Annex_VIII_CaseStudy0401_Dashidaira_Japan.pdf

IE A Hydr opow er Imp leme nti ng A gr eement Annex V III

Hydr opow er Good P rac tices: E nvi r onmenta l M iti ga ti on M eas ur es a nd B enef its

Cas e study 0 4- 01 : Res er voir Sedimenta tion - Das hidaira Da m, J apa n

Key Issue:

4- Reservoir Sedimentation

Climate Zone:

Cf: Temperate Humid Climate

Subjects:

- Sediment Flushing of Reservoir by Large-scale

Flashing Facilities

- Method of Forming Reasonable Consensus of

Local Community Concerning Sediment Flushing

(Photo by the KANSAI Electric Power Co., Inc.)

Effects:

- Long-term Removal and Reduction of Sediment in Reservoir

- Forming of Consensus Concerning Way of Sustainable Sediment Flushing

Project Name:

Dashidaira Dam

Country:

Toyama Prefecture, Japan (Asia) (N 36° 50’, E 137° 40’)

Implementing Party & Period

- Project: the KANSAI Electric Power Co., Inc.

1985 (Completion of construction) -

- Good Practice: the KANSAI Electric Power Co., Inc.

1995 (Commencement of improved operation) -

Key Words:

Sediment Flushing, Consensus of Local Community, Environmental Impact

Abstract:

The watershed of the Kurobe River Basin yields a lot of sediment and the Dashidaira Dam

constructed in 1985 is equipped with sediment flushing gates. Through the precise prediction

of environmental impact and the meeting with stakeholders including local residents and

technical experts, appropriate mitigations for environmental impact has been established.

1. Outline of the Project

The Dashidaira Dam was constructed by the Kansai Electric Power Co. (KEPCO) on the midstream

stretch of the Kurobe River (approximately 26 km from the river mouth) for the regulating reservoir of

the Otozawa Power Station, and is the first dam in Japan equipped with large-scale sediment flushing

facilities.

Streams of the Kurobe River Basin have extremely heavy sediment loads and a major concern when

planning construction of the Dashidaira Dam was how to solve the sedimentation problem as there

were strong demands from the local community for prevention of coastal erosion. In general,

measures against sedimentation at a dam consist of installing sediment storage weirs, dredging, etc.

At the Dashidaira Dam, however, 1) a very large volume of sediment is brought down from the

upstream catchment area, 2) even if dredging were to be done, transportation of dredged sediment

would be difficult because of the constraints imposed by the site consisting of a gorge, and 3)

conventional methods such as dredging would be unable to solve problems of degradation in the

downstream area and erosion of the coastline. KEPCO, noting the importance of mitigating riverbed

degradation and coastal erosion, decided to adopt a flushing method whereby sediment would be

discharged downstream to the same extent as before construction of the dam.

Specifications in outline of the dam and the sediment flushing facilities are given in Table-1 while an

outline view of the Dashidaira Dam is shown in Fig.-1. The Dashidaira Dam has two large-scale

sediment flushing tunnels in its body. Their structures are that when it is desired to release sediment

downstream the water level at the dam is lowered for free flow of water inside the reservoir so that

accumulated sediment will be discharged.

The construction of the Otozawa Power Station was begun in 1982 and operation was started in 1985.

Table-1 Specifications of Dashidaira Dam

Item

Specification

River system

Kurobe River, Kurobe River System

461.18 km 2

Name

Catchment area

Otozawa Power Station

Max. output

124 MW

Power station

74.0 m 3 /s

Max. discharge

Effective head

193.5 m

Type

Concrete gravity

Height

76.7 m

Dam

Crest length

136.0 m

203,000 m 3

Volume

* 9.01x10 6 m 3

Gross storage capacity

* 1.66x10 6 m 3

Reservoir

Effective storage capacity

Available depth

18 m

2 lines

(steel lined)

Quantity

Flushing channel

Dimensions

5.0×5.0 m

Upstream side

Slide gate

Sediment flushing gate

Intermediate

Roller gate

Downstream side

Radial gate

*:when constructed

Dam Axis

①

Overflow section

②

Non-overflow section

③

Drawdown range

④

Flushing channel

⑤

Fig.-1 Outline view of Dashidaira Dam (source: Ref. 1)

2. Features of the Project Area

The Kurobe River springs from the Northern Alps

Mountain Range in the Chubu Sangaku National

Park, runs down from mountainland of elevation

from 2,000 to 3,000 m cutting steeply graded, deep

gorges before dropping into the Sea of Japan.

