The Santo Antonio hydroelectric development in Brazil is located on the Madeira river rapids, some 6km upstream from the cities of Porto Velho and Vila de Abunã on the Brazilian and Bolivian border. The Madeira river is a tributary of the Amazon.
State-owned Furnas Centrais Elétricas S.A. and the construction company Construtora Norberto Odebrecht S.A. developed the feasibility studies for the scheme.
With an installed capacity of 3150MW the project will have 44 generators and 44 water intakes. Compared with the 271km2 area the whole development will occupy, the plant has a relatively small capacity reservoir. Some 164km2 will be covered by natural flooding which will result in an increase of the flood area of only 107km2, corresponding to an area/installed power relation of 0.086km2/MW.
The layout of the hydro complex has been designed so as to allow for continual testing and operation of the generator units, from first to last at a pace of one unit per month. The water intake/powerhouse complex is split into three sets.
Two sets of 12 units each will be positioned over the left bank. Each unit will have the following proportions: width – 21.5m; length – 87.6m; maximum height – 57.8m.
They will be separated by a dividing wall that will allow for operation to start in the first set while construction is still in progress on the second. After these two sets, towards the river’s left bank, the spillway structure (formed of 15 spans, 20m wide each) will be positioned near the second set. Another spillway of five spans similar to the first will be built in the channel between Presídio Island and the right bank.
The third water intake/powerhouse set formed of 20 units will be placed in the riverbed and will be built at the river deviation caused by the spillway structure. An RCC dam and walls will interconnect these structures and an embankment earth/rockfill dam will be built by the left shoulder.
The fish transposition system is expected to be built at Presídio Island. In addition to these structures, room for the future construction of a sluice gate and navigation channels will be provided on the left bank.
The spillway will have a Creager type low sill and 20m wide segmented gates. It will bear 20 spans for project discharge with a return period of 10,000yr at 84,000m3/sec. The spillway will be built in two structures, one with 15 spans at the left bank and the other with five spans at the channel between Presídio Island and the right bank.
Construction
Throughout the first three years of work the river will be kept in its natural bed and work will take place at the banks protected by the cofferdams, at the left bank and at the channel between the Presídio Island and the right bank.
After completion of the spillway structure, and through the low flow seasons, the river will be diverted through the spillway structures (without ogees) to allow for construction of the water intake and powerhouse set of units on the dried up river bed.
At this stage, the first water intake and powerhouse set of units will already have their gates and grids completed and assembled to allow for the flooding of the feeding and outflow canals.
The reservoir which will allow the first generators to be tested will be formed once diversion is completed and following construction of the cofferdams on the river bed and the spillway ogee.
When the riverbed water intake and powerhouse units have their gates and grids installed, upstream and downstream cofferdam removal will begin to allow the first generators to be tested.
The environment
Santo Antonio hydroelectric complex (together with the Jirau hydroelectric complex) has been the subject of an elaborate environmental impact assessment (EIA). During 2003-5 experts carried out studies targeted at defining the effects of plant construction and operation. Altogether 118 impacts of different magnitudes and importance for the physical, biotic and anthropic environmental aspects were found.
As far as sediments are concerned, one should take into consideration that the Madeira river has very high suspended sediment concentrations, reaching values that are comparable to those found on other water courses in Brazil.
The sedimentation studies conducted to assess Madeira river values for the above mentioned EIA have covered its course since its confluence with the Beni, upstream from the future reservoirs, down to the confluence with the Jamarí River, some 50km downstream from the Santo Antonio dam.
Hydrosedimentology model studies carried at the environmental licensing stage showed that nearly 12% of suspended sediments in Madeira river waters will be kept back by the dam system. The percentage is represented by the material’s sandy portion. Computation shows that with time this condition tends to evolve to a new balance pattern. Results from the modelling show that the retention process will gradually reduce throughout the years and, after 22 years, the retention rate will be merely 1%.
Designers are currently looking at ways to reduce the amount of sediment retained to ensure easier fish passage.
The Madeira river sediment concentration behaviour will be simulated into sound mathematical models that have been designed for the areas up and downstream of the dams. Such simulations will provide important data for project engineers designing sediment solutions for the scheme. Sedimentation will be continually monitored throughout construction of the hydro complex, and for some years afterwards.
Mercury levels
The Madeira river upper basin was the second largest region for gold production in the Amazon area from the 1970s through to the 1990s. This activity was at its peak in the 1980s. As a result of this activity, it has been estimated that between 1979 and 1990 over 90 tons of Mercury (Hg) have been absorbed by the environment, with some 60% lost into the atmosphere and the remaining left as metallic Hg in the river bed. Part of the atmospheric emissions would have been deposited into water bodies and soil near the Madeira river banks, taking into consideration that deposition of gaseous Hg arising from this activity takes place up to 60km from the emission source.
The Madeira river gold mining activity has been significantly decreasing in the Brazilian portion of the river in the last ten years. However, at the Bolivian portion (Beni and Madre de Dios rivers) the activity is growing. The natural origins of the Hg, which in the case of the Madeira river may be the Andes Cordillera, due to natural volcanic erosion and from the ground in the region itself, have also been taken into account.
Implementation of the Santo Antonio complex will disturb material at the bottom of the Madeira river, especially when the excavations necessary to build the dam begin. Material resulting from such work will be temporarily placed in an environmentally insulated area to allow for the presence of Mercury to be monitored. The portions presenting excessive concentrations will be decontaminated before final disposal at the excess material deposit.
The presence of mercury will be continuously monitored throughout the project in both biotic and unbiotic environments. Residents living in two local communities will also be closely monitored for exposure to the metal.
Demographic changes
Implementation of the Santo Antonio project will cause the involuntary displacement of 643 people who are presently living in 347 houses inside the plant’s worksite and reservoir areas. They will be offered fair compensation choices that should suffice for the full reconstruction of their lives (house, generation of income, and access to public services infrastructure) according to Brazilian law and to the rules established by the financial institutions who acted as signatories of the Equator Principles.
Construction related impacts have also been the subject of special attention in this development project. In order to mitigate the demographic changes arising from the expected mass migration to the area – due to the increase in regional job positions on offer – the project will implement an educational and training centre which will be up and running before work on the project starts. This centre will prepare residents in Porto Velho so that they can utilise the job opportunities arising from the project, thus helping to maximise local benefits arising from the scheme.
Some 9000 people are expected to receive qualification and training at the centre. To welcome workers migrating from other areas in search of jobs, a centre for migrants will be also be established. This will provide support with social workers prepared to render information on the local job situation. The expansion in the demand for public services will be met with a proportional increase in the offer of health, education, leisure, safety and sanitation infrastructures.
Project status
During 2006 and 2007, several public hearings were held in the many communities located around the project site (mainly the work site and the reservoir) and these have been presented and widely discussed. As a result, a series of improvements and commitments were produced that are being incorporated into the Environmental Basic Planning.
In December 2007 the basic engineering and environmental plans were submitted to government agencies for approval. After approval the installation permit will be issued no later than August 2008 after which the area will be released for the work to commence. Work should start in September 2008 and the first generator is expected to start commercial operation in September 2012.
From that date on, one generator per month will start commercial operation, the last one scheduled to commence commercial services by March 2016.
BNDES, together with a union of private banks, will be providing funding for the scheme. Madeira Energia Consortium (MESA) will build, operate, manage and trade the hydroelectric energy produced at the Santo Antonio hydroelectric complex for a 30-year term. Table 1 shows the companies forming MESA and their respective participation in the joint venture.
Key to development
Hydroelectric energy is still the cheapest form of power in Brazil. Energy is key to the continued development of the whole country and this particular region as well. The project location is suitable with minor environmental impacts and good access to an ample energy source. This scheme has unmistakable benefits for both regional and national development.
The entire process of technical feasibility study, environment studies and others was developed in compliance with the rules set by government agencies and other nationally and internationally acknowledged institutions. Environmental impacts will be easily mitigated and only small areas – just a little larger than the natural flooding expansion of the river – will need to be flooded.
The authors are Jose Bonifacio Pinto Junior, Director, and Walter Ajeje, Engineering Manager, Construtora Norberto Odebrecht AS, Praia de Botafogo 300-11. Andar, Botafogo, Rio de Janeiro –RJ 22,250-040, Brazil. Email: jboni@odebrecht.com; wajeje@odebrecht.com, www.odebrecht.com
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