By Castillejo Ron, C.V.G. Edelca
Tauseef I. Choudry, Hydro South America Montgomery Watson Harza (MWH)

The 2160 MW Caruachi power plant in Venezuela is one of the most talked about projects in South America. Industry experts reveal construction plans for the $2.1bn station, which will start operation next year.

C.V.G. Electrificación del Caroní, Edelca, is currently constructing the Caruachi power plant, the third facility of its kind on the Lower Caroní River in southeast Venezuela in the Bolivar State. Caruachi, with a projected 2160 MW contribution to the Venezuelan electric supply, will thus help meet the ever increasing needs within the sector, and will allow for some 76 000 barrels of oil to be added to the exportation quota.

Caruachi, together with the currently operating facilities of Guri (10 000 MW) and Macagua (2900 MW), as well as Tocoma (2160 MW, in development), is part of the Lower Caroní River Hydroelectric development programme, with an estimated potential of electric energy generation of 17 GW.

The first generator of the Caruachi hydroelectric power plant is scheduled to begin operation in 2003. The estimated cost of the development is $2.3bn, 54 per cent of which is provided by Edelca. The remaining capital was granted through loans by the Inter-American Development Bank (23 per cent), as well as other financial institutions (23 per cent) such as five other commercial banks, the Nordic Investment Bank and the Corporación Andina de Fomento (CAF).


The Caruachi project will cost an estimated $2.3bn and is targeting 2003 to start up
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The transmission systems presently in operation transport the electric energy generated in Guri, Macagua and Caruachi along the National Interconnected System in order to reach both local and international customers.

The Caruachi hydroelectric plant is located about 59 km downstream from the Guri reservoir and 25 km upstream from Macagua, in the municipality of Caroní, Bolívar State. The Caroní River exhibits a south/north 912 m drop along its 640 km length. It flows over bedrock of gneiss granite, which makes it suitable for large-scale construction. The lower Caroní River can develop the greatest hydraulic potential, as it drops 1.42 m per km. The river is 1700 m wide and flows to an elevation of 55 m above sea level in the area where Caruachi is being developed.

Planning

Montgomery Watson Harza (MWH) provided services for the project’s development. As part of the master plan for the Caroní River development, MWH participated with C.V.G. Edelca in identifying the hydroelectric site, developing the river diversion sequence, and preparing feasibility level studies. MWH participated with C.V.G. Edelca in providing several continuing technical support services for preliminary design of the structures, preparation of contract documents, pre-qualifying construction contractors, bid evaluation, preparation of detailed design and construction drawings, and assistance with Edelca’s environmental programmes for the project.

MWH’s services continued during construction. MWH participated with Edelca’s high level commission to identify the contractor’s construction plant deficiencies and recommend modifications, resolve construction claims, mitigate construction delays and scheduling conflicts, and recommend contractor’s organizational and operational changes to improve efficiency. MWH also provided field supervision and shop inspection for quality control/assurance of concrete works, fill dam construction, turbine-generator and other electrical mechanical equipment installations.

River diversion

The first river diversion stage was completed in February 1991. The Caroní River was diverted through a gap of 350 m from its natural path via embankment cofferdams at elevations of 62 m.s.l. This allows a maximum diversion flow of 13 000 m3/s, which is regulated by the Guri dam upstream of the construction site. The first diversion stage was used to construct the powerhouse consisting of 12 Kaplan units, the nine radial gated spillways with 18 low-level sluices, the powerhouse tailrace channel, and the right concrete-faced rockfill dam (CFRD).

On April 15, 2001, the second stage river diversion was initiated by completing the secondary cofferdam protecting the intake works, removing the cofferdams upstream and downstream of the spillway, and diverting the river through the 18 low-level sluices designed to discharge the maximum flow during construction. The 350 m river gap was subsequently closed. The final portion of the left dam is currently under construction.


The Caruachi powerhouse will comprise 12 Kaplan units
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All the steps involved in the river diversion process were tested in advance in a 1:80 comprehensive scale model located at the Edelca hydraulic lab in Macagua, which occupies an area of 1170 m2 corresponding to approximately 750 hectares of the prototype.

Main structures

The elements that represent the backbone of the project, as determined by their role in the process of electric energy generation, were established in consideration of fundamental geological, topographical, hydraulic and energy efficient conditions.

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The main dam (55 m high and 360 m long), which is an integrated powerhouse, including the intakes, consists of twelve 30 m long monoliths. The intakes will be provided with operation gates and maintenance bulkheads.

The powerhouse is comprised of 12 monoliths that will house 12 Kaplan turbines in a generation hall that is 25.65 m wide and is at an elevation of 64.50 m above sea level. The transformer platform and service bridge is located directly downstream at the same elevation and is 32.15 m wide. There are two erection bay monoliths, each 30 m wide on the right side, with the necessary maintenance and service rooms for the equipment. Another lateral service bay (50 m wide) is on the left side of the powerhouse. It has complementary service rooms and serves as a foundation for the control and operations building above.

The right transition concrete dam, located between the right CFRD dam and the erection bay, is occupied by three 30 m-wide concrete monoliths as measured along the foundation’s baseline. The intermediate concrete dam (51.15 m wide and 41 m long) is placed between the spillway and the powerhouse. The left transition concrete dam serves as an abutment for the left embankment dam and is located on the left side of the spillway. It consists of five monoliths, 176 m in total length and 50 m wide at the widest base.

