoir Operation
capacityof reservoir waterA

Storage

89,000,000m3

9.5

B

Storage

72,500,000m3

3.7

CStorage

22,800,000m3

8.9

DStorage

205,444,000m3

15.0

E

Storage

458,000,000m3

2.6

FRegulating

11,000,000m3

44.8

G

Regulating

980,000m3

192.2

90%

80%

69'～80'81～'90'91'～00'01'～10'

70%
0%

5%

10%

15%

20%

25%

Sedimentation ratio in active water capacity

Figure 7. Relation between sedimentation in active water
100%

Water use efficiency (yearly ratio of total turbine
discharge to total inflow )Fig. 7 shows relation between sedimentation in active
water capacity and water use efficiency at C reservoir. In
vertical axis, water use efficiency means yearly ratio of
total turbine discharge volume to total inflow volume.
Plots in the figure are distinguished by every ten years.
This figure indicates that sedimentation actually
influences on hydropower plant operations since water
use efficiency is lowering according to the decrease ofactive water capacity. It is not only by sedimentation but
also by some other events such as change of operation
rules, maintenance and climate change.

Water use efficiency (yearly ratio of total
turbine discharge to total inflow )100%

81'~90'60'~70'01'~10'01'~10'90%

71'~80'60'~70'01'~10'60'~70'80%

01'~10'60'~70'60'~70'70%

81'~90'60'~70'60%

01'~10'50%

01'~10'A reservoir
C reservoir
E reservoir
G reservoir

B reservoir
D reservoir
F reservoir

5%

15%

40%
-5%

0%

10%

20%

Sedimentation ratio in active water capacity

25%

Figure 8. Time history of the relation between sedimentationin active water capacity and water use efficiency

lowering as time goes. Reservoir C has relatively highwater use efficiency and it is important to store water
during floods and to keep reservoir capacity as much as
possible without sedimentation. Moreover, daily
operation for power generation is based on the standard
dam water level for each period. If dam water level ishigher than the standard water level, it is needed to makedaily turbine discharge more. Therefore it would be themore strict operation for power generation if reservoir
has the more sedimentation in active water capacity.
Reservoir A and B are in tendency that water use
efficiency is almost the same level though sedimentation
is increasing. In resent 20-30 years, two reservoirs havebeen losing their water use efficiency gradually.
Reservoir B has relatively high water use efficiency, andit could lose its water use efficiency like reservoir C
hereafter. Reservoir A does not have high water use
efficiency, but has low annual turnover rate of reservoir
water shown in Table 5. For reservoir A, it is also
important to storage water like reservoir C.
In reservoir F and G, sedimentation in active watercapacity and water use efficiency are not related eachother. Since annual turnover rates of reservoir water are
high and much water always flows into them, they do notneed to storage water relatively.
In reservoir D, although sedimentation has been
increasing, water use efficiency is maintaining the samelevel. Reservoir D is large enough to be able to continue
the same operation even if reservoir sedimentation isincreasing.
In reservoir E, sedimentation has been almost
maintaining same, and water use efficiency has been
lowering about 5% in 50 years. There is no relation
between them. This indicate that there are some reasons
except sedimentation to lower water use efficiency.
Relations between sedimentation in active water capacity
and water use efficiency are various. Power generation insome reservoirs have been influenced by sedimentation
but others have not. Also mechanisms of influence arevarious. We found that some reservoirs needsedimentation management measure conducted now or in
the future.
3.3. Sediment Influences on Power Production
It is found that there is a relation between sedimentationin active water capacity and water use efficiency in somereservoirs. If annual turnover rate of reservoir water islow, water use efficiency could be influenced by
sedimentation. Lowering water use efficiency has been
occurred not only by sedimentation but also by change of
operation rules, maintenance and also climate change.
In purpose of planning countermeasures against loss ofgeneration in hydropower plants, we have to continue toanalyse relation between sedimentation in active watercapacity and water use efficiency, and to expand
analysing scope to dam operation and cascadehydropower system in the river.

4. CONCLUSION
The paper presents the reservoir sedimentation inhydropower stations, considering operation rules ofstorage type regulating reservoirs. Based on specificcharacteristics and dam facility condition of
sedimentation, we can propose suitable countermeasuresagainst reservoir sedimentation by classifying theregulating reservoir to 3 types. Also influence bysedimentation on power generation is discussed on thispaper. Result includes some useful findings as follows.
1) It is more important to manage sedimentation
control in regulating reservoirs than storage ones.
Because that sedimentation in regulating ones isdominant to surrounding infrastructures more than
the storage ones.
2) There is clear relationship between reservoir
operation and sedimentation progress.
3) It is effective to classify regulating reservoirs into 3
types; lake, river, and intermediate types, andpropose suitable countermeasures depending oneach conditions. The measures for river type andintermediate type are mainly conducted bydraw-down operation which is physically effective
and economically feasible comparing to otheralternative measures.
4) There is a clear relation between sedimentation in
active water capacity and water use efficiency. If
annual turnover rate of reservoir water is low,
water use efficiency could be influenced by
sedimentation.
5) Some reservoirs need sediment management tominimize loss of power generation.
REFERENCES
Electric power civil engineering association in Japan(2006):
Research and enlightment on reservoir sedimentation inhydropower station, 2006
Kazuo Ashida, Michiue Masanori.(1972): Study on Hydraulic
Resistance and Bed Load Transport Rate in Alluvial Stream,
Jurnal of Japan Society of Civil Engineers, vol.206 pp56-69
Hirofumi Okumura, Tetsuya Sumi, S.A. Kantoush. (2011):
Reservoir sedimentation management in hydropower
station considering properties of sedimentation and facility
condition, International Symposium on Modern
Technologies and Long-term Behavior of Dams