Postcards from AFRICA15 January 2001
A tour of hydro projects around Africa
ZIGA DAM Nakamé river valley, Burkina Faso, West Africa
lahmeyer International was appointed by the Office National de l’Eau et de l’Assainissement of Burkina Faso as the lead firm in a joint venture with BERA-STUDI KCIC, to supervise the construction of the Ziga dam after the feasibility study, tender design and tender documents for the project were completed in 1977. The main purpose of the Ziga dam is to store water in the Nakamé river valley to supply the capital city, Ouagadougou.
The project involved the construction of a 19m high and 2730m long zoned earthfill dam with a central clay core. Due to adverse foundation conditions and pervious alluvial sediments, the dam foundation was sealed with a one phase slurry trench cutoff wall.
The spillway is an unjugated structure with a downstream stilling basin. It is120m long and has been designed for a 1 in 5000 years flood of 2200m3/sec. For economic reasons, roller compacted concrete was selected to build the spillway as well as the ground floor of the stilling basin. The volume of the spillway and stilling basin structure is about 40,000m3. The water intake is designed for a maximum capacity of 2.9m3/sec. Water is transferred through a 1m diameter and 43km long pipeline from a connected pumping station and water treatment plant to the distribution network of Ouagadougou.
Construction of the dam began in 1998 and was due to be completed in 2000 for impoundage. The reservoir will have a total volume of 208M m3, a usable volume of 185M m3 and an area of 72km2. The dam and appurtenant structures were estimated to cost around US$40M.
HARDAP DAM Fish River, Namibia, South West Africa
The Hardap dam was completed in 1963 and is located in the southern region of Namibia on the Fish river. Its main purpose is to store water which is used to irrigate 3000ha of farmland near the town of Mariental. The dam is 33m high and has a crest length of 855m. It is constructed from river gravel and the downstream toe is firmed by quarried basaltic rockfill with an asphalt concrete sealing on the upstream face. The spillway has a capacity of 5000m3/sec and a width of 55m. The reservoir capacity is 252M m3.
After 30 years of operation, the dam needed rehabilitation to ensure the future safe supply of irrigation and potable water for Mariental. Permanent and periodic leakage through holes and cracking in the asphalt concrete sealing has occurred since 1969. In 1981 the concrete asphalt sealing was covered with a bituminous mastic coating from the dam crest to elevation 1120m asl. This was followed by the application of a painted coating from the dam crest down to elevation 1120m asl in 1983.
Cracking was recently observed in the upper part of the sealing on both dam wings and also down to water level. Replacing the aged concrete sealing was believed to be the only reasonable solution to guarantee safety and impermeability of the dam and was executed in 1994-1996. The surface of rehabilitated asphalt concrete sealing was 22,000m2. The inspection and safety study, rehabilitation design, tendering and contracting and supervision of rehabilitation was undertaken by Lahmeyer International for the Ministry of Agriculture, Water and Rural Development of Namibia.
KATSE DAM Malibamats’o river valley, Lesotho, South Africa
The Katse dam is a 185m high and 710 m long double curvature arch across the Malibamats’o river valley and was constructed as part of the phase 1A works for the Lesotho Highlands Water project.
The Highland Water Venture won a 72 month construction contract for the dam in December 1990 for a tender price of about US$480M. In 1993, a dam acceleration programme was agreed to increase concrete placing from 80,000m3/month to 90,000m3/month to mitigate delays arising from three major design changes and to overcome some unforeseen conditions in the foundation excavations. The final cost of Katse was estimated at around US$0.18B when the dam was completed in 1997.
The removal of poorer than expected autobrecciated rock in the dam foundations increased excavation by about 200,000m3 to about 1.1M m3. As a consequence, a larger volume of concrete (2.32M m3) was needed for the dam. The shape of the valley was also altered, giving the dam a flatter more u-shaped configuration than is usual for concrete arch applications. These design changes also raised concern about the ability of the concrete arch to withstand extreme loading exerted by the reservoir. As a precaution, the dam owners opted for a preformed joint and shear keys in the foundation flanks. The preformed joint will anticipate the formation of a crack in the upstream heel of the arch as the central cantilever rotates forward under full reservoir loading, allowing the crack to be managed and controlled.
Impounding the 1950M m3, 35.8km2 reservoir, induced tremors which damaged homes at the Ha Mapelong village, about 10km upstream from the dam. However, villagers have decided to stay in the area and damaged houses were replaced as a project cost while others with minor damage were repaired. More instrumentation has been installed to monitor a wider area around the dam and no further activity has been recorded as the reservoir has continued to fill.
Katse dam and the base of the intake tower were designed to withstand accelerations of 0.30g on the horizontal plane and 0.1g on the vertical which is equivalent to an earthquake of about 6.5 on the Richter scale.