The catchment area is 682.5 km 2 , and with a

length of 86.0 km, that is, the Kurobe is one of the

swiftest rivers even in Japan. The river basin has

an annual mean precipitation of approximately

4,000 mm to make it one of the most rainy and

snowy areas in the country, and there is an

abundant flow of water throughout the year.

The entire catchment of the Kurobe River consists

of new and old granites which are low in water

retention capacity, and the runoff ratio is extremely

high. There are approximately 7,000 collapse areas totaling 31 km 2 out of the 667 km 2 of

mountainland in its catchment area. It means that this area is characterized by heavy sediment load

extremely.

The mountainland part of the Kurobe Basin is designated a special area of the Chubu Sangaku

National Park and a pure natural state is maintained. The Kurobe River, with its abundant water is

looked upon as a rich source of electric power and a stable fountain of domestic and agricultural water

supply. It also contributes greatly to the local economy as a tourism resource.

Toyama Pref.

Dashidaira Dam

Fig.-2 Dashidaira Dam location

3. Major Impacts

As of June 2000, sediment flushing had been carried out a total of eight times at the Dashidaira Dam.

Annual sediment discharges and the cumulative discharge are shown in Fig.-3.

In December of 1991, six years after completion of the dam in 1985, the sediment accumulated had

reached approximately 3 million m 3 , making it possible for discharge to be done from the gates, and

the first flushing was carried out. The result, contrary to expectations, was that turbid water of a dark

gray color and with a putrid odor was discharged, and moreover, this turbid water spread out into the

sea area and discharge was discontinued at the request of the local community.

A committee including local representatives and knowledgeable persons was organized in order to

study the impacts and suitable methods of the sediment discharge. The committee carried out

investigations of the impacts on fisheries and agriculture, and of the impacts on the environment, and

also conducted sediment-flushing tests (February 1993). It was learned that organic matter had become

degenerated by long-term deposition of sediment in the dam reservoir, and this had affected the

downstream environment when the sediment was discharged. Consequently, comparisons were made

with alternatives such as 1) leaving sediment untouched instead of removing it (for example, removing

the dam and returning the river to its original state), 2) discharging sediment without using sediment

flushing gates (for example, preventing sediment from entering Dashidaira Reservoir), and the final

conclusion was that to discharge sediment using sediment flushing gates would be the most suitable

measure for dealing with sedimentation at the dam. Table-2 gives the environmental impact items

investigated.

Table-2 Environmental impacts investigated

Site investigated

Items Investigated

Contents of investigation

Dam

River

Sea

Water temperature, pH, SS, Turbidity, BOD, COD, T-N, T-P,

etc.

Water quality

●

Bottom material

(sediment)

Appearance, Mud temperature, Smell, pH, COD, Ignition loss,

T-N, T-P, Grainsize distribution, etc.

●

Fish, Attached algae, Chlorophyll-a, Benthic organisms, Zoo/

Phytoplankton, etc.

Aquatic organism

●

Sedimentation condition

●

Cross sectioning

4. Mitigation Measures

The method of discharging sediment from the Dashidaira Dam is to temporarily lower the water level

of the reservoir and wash out deposited sediment by free flow of river water. The

previously-mentioned committee recommended that in order to minimize impacts on the downstream

environment, discharge should be done during floods when the volume of river flow is large so that the

discharge of sediment from the dam would be close to natural conditions, and sediment flushing was

done during floods according to the committee recommendation.

