Kihansi Falls pave the way for power development

Tanzania in East Africa was formed in 1964 when Tanganyika, after gaining its independence from British rule in 1961, merged with Zanzibar. Internationally, the country is well known for its wildlife parks, the explorer Dr Livingstone and the highest mountain in Africa (Kilimanjaro). But despite being rich in mineral and agricultural resources, the economic development of Tanzania has been very slow. The gross domestic product (GDP) is under US$9B and the income per capita is only US$120 a year. Nevertheless, the implementation of economic reforms in the 1990s led to a growth rate in GDP of about 4% and this is expected to improve to 6.9% in 2003. Electric power development has also been lagging. With a total installed capacity in 1990 of 395MW serving just 5% of the population, it was clearly necessary to embark on an aggressive power development programme involving assistance from international donors.

Lower Kihansi

The Lower Kihansi hydro power project (LKHP) in Tanzania is located in the Rufiji basin, some 550km southwest of the principal city of Dar-es-Salaam.

It is owned by the Tanzania Electric Supply Company (TANESCO) which is practically the only entity at this time which generates, transmits and distributes power in the country. The current system has a total installed capacity of 583MW, of which 381MW is hydro. The high head LKHP will add 180MW to these figures in the first stage, ultimately reaching a total installation of 300MW, making it the largest power facility in Tanzania.

The project was conceived by Norconsult in the Rufiji Basin Hydropower Study of 1984, which also suggested that the Kihansi river was the most favourable for future hydro power development in the basin. Subsequent studies considered the development of the site in two steps:

•The Upper Kihansi project, which has a large storage dam for regulating the river flows and a relatively small low head power plant.

•The Lower Kihansi project with a low diversion dam and a high head power plant as the main energy producer.

The investigations for the latter scheme were undertaken by EPDC of Japan, financed by JICA. The report, which was published in 1990, demonstrated the technical feasibility and economic viability of the Lower Kihansi project, while the economics of the Upper Kihansi project were very marginal. The World Bank and TANESCO subsequently agreed to incorporate the implementation of the Lower Kihansi project into a sector loan package — the Power VI Project Programme — and proposals were invited from consulting engineers.

In December 1991, Norplan of Norway was selected to carry out a feasibility review of the Lower Kihansi scheme, and if this proved positive, to continue with the final design, tendering and supervision of construction. Norplan's Inception Report, issued in June 1992, concluded that the most favourable development of the site was as defined in the EPDC study, and that the initial stage should comprise the Lower Kihansi project but with a different underground layout and design.

Simplicity

The LKHP exploits the steep escarpment where the Iringa plateau drops some 1000m to the Kilombero flats, as the Kihansi river rushes over the cliffs to form the magnificent Kihansi Falls. This topographic feature permits the economic development of a hydro scheme by providing the high head needed for the relatively low streamflow.

The beauty of the project lies in its simplicity. A low concrete gravity dam diverts the river flow into a vertical unlined intake shaft. This connects to an unlined headrace tunnel with short steel penstocks at the downstream end, leading to the underground power house. The power is generated by Pelton wheels and the spent water is evacuated through a tailrace tunnel and an open-cut tailrace canal. Although conceived as a run-of-river development, the hydrologic regime of the Kihansi river at the project site shows a large degree of natural regulation. The long term average streamflow is 16.4m3/sec, and the dependable flow is 7.5m3/sec at 97% reliability.

Considering a gross head of 853m and the existing Kihansi streamflow records, power studies showed that in the mixed TANESCO hydro/thermal system, LKHP with an installation of 180MW would add a firm energy output of 730GWh/year. The additional average energy output is estimated at 945GWh/year. It is recognised that any additional installation at Kihansi would essentially be for peaking purposes.

Finance

It should be noted at this point that although the World Bank had enough funds to finance the whole of the Lower Kihansi project, there were talks at the outset with other donors about sharing the involvement. The interested parties were the Norwegian Agency for International Development (NORAD), the Swedish International Development Agency (SIDA), the European Investment Bank (EIB) and Germany’s Kreditanstalt für Wiederaufbau (KfW). The consultant, Norplan, split the entire project into components to match the individual donor’s committment, resulting in the formation of 11 different contracts.

Civil works

The main components of the civil works at Lower Kihansi include a 25m high concrete gravity dam which impounds a small reservoir with a total storage of 1.6M m3, of which 1M m3 is used for daily regulation. An intake structure with trashracks and gates is located at the left abutment, while adjacent in the dam proper are the sediment flushing gates, the larger one being a low-level radial gate. The intake connects to the pressure waterway via a transition elbow section to the circular unlined vertical headrace shaft (25m2), some 500m deep. This in turn connects to the headrace tunnel, which is unlined, and slopes downstream at an inclination of 1:7. At the downstream end are the stonetrap and the transition section to the steel penstocks. The tunnel is 2200m long and has a cross-section of 30m2, except in the last downstream 600m where its cross-section is 37.5m2 to allow for the sectional lining in the zone of the highest pressure. No surge shaft or chamber had been provided due to the relatively short tunnel length and the low water velocities, combined with the use of Pelton turbines. The high pressure tunnel plug is also located in the by-pass section used during construction as a connection between the headrace and access tunnels.

The penstock section provides for two branches, one for units 1, 2 and 3, and the other for the future units 4 and 5. The tailrace tunnel has a length of 2400m and a cross-section of 40m2. It slopes gently downstream at an inclination of 1:900 (it is designed as a free-flow conduit). This connects to an 800m open-cut canal which evacuates the water into the existing Kilombero river system.

The power house cavern is excavated deep into the mountain massif and is 12.6m wide, 70m long and 30m high. The excavation for units 4 and 5 was included as an option in the tender and TANESCO exercised the option for future convenience, since the quoted price was very reasonable. The power house is connected to the outside by a 1900m access tunnel (40m2 in cross-section) and also by a separate cable tunnel which was chosen as an extra security measure for the 220kV cables passing from the underground power house to the outdoor switchyard.

The switchyard at Kihansi comprises SF6 switchgear, including a 20MV shunt reactor for voltage regulation, and a 220/33kV transformer for the local supply. For additional reliability of power transmission, two 220kV lines were built from Kihansi to connect with the TANESCO grid. One line, which is 97km long, runs to the west and joins the system at the Iringa substation. The other line, which is 178km long, runs to the north and joins the grid at the Kidatu substation (the location of the 204MW Kidatu hydro power station). The first was built early on and served to import power from the grid for the construction work during the last year.

Engineering challenge

A head of around 850m can be achieved with the location of the tunnel in the escarpment of the Udzungwa mountain range. This escarpment, belonging to the eastern branch of the east African rift system, is formed by large scale block faulting. The rocks are mainly competent gneisses of the Pan African Mozambique Belt, subjected to high grade metamorphism. The degree of faulting and jointing is in general moderate to low. Most pronounced is a joint/fault system oriented perpendicular to the tunnel system with partly high permeability. The lower end of the headrace tunnel is located about 730m below ground.

Although the overall concept of the scheme is simple, the very high water pressures in an unlined headrace tunnel proved to be a real engineering challenge. Assistance from the Panel of Experts appointed by the World Bank and specialists in the field, including the Norwegian institute Sintef and Swiss Solexperts was required. The primary aim of the field investigations was to ensure that there were sufficient internal stresses in the rock mass for adopting an unlined design for the headrace tunnel which gives large cost savings within the concept of acceptable water leakages.

Stress measurements by use of hydraulic fracturing methodology were first conducted in deep, core drilled holes from the surface. After some costly and time-consuming attempts, where test equipment was lost in deep drill holes, a different approach was chosen. Hydrofracturing tests would be done from short holes drilled from within the tunnel during excavation. If the results from these tests were unsatisfactory, the layout would have to be modified.

The contract conditions were written to allow the power station to be sited deeper into the rock massif if necessary, since this would result in larger rock cover and probably improve the rock mass stress conditions for the critical part of the headrace tunnel. Hydrofracturing testing began when 800m of the access tunnel had been excavated and 300m remained to the original power house location. Initial testing gave insufficient minimum principal rock stresses and it was decided to relocate the power house. The encountered stress pattern was characterised by sub-horizontal minimal principal stresses oriented north to south, parallel to the tunnel axis and perpendicular to the main joint orientation. This pattern is assumed to reflect the original stress situation so that the low minimum principal stresses can be explained by extensional tectonism.

Continued stress measurements by hydrofracturing/hydrojacking and by overcoring methods indicated an improved stress situation deeper into the rock massif. After critical analyses of the test results, it was decided that the

results were satisfactory and in general sufficiently high stresses were available within the rock mass to proceed with the unlined headrace tunnel design, provided that the power station complex was moved 730m into the mountain beyond the initial location. Provisions also had to be made for concrete lining in the critical sections of the headrace tunnel. An emergency plan with a conventional high level combination and lined shaft was then put aside.

Economic analyses were made to obtain parameters for reducing leakages from the headrace tunnel. In the section with the highest water pressure, cement grouting ahead of the tunnel face was conducted in sections where unacceptable permeability was detected by water pressure testing in sounding holes. A 120m long horizontal steel penstock liner from the power station to the headrace tunnel was designed to give an acceptable pore pressure gradient.