MUELA HYDRO POWER COMPLEX Ngoe Valley, Lesotho, South Afica
The Muela hydro power complex is located in the Ngoe Valley, 122km north east of Maseru near the village of Muela in Lesotho. It was constructed as part of the phase 1A works for the Lesotho Highlands Water Project and is linked to the Katse reservoir via a 45km transfer tunnel. The tunnel is capable of transporting 26.9m3/sec of water from the Katse reservoir to the Muela power house and then on to South Africa through a delivery tunnel.
Lahmeyer International performed a feasibility study for the Muela hydro complex from 1983-1986. The company was also responsible for tender design and documents, the construction study and supervising the construction of the project as part of a joint venture, which took place from 1992-1999 for the Lesotho Highlands Development Authority.
Muela is an underground power complex. The upstream surge shaft has a height of 160m and a diameter of 9/16m to accommodate the transient hydraulic behaviour of the 45km long headrace tunnel. The steel lined vertical penstock shaft has a diameter of 2.5m and trifurcates just upstream of the power house cavern. The upstream surge shaft has a height of 160m. The power house is 58.8m long, 14.5m wide and 27.5m high and accommodates three identical generating units with a total installed capacity of 72MW, equipped with three vertical Francis type turbines with synchronous generators.
The surge chamber is located 40m downstream of the centre-line of the turbines and regulates the tailrace flows of the turbines. The tailrace tunnel has a diameter of 3.75m and a length of 1400m with a tailrace channel which conveys the outflow from the tunnel to the tailpond. The downstream surge shaft has a diameter of 10.4m and a length of 58.25m. The double curvature arch tailpond dam or Muela dam on the Ngoe river is about 500m downstream of the tailrace outfall and is 55m high and 200m long.
TEDZANI III Shire River, Malawi, South East Africa
The stage III of the Tedzani Falls hydro power station on the Shire river, about 7.5km downstream of the Nkula Falls hydro scheme, is an extension of the existing 40MW Tedzani I and Tedzani II hydro power schemes which were constructed between 1971 and 1977 in Malawi. Lahmeyer International carried out the preliminary phase, review, model tests and field investigations for the Tedzani stage I and was responsible for the tender design and preparation of tender documents for Tedzani stage II.
The installed capacity of Tedzani III is 50MW, generated by two 25MW Francis turbines, each with a discharge of 78.8m3/sec and a design net head of 41.7m. The project has an intake structure, a 6.5m diameter and 1010m long concrete lined headrace tunnel, a surge tank, a 136m long and 5.3m diameter pressure shaft, a bifurcation manifold system, a surface power house structure and 6.4km of overhead transmission lines. A new intake structure is planned which will be located on the left bank of the river, utilising the existing Tedzani barrage.
To guarantee the satisfactory combined operation of the existing intakes and the new intake of Tedzani III, hydraulic model tests were performed to verify and optimise the design of the new intake and guiding devices. Lahmeyer International was responsible for the construction design and site supervision of Tedzani stage III from 1990-1995.
The work was performed for the Electricity Supply Commission of Malawi and commissioning of the new intake structure took place in 1995.
MAGUGA DAM Komati river basin, Swaziland, South Africa
The construction of the Maguga dam is a key element in developing the irrigation potential of the Komati river in Swaziland and in South Africa. The reservoir created by the dam is estimated at 332M m3 and the full supply water level is 101m above the foundation.
Lahmeyer International is a partner in the Maguga dam joint venture for the Komati Basin Water Authority. The company reviewed the feasibility study and participated in design selection, tender design, tendering detailed design and supervising construction from 1996 -2000.
The feasibility study was reviewed in 1992 and one of six dams had to be selected for the site. The potential dams included a roller compacted concrete (RCC) arch dam, a gravity RCC dam, three rockfill dams (inclined clay core, asphalt core, concrete face) and a composite RCC rockfill option. Different dam types require different types and location of the respective spillways, diversion works and hydro power components so all dam types and appurtenant structures were economically assessed after preliminary stability and hydraulic analyses were completed.
The analyses revealed that the three rockfill dams would be most suitable for the site and they were tendered. Construction was scheduled to begin in September 1998.
BELES, CHEMOGA-YEGA and HALELE-WERABESA Ethipian Highlands, Ethiopia, East Africa
The Ethiopian Electric Power Corporation commissioned Lahmeyer International to lead a joint venture of consulting engineers to carry out feasibility studies for the following high head hydro power schemes at different locations in the Ethiopian Highlands:
• Beles hydro power plant will have a capacity of 195MW, a gross head of 270m exploited in a single stage, 15km of tunnels and an underground power house.
• Chemoga-Yeda scheme: will have a capacity of 440MW, a gross head of 1400m exploited in two stages, three embankment dams, 6km of interbasin transfer tunnels, 17km of power tunnels and shafts, and one underground and one surface power house.
• Halele-Werabesa scheme will have a capacity of 374MW, a gross head of 400m exploited in two stages, two embankment dams, 15km of power tunnels and shafts, and two under-ground power houses.
