DAM DEVELOPMENT IN VIETNAM - ACHIEVEMENTS AND CHALLENGES

DAM DEVELOPMENT IN VIETNAM - ACHIEVEMENTS AND CHALLENGES

DAM DEVELOPMENT IN VIETNAM - ACHIEVEMENTS AND CHALLENGES

ABSTRACT

Vietnam is a South - East Asian country, which is affected by the subtropical humid moonsoon climate Its annual rainfall is high and most of its territory (especially in the North and the Central parts) is mountainous and has good potential for water reserves and hydropower Moreover, rainfall, almost the only source of surface flow, concentrates in a few months yearly during the rainy season Therefore, there is an urgent need to be regulated by reservoirs The Dam Development Strategy has a key role in Vietnam economic-social programs: power generation; flood disaster mitigation; irrigation, water supply for life and industry; transport; and environment promotion A number of large dams (higher than 50m) are under construction now or in the near future Dam development in Vietnam faces chalenges relating to the extreme floods and drought due

to the climate change, the upper river basins outside the territory, the environment

impacts, the land inundation and resettlement

1 Natural Characteristics and Major River Basins

Location:Vietnam is situated in South - East Asia with its main land lying between north

population of 83 mil The country’s mountainous topography (three-quarters are mountains and hills) and subtropical humid monsoon climate profoundly affect the quantity and distribution, both temporally and spatially, of water

Water Resources Redistribution: Mean annual rainfall, which almost is the only source of surface flow, is about 2,000 mm; but about 75% acumulates in only three months (more than 30% usually in only one peak month) This occurs from July to September in the Northern and Southern areas and from September to December in the Central area

1

Professor, Former Vice-Minister, Chairman of Vietnam National Committee on Large Dams; 95/12G Chua Boc Str., khu DH Thuy Loi, Dong Da, Hanoi, Vietnam.

2

MSc., Water Resources University; Tay Son Str., Dong Da, Hanoi, Vietnam

3

MSc., Vietnam Water Resources Academyr., Tay Son Str., Dong Da, Hanoi, Vietnam

Figure 1 Location of dams mentioned in the text

Cua Dat CFR Dam

Hoa Binh CCR Dam, H128m

V9.5bil.m3 , P1920MW

Ialy CCRDam,H69m

V928mil.m3,P720MW

Easoup Earth Dam H27m,V149mil.m3

Dau Tieng Earth

Dam,H28m,

V1.6bil.m3

Phuoc Hoa Weir, H28m

Dinh Binh RCC

Dam , H53m

V226mil.m3

Ban Ve RCC dam H1379m, V1.8bil.m3

P400MW

Son La RCC Dam H138m, V12.5bil.m3

P2400MW

Tri An CCR Dam H40m , V2.7bil.m3

P400MW

Ca Giay

Earth Dam

H26m V36mil.m3

Phu Ninh

Earth Dam

H40m V344mil.m3

Trang Vinh Earth Dam H28m , V75mil.m3

The mean annual runoff totals 880 billion m3 (ranking 12th in the world), of which 70% is generated outside the border lines and flows downstream into Vietnam The mountainous landscape offers substantial potential for hydropower and water storage In wet seasons this promotes rapid flood concentrations, makes heavy inundation in alluvial plains and deltas, where most big cities are located In dry seasons, there are water shortages (the minimum monthly flow of most basins is just 1% of annual runoff) and drought, which threaten the water supply and living condition of millions people with

impacts on environment, agriculture, aquaculture, etc… General task in the water sector

is flow redistribution to water storage in wet season for flood mitigation then flow promotion in next dry season by use of both construction (dyke, reservoir,

weir,sluice,canal,…) and non-construction (afforestation, inhabitant relocation , )

measures

Major River Basins: As for characteristics of topography and river basins, Vietnam is considered to be composed of three regions: Northern, Central and Southern ones (fig.1)

rest is outside the border) and taking more than 15% of total runoff of the country The

the task of power generation for the whole country, big reservoirs therein must have an important part in flood mitigation in the delta together with an embankment river dyke system, which have been built for many centuries and also directly protects the delta from floods

Central Vietnam stretches from the North ( parallel 20o) to the South ( parallel 11o) and is

composed of two subregions: the East Coastland and the South –West Highlands The

coastal subregion, a very narrow land strip between mountains and sea, is composed of a

considerable hydropower potential due to their high stream slopes as sources of the rivers are in the mountainous areas close to the coastline This subregion faces yearly 7÷10 typhoons and tropical storms coming from East Pacific Ocean with high rainfall and flood peaks Sources of Mekong tributaries are in the highlands They flow towards the main stream outside the Western border Floods and droughts usually come suddenly right after each other This situation can be mitigated by reservoirs only in connection with power generation

The South Vietnam can be divided into two subregions The East subregion is the Dong

Nai River Basin (44,100 km2) with a big potential of power energy and water supply Rapid downstream industrial and urban development (including Ho Chi Minh City)

urgently raises the issue of water requirements To the West is the downstream Mekong

Delta of 39,000 km2 in Vietnam territory only (5% of the whole Mekong basin) It takes a

low and plane delta yearly meets with serious challenges of big floods in the wet season and drought with salinity intrusion in the dry one No reservoir but many big water systems (long canals, sluices, field embankment dykes,…) for flood control, inundation mitigation, irrigation, fishery, aquaculture and water way are needed

Overview of Dam and Reservoir Development

Vietnam has a historical tradition in making water constructions Old earth dyke systems

in the Mekong Delta 300 years ago Some large irrigation systems composed by weirs, canals and sluices were built in the 1920s and 1930s More than 2500 pools (each of them

the downstream Da River in North-West Vietnam (70 km to Hanoi) there has come into

height H=128m to generate power of capacity P=1920 MW Now a more enormous

generate power P=2400 MW

Reservoirs in Vietnam are of two categories, in which the first ones especially for energy and the other multipurpose reservoirs for combination of flood mitigation, water supply,

irrigation and power generation There are presently 3 National Programs with more than

10 bil USD investment being implemented in 10 years (2005-2015):

• Program for Medium and Small Water Systems in dispersed areas in highlands and mountains aiming at satisfying local water demand and reducing poverty;

• Program for Large Multipurpose Water Systems with big reservoirs (V=0.3÷1.5

• Program for Large Hydro Power Plants with high power capacity (P=150÷2400

MW)

TECHNICAL ASPECTS OF DAMS

In Vietnam the largest earthfill dam is of H=60m; clay core-wall rockfill (CCR) dam

H=128m; concrete face rockfill (CFR) dam H=119m; traditional gravity concrete dam H=46 m; roller compacted concrete (RCC) dam H=53÷139m (fig.2) Beside common technical demands, the dam construction therein needs much special technical research works such as: flood frequency design; optimum combination of local material dam and

57%

14%

3%

19%

7%

earth dams Rockf ill w ith core Rockf ill+RC f ace RCC dams Concrete

For many years the frequency of extremum flood design has been taken as 0.1% based on

the Statistics Modelling (SM) for large dams (more than H=40m) according to the

Vietnamese State Standard However numerous factors have affected flooding: global and regional changes of climate; decline of forests; human socio-economic activities;… Therefore some recent big floods have seriously threatened dam safety The flood design

spillway sizes; river diversion versus peak flood; dam material treatmen;…due to

natural conditions

Flood Frequency Design

2

4

6

8

time (10 hours)

flood discharge (1000 cub.meters/s)

― design

― PMF

Figure 3 Flood process analysis of Phu Ninh Reservoir

value has been urgently reviewed to promote the safety of existing dams and amended the Standard for forthcoming dams The Probable Maximum Precipitation (PMP) procedure

has been discussed and applied for the Probable Maximum Flood (PMF) value

For example, flood design value of Phu Ninh Dam has been reviewed This earthfill dam

of H=40 m was built in 1982 on the Tam Ky River in Quang Nam Province with its

Since then data have been added to the annual flood and rainfall series The statistical analysis of the added series shows considerable difference compared with the design values of 30 years ago Flood putting the dam in much danger in 1999 is only of 1.8% frequency The PMF analysis has been carried out and numerical results are showed as follows (fig.3):

Frequency

(%)

Flood Discharge

Total Runoff in

72 hours (106 m3)

It is seen that SM could only give more exact results when the series is sufficiently long

In the case of this dam, the SM design flood discharge and total runoff (in 72 hours) values are 17% and 25% forcer after updating the series The PMF value shows itself to

be much safe for design It is proposed that the SM be applied for design and the PMF for

utmost safety Much more research and investigation, including flood morphology

analysis, will have to continue for a satisfactory conclusion

Optimum Combination of Local Material Dam and Spillway Sizes

Local material (LM: earthfill, rockfill with clay core,…) dams, which would be damaged

by overflow, are in flood safety in combination with spillway capacity Formely such a

dam was constructed almost by primitive tools so its size, especially its height, usually had to be designed downwards but spillway upwards This fact decreased the reservoir stockage volume because a lot of water would be wasted downstream in rainy season The investment effectiveness has been limited, particularly in situations where water becomes more and more needed Nowadays, advanced technologies in construction equipment, material, foundation and seepage treatment,…facilitate large dams safely, The extension of LM dam size is often not more expensive than spillway including weir,

gate, waterchute, stilling basin, inlet and outlet channel, etc…The optimum combination

of LM dam and spillway sizes, which is selected among variants, make the whole reservoir headwork safer, more reasonable and effective For example, Song Sat Dam,