Approximately 3.4 million m3 of sediment were newly deposited at the Dashidaira Dam by severe

local rain in 1995 so that the stability of the dam was endangered, so that it was decided that

emergency flushing for disaster recovery should be carried out over three years (1995-1997). These

emergency discharges were also carried out during floods in accordance with recommendations of the

committee.

5. Results of the Mitigation Measures

As of June 2000, sediment flushing had been carried out a total of eight times at the Dashidaira Dam.

Annual sediment discharges and the cumulative discharge are shown in Fig.-3.

9000

T o ta l sto ra g e o f re se rvo i r:

a b o u t 9

10 3 m 3

EF : 1720

1 0 6 m 3

8000

10 3 m 3

EF : 80 0

Fl o o d o f K URO B E ri ve r (1 9 9 5 )

7000

10 3 m 3

F : 700

6000

5000

10 3 m 3

F : 8 0

4000

3000

10 3 m 3

EF : 460

2000

10 3 m 3

F : 20

10 3 m 3

F : 340

10 3 m 3

1000

F : 460

'85 '86

'87 '88

'89

'90 '91

'92

'93

'94 '95.6

'95.7

'95.10 '96 '97 '98

'99

* F : F lus hing, EF : Em ergenc y F lus hing

Fig.-3 Temporal change of reservoir sedimentation

The sediment flushing facilities of the Dashidaira Dam produced results as expected in the aspect of

flushing sediment out of the reservoir. As a result of adopting the procedure of releasing sediment to

coincide with flood discharge, it became possible to carry out sustainable sediment flushing without

causing any great problem in the downstream area. It was found on investigating sediment flushing

records and the environment when flushing that 1) the sediment flushing facility of the Dashidaira

Dam is effective as a means of discharging sediment from within the reservoir, 2) the sediment

flushing carried out to coincide with flooding is effective as a measure for mitigating the impact on the

downstream environment, and 3) if a proper sediment flushing method is adopted, the environment is

not greatly affected even when large-volume sediment flushing is hurriedly done. Flushing has since

been done annually at the same time as flooding in accordance with the advice of the committee.

Meanwhile, in order to ascertain the environmental impacts, KEPCO has carried out investigations on

the river and sea area downstream of the dam.

The results of the investigations may be summarized as follows:

1) In water quality investigations of the river, turbidity conditions of the downstream river are

showing improvement year by year, and prominent impacts due to sediment flushing are not seen.

2) In water quality investigations of the sea area, indices of turbidity and organic matter were

temporarily high in the vicinity of the river’s mouth during sediment flushing, but in investigations

one day after flushing, the conditions were seen to have returned more or less to normal.

3) In bottom material investigations, changes in conditions before and after sediment flushing were

not seen and impacts on bottom materials were not detected.

4) Regarding aquatic organisms (benthic animals), as a whole, there were reductions in populations

immediately after sediment flushing, but in investigations one month later, the situation had

returned more or less to the condition before sediment flushing and close to a natural flood

condition.

Table-3 Results of sediment flushing impact investigations (water quality-SS measurements)

(Unit: mg/L)

Immediatel

y below

dam

Shimokurobe

Bridge

(near estuary)

Point C

(sea area)

Point A

(sea area)

Before flushing

230

490

Max. observation

103,500

26,000

1,000

1995

emergency

flushing

During

flushing

Average

18,000

7,500

After

flushing

1 day after

193

Before flushing

764

1,520

1,500

Max. observation

56,800

6,770

1,200

During

flushing

1996

emergency

flushing

Average

10,000

2,900

1 day after

194

879

After

flushing

1 month after

Before flushing

Max. observation

93,200

4,330

3,500

During

flushing

1997

emergency

flushing

Average

10,000

2,200

1 day after

108

757

After

flushing 1 month after 35 22 6 6

Note: Maximum observations of turbidity and SS at Point C in 1997 emergency flushing being higher than for

previous two emergency flushings are because in 1996 emergency flushing, rough weather during peak of

turbidity of river made sea area investigations impossible.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: