(space)

Optimization of dam operation rule to improve water environment
in Monobe River
Hirotoshi Kurata* and Tsunemi Watanabe*
Kochi University of Technology*

ABSTRACT: In the basin of Monobe river, one of the major rivers in Kochi prefecture in Japan,
environmental conditions are being worsened. In those environmental problems, draught water flow isespecially a fundamental issue because it often decouples and decreases habitat of water creatures.
Draught water flow is caused by water right and dam operation rule with emphasis on hydraulic power
generation and flood control. In our study, the authors focused on dam operation. Purpose of this study is toquantify tradeoff relationship between hydraulic power generation and protection of river environment. Theauthors simulated dam operation to evaluate hydraulic power impact for power generation and riverenvironment by changing dam operation rule.
KEYWORDS: draught water flow, river environment, hydraulic power generation, dam operation
1. BACKGROUND AND OBJECTIVE OF
However, the development has large negative

RESR|EARCH

impacts on river environment, which are muddy
The Length of Monobe River is 71km, and the2water, draught water flow, and shore erosion. Thebasin area of Monobe River is 468km . There areauthors focus on the draught water flow because thisthree dams, six hydraulic plants, two intake weirs fordecouple and decrease habitat of water creatures.

irrigation. Those infrastructure facilities were madeThroughout the month of October to November inin 1950’s. The river development gives the society

2007, mean daily flows in most days are below 1 m3

social benefits in terms of power generation, water

per second. As a result, most of sweet fish cannot

use and flood control.

move to spawning grounds.

water intake
15.5㎥/s（7㎥/s）

・導流提の検討（国土交通省）

Goudou dam

Nagase dam

Tougou dam

Yoshino dam
Suita dam
Goudou dam
Tougou dam
Stream flow
observation

Capacity(1000m3)
Nagase dam 40710
Yoshino dam 480
Suita dam
5960

Figure 1 Monobe river basin

Minimum flow rate to maintainthe river environment indownstream of Tougou dam
1/1～10/15

1.9㎥/s

10/16～12/31 2.9㎥/s

Stream flow
observation

Figure 2 Minimum flow rate to maintain the riverenvironment

InflowRequisite
water flow

01

97（30㎥/s）

Discharge

2093

1989

1985

1981

1977

1973

1969

minimum flow

1965

fish to run upstream and low rainfall term.
Inflow/Discharge
Maximum discharge(m3/s)
for power generation

drought flow
(355days flow)

19

19

1961

m3/day543210Figure 3 stream regime in Monobe River

Reservoir level（ｍ）

As demonstrated in Figure 2 and Figure 3, 355days

Targetlevel

Time

Reservoirlevel

flow and the minimum flow were below theminimum water flow rate to maintain the riverenvironment in most years. Furthermore, theminimum flow of some years records 0 m3 per

Time
Figure 4

Actual dam operation

second. Causes of draught flow are irrigation right

1.1 Objectives of this studyand dam operation. The authors focus on dam

Purposes of this study are to quantify tradeoff

operation in this study.

relationship between hydraulic power generation andTheauthors

will

explaindocumented

dam

river environment and to evaluate risk of drought

operation rule. To reduce probability of drought

under conditions of the updated irrigation right.

water flow, documented dam operation rule is

2. METHODOLOGY

obligated to maintain certain water level. However,

Figure 5 shows an outline of this study. First, theactual dam operation doesn’t conform to the ruleauthors simulated the two output variables, thebecause maintaining a certain reservoir water level

hydraulic power generation and stream regime, that

increase gate discharge. Discharge from dam isis, drought days, when dam is operated according todivided into gate discharge and water consumption

the documented operation rule. Next, an attempt isfor hydraulic power generation; therefore, increasing

made to evaluate changes of these two output

gate discharge means decreasing water consumption

variables when dam is operated differently.

for①Evaluation of actual operation

hydraulicpowergeneration.

Inaddition,

maintaining certain reservoir water level may
increase difficulty for flood control. However, thefollowing dam operation may causes depletion of

Operating data ofdam
・reservoir level・discharge amount②Evaluation of documented operation ruleconditions of new irrigation right. Irrigation right isof rice makes irrigation period about two weeks

Inflowearlier than the conventional irrigation right.

Parameter・Target
reservoir level・Regulatewater flow

environment, because the starting period of the newPowergeneration
model

Reservoir level,
discharge amount

updated in this year. Farmer’s demand for early crop

However, it may bring negative impacts for river

Productionof electricity

Stream regime

reservoir storage.
The authors should evaluate risk of drought underPowergeneration
model

Dam
operation
model

Streamregime

irrigation period coincides with the period for sweetFigure 5 Outline of this study

Productionof electricity

2.1 Dam operation model
The pricipal author developed a dam operation

In this study, the relationship between output ofmodel based on realistic dam operation rules bypower

Visual Basic for Application. This model repeats

generation and reservoir level is approximated with aconditional branching. If the reservoir level is higher

regression model using basic data for hydraulicthan the target water level, the dam discharges thepower generation planningsum of inflows to the dam and the regulate flow..TheFigure 6 shows accuracy of the hydraulic power

regulate water flow is a parameter to lower thegeneration evaluation model. Error is within +2.3%

reservoir level. If heavy rain appears, dam eases

to -2.5%. First, this error may be caused by error ofdischarge by amount of space of dam until thedata. In flow, discharge and reservoir level data isreservoir level rises the above limit water level. As adaily value averaging short term changes. Thus,result, water flow in downstream is decreased. Whenusing such a dayly averaged data may become ainflow to dam is insufficient for requisite water flowcause of error. Second, ignoring flow control

in downstream, dam supply water flow by expending

functions of Suita and Yoshino dams may cause error

storaged water. As previously mentioned, the modelof hydraulic power generation evaluation model.

generation,discharge

flow

forpowerhas two parameters, the regulate water flow and thepower generation （100milion kwh）

target reservoir level.
The model is run for only Nagase dam in theupstream. The model does not consider flow control
functions of Yoshino and Suita dams in themidstream because flow control functions of thosedams are very small in contrast to Nagase dam.
If discharge of Suita dam is less than the requisite
water flow amount, which is the sum of irrigation

250200150
10050

actual
model

2002
2003
2004
2005
Figure 6 Total power generation production

right of the minimum flow to maintain riverenvironment, that day is counted as a day of drought

2.3 Statistical estimation process of output fromwater flow in the model.

models
The authors estimated drought days in downstream

2.2 Hydraulic power generation evaluation modeland power generation each day for 12 years fromIn general, production of electricity is estimated by

1994 to 2005. Next, we estimated sum of those eachformula.

equalsyear. Next, we calculated average values andgenerating efficiency “μg” times discharge amountstandard deviation of those. Finally, we obtainedfor power generation “Q” times drop ”H”. In reality,“best” parameters to try to simultaneously optimize

generating efficiency is a function of discharge forpower generation production and days of drought bypower generation and drop. In short, the formula isthe use of goal programming.

Outputofpowergeneration

very complicated.
3 Result of analysis under old irrigation rightΡ = 9.8 × Q × H × μg
Formula 1 Output of power generation equal

We first solved the above-mentioned optimization
problem under the old irrigation right and analyzed

tradeoff relationship between power generation andwater environment.

3.3 Relationship between power generation andwater environment in downstream

3.1 Power generation production

Figure 9 shows the average and standard deviation

Figure 7 shows average values of and standardof the optimized solution by goal programming anddeviations of power generation. The authors first

actual value. X axis means days of drought, and Y

prepared 100 sets of parameters and evaluated power

axis means power generation production. Under oldgeneration for each parameter. However, some sets

irrigation right, the average and standard deviation ofof parameters increased difficulty for flood control.

power generation decreased by 2.7% and by 4.5%,

Thus, sets of parameters creating the situation inrespectively. In short, considering the magnitude ofwhich dam water level exceeds 191 meters of thestandard deviation of power generation, when thetarget reservoir level are removed from the analysis.

dam is operated subject to the optimum rule,possibilities of lowering the power generation below

Average of power generation（100million kwh）

its actual value is small statistically. Thus, the168
166
164
162
160
158
156
154

optimum dam operation would improve water
documented rule
actual operation

environment dramatically without sacrificing power
generation.

25

26

27

28

29

30

Standard deviation of powergeneration（100million kwh）

Figure 7 power generation under
old irrigation right

Average of power generation（100milion kwh）
20015010050

3.2 Days of drought
Figure 8 shows drought days under the old irrigationright. Average of drought days decreased. Standard
deviation of drought days increased in contrast toactual dam operation.