Province (Central Highlands) Several variants of dam and spillway sizes were proposed

No1 No2

Height of dam H (m)

Flood discharge over spillway for

Number of gates of (5m × 5m)

Earth dam cost (million US$)

Spillway cost (million US$)

31

51

601

764

3 1,434 1,404

34

70

428

535

2 1,809 1,093

River Diversion versus Peak Flood

As mentioned above, river diversion is very complicated and difficult during dam

construction in Vietnam, because flow discharge in rainy season is about 50÷80 times

higher than in dry season No large dam can be finished in only one dry season

Therefore, river diversion must be considered strictly for two types of flow discharge:

“normal” in 9÷10 months yearly and terrible “peak” occurring suddenly sometimes in 2

heavily rainy months The “normal” discharge can usually flow downstream by intake, tunnel, new channel,…to be put at suitable levels allowing dam construction yearly without any particular concern Because of its huge value, the “peak” discharge can be solved by natural open channel at first and by spillway at last stage of the construction During the first stage, parts of dam on shoal banks are built while the natural stream channel is still widely open and reserved for peak flooding

The second stage begins with closure when finishing cofferdam for river diversion

As for a non-overflowable dam (earthfill or CCR dam), most of the dam volume is

implemented at the first stage At the second stage, which is strictly permitted in one dry season only, the remaining dam parts must be rapidly built so that the top dam is always higher than the level of water stored in the reservoir and to be sure that the next peak

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X

X

X

X

X X

X X

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X X X

flood can flow downstream over spillway The dam consists of three portions: two on shoal sides (right and left) and one, which is the highest in the middle on the bottom of the natural channel The transition zones between those dam portions built in different times must be carefully done as they stretch all over the dam cross section at the most vulnerable places For example, it was very difficult to overcome an extreme danger

when considerable cracks occured in such transition zones of Ca Giay Earth Dam

closure by immediately excavating an emergency spillway to lower the water level in the reservoir, hoeing and retamping the transition zones, expanding the cross section of the vulnerable middle portion of the dam with added soil mass at downstream side

Figure 4 Ca Giay Earth Dam portion in danger after closure

Figure 5 CFRD Cua Dat under construction with peak flood

As for concrete or CFR dams, peak flood can flow over a dam under construction One needs to pay attention to water energy dissipation for dam safety For example, Cua Dat

Dam (fig.5) on the Chu River in Thanh Hoa Province, the largest CFR one in Vietnam, is

in a natural stream channel The cofferdam was finished at the beginning of the third dry season and a low level diversion tunnel started to work In the third rainy season, water overflowed the dam and its energy was dissipated by gabion stepped drops In the fourth rainy season, the spillway would start to lead peak flooding downstream

Dam Material Treatment

It is well known that dam structure must absolutely guarantee slope stability and protection from erosion by seepage flow inside the dam and its foundation Aiming at this object, essential dam materials (soil, concrete,…) need a special process in connection with other measures under complicated climate conditions such as in Vietnam

Formely, earthfill dams were not large (H<30m) so they used to be built as homogeneous

ones, especially in the North Vietnam because of wide availability of suitable soil material from borrow pits and regular humidity almost the year round for rolled-fill process In Central Vietnam, where many large earthfill dams have being built, weak soils (pervious, cohesionless, swelled,…), extremely changeable humidity, frightful flooding,…must be considered seriously Many catastrophes happened in past decades Some dams were damaged and others were threatened with danger, especially in Southern provinces of Central Vietnam It is more and more difficult to find sufficiently impervious soil A reasonable solution includes multizoned earth dams made with locally available materials, sufficient size for slope stability and special measures (impervious diaphragm wall, grouting,…) to cut off seepage flow A thin reinforced

Ninh Province (North Vietnam) Cement-bentonite (CB) diaphragm walls have been

Dak Lak province (Central Highlands) has been added a CB diaphragm walls inside with

recommended that the impervious diaphragm walls should be considered very carefully

to apply to dams right after construction, because some cracks are induced by different displacements between diaphragm wall and soil zones by sides at that moment It would better to use advanced grouting technologies for dams just completed Those diaphragm walls need to be implemented effectively to cut off seepage flow in foundation or in old dams being depressed

