Annex_VIII_CaseStudy0202_Tsuga_Japan.pdf

IEA Hydropower Implementing Agreement Annex VIII

Hydropower Good Practices: Environmental Mitigation Measures and Benefits

Case study 02-02: Hydrological Regimes - Tsuga Dam, Japan

Key Issues:

2- Hydrological Regimes

1-Biological Diversity

Climate Zone:

Cf : Temperate Humid Climate

Subjects:

Discharging of Ecological Flow from Existing Dam

The Tsuga Dam

Effects:

- A remarkable increase in species of benthos in reduced flow river caused by diversion owing

to the discharging of ecological flow.

Project Name:

Tsuga Dam

Country:

Kochi Prefecture, Japan (Asia) (N 33

15’

、

E 132

58’)

Implementing Party & Period

- Project:

Sikoku Electric Power Co., Inc.

1944 (Completion of construction) -

- Good Practice:

Sikoku Electric Power Co., Inc.

1989 (Commencement of operation) -

Key Words:

Ecological Flow, Reduced Flow River Caused by Diversion, Benthos

Abstract:

To check the effect of discharging the ecological flow in the Tsuga Power Plant, habitat

conditions of benthos in the reduced flow river caused by diversion were investigated ten years

after the start of discharging (1998/99) and results were compared with survey data collected

before the start of discharging (1988). From the comparison, a remarkable increase of species

of benthos after the start of discharging was confirmed. Comparison between the affected area

and other areas of the main course and tributary streams also confirmed that there were no

distinct differences in the number of species, the number of individuals, diversity, water quality

and the like. The findings suggest that the present river environment in the reduced flow river

caused by diversion is being improved or recovered by the effect of the ecological flow.

1. Outline of the Project

The Tsuga Power Plant (No. 1 and No. 2 power generators: 18,000 kW) is a dam and conduit type power

plant opened in 1944. Water is taken from the Tsuga Dam, a concrete gravity dam built in the Yusuhara

river, a feeder to the Shimanto river, through an about 5.7 km- long headrace tunnel. The dam gives an

effective head of 96m.

While the Tsuga Power Plant had had a reduced flow river caused by diversion of about 22 km in total

length from the dam to the outlet of the plant, in

1989 when the water right was renewed,

discharging an ecological flow was decided with

the aim to improve or restore river environments in

the area. In April 1998, modification work to

change the outlet for the discharging from the dam

spillway gate to the headrace tunnel directly below

the dam was completed and the No. 3 power

generator was put into operation for better use of

the ecological flow (1.91 m 3 /s, effective head: 37.1

m).(See Table-1 and Figs.-1 and 2.)

Shikoku

Tsuga Dam

Fig.-1 Map of the basin of Shimanto River

Table-1 Particulars of Tsuga Dam

Item

Particulars

River System

The Yusuhara river, a feeder to the Shimanto river of the Watari River System

381.0 km 2

Catchment Area

Tsuga Power Plant power

generators Nos. 1 & 2

Tsuga Power Plant power

generator No. 3

Name

Maximum output

18,100 kW

550 kW

Power Plant

22.00 m 3 /s

1.91 m 3 /s

Maximum water consumption

Effective head

96.00 m

37.10 m

Type

Concrete gravity type

Dam

Crest height

45.5 m

Crest Length

145.0 m

19.3 × 10 6 m 3

Total storage capacity

14.0 × 10 6 m 3

Reservoir

Effective storage capacity

Effective depth

15.0 m

2. Features of the Project Area

The Shimanto River, in which the Tsuga Power Plant is built, has its source in Mount Funyu in the north

of Higashituga Village on the boundary between Kochi and Ehime Prefectures. The river grows while

winding its way by collecting water from a number of tributaries including the Yusuhara River and drains

into the Bay of Tosa in Nakamura City. As it meanders much, the river flows in a variety of ways and

benefits the fishery and forestry in communities along the course. The Shimanto River is also known

widely as “the last limpid stream in Japan.”

Meanwhile, national consensus arose sometime around 1985 for the improvement of river environments.

In 1988 the Ministry of International Trade and Industry (the present Ministry of Economy, Trade and

Industry) and the Ministry of Construction (the present Ministry of National Land, Infrastructure and

Transport) set the so-called “Guideline for Ecological flow with respect to River Right,“ which makes it

an obligation for power plants that fall under certain conditions to discharge appropriate ecological flows.

Under such circumstances, on the occasion of the renewal of this plant’s water right, a movement

requesting dam removal developed to become a newspaper topic. To deal with such a trend, Kochi

Prefecture formed “the Tsuga Dam Review Panel” made up of persons of learning and experience and

representatives of the national agencies concerned, the city, town and village interests as well as business

entities, the prefecture itself serving as the secretariat. The council discussed technical matters

concerning the dam, such as the discharging of ecological flow and the quality of water in the dam

reservoir and an agreement was reached smoothly to take the steps for renewing the water right on the

basis of the ecological flow as deliberated by the panel.