Average of drought days
160
1401201008060
documented rule40
actual operation
200020406080Standard deviation of drought days

Figure 8 Drought days under
old irrigation rightactual operation
documented rule

0.00

50.00
100.00150.00200.00
Average of days of drought

Figure 9 Relationship between power generationand stream regime under old irrigation right4 Result of analysis in condition of new irrigation

In analysis under the new irrigation right, theauthors first evaluated risk of drought in spring,
estimated power generation production and days ofdrought. In addition, we optimized those values andanalyzed

tradeoff

relationshipgeneration and water environment.

betweenpower4.1 Risk of drought in spring

We calculated non-exceedance probability of

As a result of simulation under the new irrigation

monthly precipitation which is equal to precipitationright, days of drought decreased generally, however,in 2001 spring based on an assumption that

results of simulation indicates increasing risk of

precipitation distribution can be approximated bydrought in spring. Figure 10 shows cumulative water

lognormal distribution. Non-exceedance probability

budget of Nagase dam in 2001 spring. Under theof monthly precipitation, which is equal to

new irrigation right, water budget of Nagase dam

precipitation of April in 2001, is 1.1%. However, wefalls into the red on March 21 when the irrigationconsidered this calculation of non-exceedance

period is started. 23,500,000m3 of reservoir storage,

probability is underestimated.

which is 70% of max reservoir storage, is depletedFigure 12 shows monthly precipitation of April

until early May. As a result, reservoir storage isfrom 1954 to 2006. As demonstrated in the figure, it

reduced to almost zero on April 29.

is possible that long term trend of monthly
precipitation of April shows decreasing trend. In

Water budget（1000㎥）
15000

new irrigation addition, monthly precipitation which is equal toright2001 had appeared in 1989.
old irrigation
Therefore, we thought rainfall as small as that inright

10000
5000

probability of 1.1%.

of Nagase dam in 2001 spring
As demonstrated in Figure11, monthly precipitationis less than the average in 2001 spring. As can beseen, it is probable that amount of rainfall less thanor equal to rainfall in 2001 spring gives large
negative

impacttoagriculture

andriverenvironment..

Figure 12 precipitation of April in upstream

Monthly precipitation（mm）of Monobe river

600

2001
average( 52years)

500

2005

cumulative water budget

2000

Figure 10

1995-200001990-15000monthly precipitation（mm）
700
precipitation of April
600
500
400
300
200
100 y = -3.8605x + 376.82
R 2 = 0.2434
01985-10000

1980

5/1

1974

4/1

1969

3/1

1964

2/1

1959

1/1

1954

-5000

2001 could occur at higher than the estimated

4.2 Power generation

400

Figure12 shows power generation associated with300

proposed sets of parameters under the new irrigation

200

right and the old irrigation right. The power

100

generation associated with the current operation rule0

is also included.
DecNovOctSepAug

Jul

JuneMay

AprMarFebJan

Figure 11 monthly precipitations in 2001

Power generation under the new irrigation right issimilar equal to power generation production underold irrigation right.

drought days decreased by 83%, and its standard

We evaluated power generation associated withdeviation decreased by 26%. Except for 2001, waterparameters except those creating the situation theenvironment may be improved.

dam water level exceeds the above-mentioned 191

average of power generation
production（100million kwh）
200

meters of the target reservoir level in the same wayof analysis as the old irrigation right.

150
old irrigation rightactual operation
new irrigation right

Average of power
generation(100 milion kwh)
168
166
164
162
160
158
156
154
152
2426
28
Standard deviation of power
generation(100 milion kwh)

100

actual operation
documented rule

50
0.00

50.00 100.00 150.00 200.00
average of days of drought
Figure 15 Relation between power generationand stream regime in new irrigation right
5. Conclusion

30

In this study, the authors quantified tradeoff
relationship

Figure 13between

water

environment

andhydraulic power generation. The results shows aPower generation production

possibility of reducing occurrence of drought water
4.3 Days of drought

flow without sacrificing much of hydraulic power

Figure 14 shows drought days under the newgeneration. Under the new irrigation right, however,irrigation right. Except for 2001, entire drought days

risk of making the dam water budget deficit exists.

decreased dramatically.

Future assignments of our study are to evaluate

Average of drought days
160.00
documented rule
140.00actual operation
120.00100.00
80.00
60.00
40.00
20.00
0.00
0204060
Standard deviation of drought days

changes of social benefit by loss of hydraulic power

Figure 14 Drought days in new irrigation right

4.4

Tradeoff

generation

Relationshipandwater

betweengeneration, evaluate risk of drought under the newirrigation right in more detail, and evaluate risk offlood control by changes of dam operation.
REFERENCES
AkihiroNagai,Naoya

Tanakamaru,

2003“Hydrology for dam operation”
Kochi bureau of enterprise, 2007, “Outline of Kochi
bureau of enterprise”

powerenvironment

indownstream

Hirotaka Nakayama, Tetuzou Tanino, 1994, “theoryand application of multi objective programming”

Under the new irrigation right, average of power
generation production decreased by 2.7%, and its

Youshi Fujimori, Masao niiduma, 1995, “EXCEL

standard deviation decreased by 3.7%. Average ofVBA standard programming”