Construction of conventional concrete or RCC dams, even CFR dams, always faces

thermostresses and cracks, especially in the Southern subregion of Central Vietnam,

noon almost the year round Thermostresses are analysed by advanced software based on finite element and boundary element methods The concrete processing is carried out at night and in the early morning Cement content is reduced, while fine ash and puzzolan is added Water is blended with ice It is always built a thin impervious reinforced concrete

diaphragm wall on the upstream side even more covered with an impermeable paint to protect main part of concrete dam out of seepage through microcracks Such combined

measures have been applied successfully at the Dinh Binh RCC Dam (H=53m, V=226

cement per RCC cubic meter Many large RCC dams, including the Son La Dam (North West of Vietnam) of 139m height as mentioned above and Ban Ve Dam in Nghe An

Province (Northern subregion of Central Vietnam) of H=137m to make a reservoir of

V=1.8 bil m3, are under construction now

ENVIRONMENTAL ASPECTS OF DAMS

This topic has brought about much discussion Dams and reservoirs are always

accused by those committed keep environment unchanged However, dams and reservoirs

have greatly contributed to socio-economic development While making some changes,

they also create a new, even better, ecological balance together with anticipatedly valuable benefits in water resources regulation for increasing water supply, irrigation,

fishery, power generation, flood mitigation, climate and environment

promotion,…Generally, mankind doesn’t live as before as in the prehistorical ages with

wild nature and environment must be better for nothing else but human life Nevertheless

it couldn’t be slight the lessons of serious negative impacts on the environment from unsuccessful dams

Downstream erosion

Flow in dam downstream including discharge value, sediment grade, changes

complicatedly It needs much long term investigation Erosion occurs confusedly due to

scour of clear water released by the dam, including expected vertical and horizontal erosion and new stream water surface levels Moreover, the flow, which is down the dam for hydropower purpose only, much depends on an effective operation for energy demand Hydroturbines won’t be operated at night but fully at peaks period during the day Water is stored even sometimes in dry season resulting in no flow downstream The

downstream flow of the big Hoa Binh Hydropower Plant (as mentioned above) has often

made heavy erosion at many places, where no threat happend on river bank safety up to

that time Operation of the Yaly Hydropower Plant (CCR dam of H= 69m, V=928 mil

beginning Regulated downstream flows of dams in Central coastal subregion have impacted on sedimentation and movement of river mouths, where there are sandy islands, large salty ponds, typhoonal and monsoon regime

Ecosystem

Ecosystem, which changes in basin from the river source to the estuarial zone during and after dam construction, includes some improvement in one side and some deterioration

on the other Owing to reservoir, a milder climate, moderate temperature, higher evaporation, promoted ground water level much favour cultivation and tree planting Regular flows in the dry season much improve downstream environment to protect large areas from drought, soil degradation, desertization and salinity penetration Fishing can

be favored in reservoirs Some studies on sedimentation have showed that no

considerable consequences are in big and medium reservoirs (V>20 mil m3) when forests are protected, such as in the Tri An Hydropower Reservoir (near to Ho Chi Minh City,

roughly affects the ecosystem of aquatic wildlife No fish waterway has been arranged

with dam construction up to now but only one planned in Phuoc Hoa Weir (H=28m) on

the Be River in Binh Duong Province (South Vietnam) In general, environment impacts are not researched and accessed enough

a Observation after 10 years b Computational modeling

Figure 6 Sedimentation analysis of Tri An Hydropower Reservoir

Human Use Values

“Water for all!” Almost all source of rural and urban water supply is now from reservoirs After a long time of exploitation, the ground water in Hanoi (3 mil people) and Ho Chi Minh city (7 mil people) is expected to be exhausted Luckily some big

the Be River) are not far from those cities and the water released right just below dams doesn’t need much purification

Dams have a key role to exploit water power with total potential capacity of more than 15

GW in Vietnam Dams and all irrigation systems have a precious contribution to the agricultural development The country, which was in critical food shortage not long ago, has become a big exporter of rice, coffee, sea products, meat, cashewnut, pepper, rubber, fruit, etc…The poverty reduction in rural and mountainous areas has reached much

progress.Reservoirs are considered as a chief measure to protect human life from flood

and thirst However, the social disadvantage of dams includes land compensation and resettlement Dam safety, which has to be paid much attention, is not researched and

sufficiently invested yet

REFERENCES

The contents of this paper are from official documents

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