Lahmeyer was involved with the management and execution of large scale field investigation programmes at remote locations covering topography, hydrology, geology and the human, plant and animal environments from 1998-1999. The company also assessed the hydro power potential of these areas, evaluated numerous scheme layouts using the company’s EVALS computer program package, undertook power system expansion planning studies to identify least cost commissioning sequences, used detailed engineering and economic analyses to define the structural layout of each scheme and performed economic and financial evaluations to confirm the feasibility of each project.
THE MIDLANDS DAM Grand River South East, Central Plateau, Mauritius
The Midlands dam is located on the Grand River South East on the central plateau of Mauritius and was constructed to improve the water supply for domestic, industrial and irrigation purposes in the northern plains of the island. Water is directed from the dam’s reservoir into the existing Nicolière water transfer system. The average annual rainfall in the 17.2km2 catchment area is 4100mm, providing the Midlands dam reservoir with an active storage capacity of 25.5M m3.
Lahmeyer International examined a number of dam types, but a rockfill dam with asphaltic surface sealing proved to be the most technically and economically favourable structure for the site. The Midlands dam is 21m high, 2500m long and 7m wide at the crest. The spillway capacity is 645m3/sec and the capacity at the bottom of the outlet is 2 x 40m3/sec.
The dam is zoned with course rockfill (<600mm), fine rockfill is used for the upstream shoulder and foundation (<300mm), a transition zone of processed rockfill is used for the sealing (<30mm) and asphaltic sealing of the upstream face consists of an 8cm binder layer, a 10cm sealing layer and a mastic coat.
A cutoff wall has been constructed to reduce seepage through the foundation and the risk of internal erosion. The wall connects the asphaltic surface sealing at the upstream toe with the impervious bedrock. The average width of the wall is 0.5m, it has a depth of 18m and covers a total area of 30,000m2.
Lahmeyer International carried out a number of services for the government of Mauritius’ ministry of public utilities, including construction design, training personnel, hydrological studies and supervising construction and installation from 1997-2000.
SÉLINGUÉ HYDRO PLANT Sankarani River, Mali, Africa
The Sélingué hydro plant is located on the Sankarani river which is a subsidiary of the upper Niger river in Mali. The plant was commissioned in 1980 and is used to generate power, improve navigation conditions on the river Niger by regulating flooding and provides irrigation water for approximately 55,000ha of farmland. The plant consists of a power house, spillway and two earth dams with a total length of 2200m, and a height of 23m. The power house accommodates four Kaplan turbines with a total capacity of 47.6MW and a head of 18.5m.
In June 1996, Energie du Mali appointed Lahmeyer International to provide consulting services for a dam rehabilitation project and overhaul of the mechanical and electro-mechanical equipment at Sélingué. The consulting services included supervision of the rehabilitation measures on the dams, overhaul and extension of the dam control system, verification and supervision of the rehabilitation measures on the four generating units, instrumentation and control devices, an emergency diesel unit and a transformer station.
Lahmeyer International is continuing to oversee the rehabilitation work which began in 1997, and is co-ordinating site activities as well as inspecting the testing of plant components. However, restrictions in Mali’s energy supply only permits the overhaul of one unit per year and during the months of April, May and June, all units have to be operating in order to meet energy demands in the dry season with decreasing reservoir levels. The total cost of the rehabilitation measures and the overhaul of the equipment is approximately US$27.2M, financed by the World Bank. The rehabilitation of the last generating unit is due to be completed in spring 2001.
OWEN FALLS Victoria Nile River, Uganda, Africa
The Owen Falls hydro power station started operating in 1954 and is located on the Victoria Nile river in southeast Uganda near Jinta. The 30m high, Owen Falls gravity dam submerged the waterfall and controls the discharge of water from Lake Victoria into the Nile basin and supplies hydroelectricity to Uganda and Kenya.
Ten Kaplan units were gradually installed at the plant and commissioned by 1968. The units were recently uprated to 18MW each, but water has spilled continuously through the dam sluices and feasibility studies have shown that the site can accommodate additional generating capacity. This led to the implementation of the Owen Falls Extension Project, 800m downstream of the existing dam.
Construction of a new power house and commissioning of two units was completed under a series of individual contracts in 2000. The installation of the third unit is still in progress. Lahmeyer International was contracted by the Uganda Electricity Board (UEB) to offer technical assistance in the equipment design and procurement works for the last two generating units and all associated equipment and works under one single contract. Protection and controls of the units will be linked with the existing plant facilities.
The picture shows the new power house which will accommodate the new generating units. The award of contract for the supply and works is scheduled for mid 2001. The supply contract will consist of 2 x 40MW generating units with propeller type turbines, generator transformers, equipment for the extension of the 132kV switchyard, all balance-of-plant equipment and associated structural works. Implementation is due to be completed by mid 2003.