The spillway (178.16 m wide) will be capable of discharging 30 000 m3/s of excess floodwater coming from the Guri Dam, according to the PMF analysis. The spillway crest is at an elevation of 70.55 m above sea level, and is controlled by nine radial gates measuring 15.24 m in width and 21.66 m in height.

The upstream concrete slab acts as an impermeable layer on the rockfill dam, which is transitioned by one filter zone. The slab is connected at the upstream toe of the dam to a plinth, which runs along its entire length. This dam type was selected to balance the materials from the powerhouse and spillway excavations and to eliminate stockpiling.

Dam and drainage

The left embankment dam (4300 m long, and up to 47 m high) will connect the spillway to the left bank. To minimize the volume, the design adapts to the natural topography, connecting existing mounds, which divide the dam into three main sectors, resulting in an almost arch-shaped embankment.

The design of the embankment sections is based on the availability of construction materials and the foundation conditions. In the section adjacent to the spillway, the foundation of the dam was excavated to bedrock and consists of a rockfill dam with an impervious central core of plastic, sandy clay. This section transitions to an earthfill segment with plastic, sandy clay material used upstream of the chimney filter and silty sand used in the downstream part. Rockfill cofferdams with impervious material on the waterside were initially used to close off the upstream and downstream sections of the dam exposed to flow, and were incorporated into the body of the embankment. The drainage throughout the embankment is provided with a sloping chimney filter in the middle, connected to a horizontal filter under the downstream shell. The foundation of the dam has leveling concrete wherever the excavation of the nucleus reaches bedrock and selective consolidation grouting was executed in that area.

The final predominant section of the embankment is located on thick alluvial deposits of superficial sand, which overlies intercalating bands of kaolonitic clays with coarse to fine sands. The superficial sands generally have low relative density and a porous structure, considered liquefiable. These sands were removed from the foundation by excavating a core trench to the top of first kaolin layer. In this area, the inclined filter was extended in the partial cutoff trench and the horizontal filters and drains were thickened. For additional drainage capacity, a 61 cm diameter perforated drainpipe was embedded at the downstream end of the horizontal filter in the area of the alluvial deposits. Additional drainage is provided by 46 cm diameter relief wells spaced at ten m intervals, with a typical depth of 20 m, to relieve any residual pore pressure near the downstream toe.

The CFRD right dam (900 m long, 50 m high) will connect the powerhouse to the right bank. The left earth embankment dam (4200 m long, 45 m high) will connect the spillway to the left bank.

When the structures are completed, a reservoir at an elevation of 91.25 m above sea level and a surface area of 25 500 hectares will be formed. In the last term of 2002, the 18 spillway bottom sluices will be closed and the reservoir will be raised to the final elevation. The first unit will be commissioned by the end of the first term of year 2003.

Environmental programmes

Ensuring appropriate consideration for the environment is part of every hydroelectric energy project. A team of natural resource experts on the Caruachi project team developed a comprehensive set of environmental programmes that collectively form the ‘Environmental Management Plan’.

This plan comprises 13 separate programmes that address the social, cultural, and ecological impacts of development of the Caruachi Project. Under each programme, strategies are developed and implemented in order to avoid, mitigate, or compensate for possible adverse effects. In addition, opportunities for beneficial effects are identified and enhanced.

The projects are:

  • Caroní River Basin integral management plan, which will guide the development of the resources of the entire basin in a rational and sustainable manner, while protecting sensitive areas.
  • Solutions for the human population affected by the project, which will provide property indemnification and relocation of those persons living in the area of influence of the project.
  • Wildlife management, which will comprise rescue of animals prior to reservoir filling, reinsertion into appropriate habitat, and monitoring.
  • Archaeological salvage, which will add to knowledge of the human settlement process of the lower Caroní River Basin, from the prehistoric era to our times.
  • Geographic information system, which will gather and organize information on hydrology, land utilization, animal and vegetation inventories, geomorphology, mining potential, climate, and other important natural resources in the basin.
  • Reservoir vegetation management, which will consist of a vegetation inventory and carefully designed clearing plan.
  • Forestry management on the reservoir shores, which will provide a detailed inventory of the shoreline forests that will be affected by the project, with the future goal of re-establishing native riparian species.
  • Restoration of disturbed areas, which will put in practice state-of-the-art techniques such as hydro-seeding in an effort to restore areas that have been disturbed by construction activities.
  • Limnology and fisheries, which will evaluate and monitor water quality and fisheries resources, and formulate plans for the management of these resources.
  • Artificial beach creation, which will offer a setting for recreational activities on the shoreline of the new reservoir to benefit the surrounding communities.
  • Public health management and sanitation, which will evaluate the epidemiological features of the communities near the project, and will provide them with primary health care.

  • Caroní Urban Park, which is a designed interconnected network of protected areas that will available for public use.
  • Participation and communication, which will make all the information on Caruachi available to the public, thus increasing consciousness of the need to protect and rationally manage both the natural environment and the structures that comprise the project, including the reservoir.

Edelca is responsible for generating 75 per cent of the electrical energy within the local market, with the commitment of always implementing stringent environmental protection measures in the process. This is one of the responsibilities Edelca has in its key role in the social and industrial development of the country.

“As part of the master plan for the Caroní River development, MWH participated with C.V.G. Edelca in identifying the hydroelectric site”