3. Major Impacts

For environmental protection in the reduced flow river caused by diversion, it was necessary to

determine an ecological flow by giving comprehensive consideration to fishery, landscape, maintenance

of underground water level, conservation of

fauna and flora, keeping rivers clean and the

like. As the level of underground water and

cleanliness of rivers would be improved when

an ecological flow was set, however,

discussion went on mainly from the

viewpoints of improvement of landscape and

reservation of animals and plants.

For the improvement of landscape, aiming at

the volume of water that would be visually

satisfactory, a rate of flow that could form a

continuous watercourse while maintaining a

certain width/ depth, etc. was examined. For

the reservation of fauna and flora, ayu fish was

chosen as a typical aquatic because of its

abundance and a water depth allowing it to

live in and go upstream was examined.

Fig.-2 The basin of Shimanto River

4. Mitigation Measures

“The Tsuga Dam Review Panel (December 1988 - February 1989)” confirmed in its deliberation that

impact on landscape and aquatics should be considered mainly in determining an ecological flow and

concluded as follows by referring to opinions of persons of learning and experience and the result of trial

discharging.

a) When 0.3 m 3 /s or so per 100 km 2 catchment area is discharged immediately below the dam, the

stream regime up to the outlet would be improved and preferable effect on landscape and aquatics

would be produced.

b) In summer, sufficient desirable effect on the area from right below the dam to the outlet should be

expected in view of the characteristics of “the limpid Shimanto.” Therefore, it would be appropriate

to raise the upper limit value of the guideline by a certain surplus.

Consequently, the ecological flow was set at 1.15 m 3 /s (0.3 m 3 /s/100 km 2 ) and between April 1 and

September 30 a surplus discharge of 0.76 m 3 /s (0.2 m 3 /s/100 km 2 ) would be added to make the discharge

1.91 m 3 /s from April, 1989.

5. Results of the Mitigation Measures

Amid the society’s increasing recognition of the value of river environment, R&D on techniques for the

analysis/assessment of living environment for fluvial ecosystems is going on actively. In the following,

how the reduced flow river caused by diversion in the Tsuga Dam changed in ten years from the start of

discharging the ecological flow is discussed, mainly with respect to benthos by taking an ecosystem

pyramid, the precision of the investigations and the like into account.

5.1 Investigation Items

The benthos chosen as subjects to be investigated were general flatworms (Turbellaria), eelworms

(Nematoda), mollusks (shellfish), annelids (Oligochaeta, Polychaeta and Hirudinea) and arthropods

(Arachnida, Crustcea and Insecta). (Fig.-3)

Fig.-3 Conceptual diagram of River Ecosystem Pyramid

Fig.-4 Locations of Benthos Investigation Points

5.2 Investigation Sites

In the reduced flow river caused by diversion, two sites were chosen, namely, the “Kominono” site,

about 1km downstream of the Tsuga Dam, and the “Onaro” site, which is almost in the middle between

the Tsuga Dam and the junction with the Shimanto and about 6km downstream of the dam. The latter

was the place where similar investigations had been carried out in the past. As controls for comparing

with the reduced flow river caused by diversion, six points, of which five were in the main course of the

Shimanto and one in the Nakatsu, a tributary of the Yusuhara, were investigated

5.3 Method of Investigation

Study was carried out by comparing the numbers of species and individuals in the reduced flow river

caused by diversion between before and after the discharging of the ecological flow and the reduced flow

river caused by diversion was also compared with other areas of the main course and the tributaries. The

investigation data before the discharging were taken from “The Shimanto River, its Nature and Life”

listed in the bibliography.

1) Method of Investigation

For the collection of benthos, a combination of the collection for quantitative determination (the

number of species, the number of individuals and the input) in rapids by means of a server net with

coderat and the collection for qualitative determination (the number of species) in shallows and slow

currents by means of a landing net (the D type frame net) was employed. On each point, collection

was carried out on two points. The coderat was 50cm

50cm in size and the server net and the

landing net were 60.5mm in mesh size.

2) Frequency of Investigations

The field investigations after the discharging of the ecological flow were performed twice, in August

1998 and January 1999.

5.4 Result of Investigations

5.4.1 Comparison between before and after discharging the ecological flow

Fig.-5 and Table-2 show a

comparison of compositions of

benthos species in the reduced flow

river caused by diversion between

before

and

after

discharging

the

ecological flow was started.

The number of benthos species

identified in the Kominono site about

1km downstream from the Tsuga

Dam was 88, an increase by 68 over

the time of investigation (1989)

before the start of discharging the

ecological flow. Even when a change

in the method of classification is

taken into account, the increase is

remarkable enough. In the Kominono

site, the species of aquatic Insectra

Fig.-5 Comparison of Composition of Benthos Species

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