During water filling of the headrace tunnel and intake shaft, which took more than one month, the pore pressure build-up in the rock massif around the tunnel was closely followed by automatic logging of some 20 piezometers installed in drill holes. The present pressures around the power station have stabilised at a satisfactory level. The power station itself may be considered dry while a grouting programme is continuing to reduce the leakage (currently at 1100 litres/min) at the by-pass plug. The outflow into the ground along the tunnel system is higher than predicted (currently at 300 litres/sec) but since the ground water level is still rising it is expected that once stabilised the value will be reduced.

Turbine technology

Designing a Pelton turbine which is resistant to abrasion is a major challenge. The erosion of the runner which is expected at Kihansi, due to the small size of the sand (60µm or less in diameter) will be moderate, compared to that of the nozzles and needles. A conventional steel quality of 13% Cr and 4% Ni has therefore been used for the runner, while it has been decided to have a ceramic coating for the nozzles and needles. This coating will increase the life time of these components by a factor greater than ten.

The choice of three four-jet Pelton turbines with a very flat efficiency curve, gives the operation of the station remarkable flexibility (rated 60MW each at 8.3m3/sec). The efficiency is kept high through a range of 2-25 m3/sec. This flexibility makes it possible to optimise the efficiency in the other stations of TANESCO’s grid system during peaking operation.

Environmental impact

The LKHP is located in the eastern escarpments and valleys of the Eastern Arc Mountains which are characterised by the presence of islands of moist evergreen forest. The great age, isolation and fragmented nature of the eastern African forests have combined to produce a great diversity in plant and animal species.

During the World Bank appraisal phase, an environmental impact assessment (EIA) was carried out by an independent expert who basically pronounced the project to be accetable. The Scandinavian and European donors insisted, in accordance with their regulations, that full baseline studies shuld be made and that mitigation measures should be undertaken in areas where the project had a negative impact.

The EIA I baseline studies (TANESCO, 1995) provided a more detailed and complete assessment of probable impacts and recommendations for mitigation of unavoidable adverse impacts. Due to the late timing of the final EIA, the environmental opportunities related to project formulation, design and construction procedure were somewhat limited. Mitigation recommendations were therefore emphasised for actions which would not fundamentally change the project.

As the owner and operator, TANESCO will be operating LKHP in compliance with relevant rules and conditions established by the environmental authorities in Tanzania (NEMC). TANESCO committed itself to carry out the mitigation measures recommended by the environmental consultant, and an environment programme was planned and initiated in 1996 constituting four projects and covering both the human and natural environment.

The environmental projects were allocated a total budget of approximately US$5M. TANESCO is funding the local components of some of these activities but most of the funds are grants provided by NORAD and SIDA, and the management and administration is supported by the World Bank Loan Agreement. The programme area extends into three districts, and covers 22 villages and more than 60,000 people today.

The public health project's target population has increased by almost 50% since the start of its activities. Malaria, which is expected to spread from the lowland to the highland, has been followed up by additional studies.

Recommendations on criteria, operating conditions and terms for water rights for LKHP were prepared by the environmental consultant and submitted to NEMC through TANESCO in September 1999. The proposal is currently under consideration by the relevant Tanzanian authorities. Among the most interesting recommendations was the proposal for minimum by-pass flow aimed at reconciling economic and environmental considerations. A central feature of the proposal is a combination of low by-pass flows with the generation of artificial spray in an attempt to mimic the natural conditions which are the basis for the unique ecosystem in the Kihansi gorge.

The LKHP environmental programme has been in progress for 3.5 years. Generally, it has been on schedule and has been able to achieve its output and objectives. The challenge ahead is to manage the transition problems expected in the demobilisation and commissioning phase of LKHP, as well as the long term impacts foreseen in the operation of the power plant.

It has been proposed that responsibilities for public health issues, catchment management for infrastructure and provision of public services are transferred to local authorities in the Kihansi project area, and that long

term biophysical monitoring activities are extended for at least five years into the LKHP operation period under supervision of the concessional authorities.

Construction

The actual construction phase of the project started in July 1994, with the mobilisation of SIETCO, who won the bid for Contract 1 (preparatory works). The scope consisted of the facilities needed by the main civil works contractor and included: the diesel power plant; the water treatment plant; housing, the office and other buildings; and the 17km dam access road, all to be completed within two years. The phase was followed in mid-1995 by Impregilo, who won the bid for Contract 2 (main civil works) and who also subsequently took over the portion of the dam access road which remained from Contract 1.

The equipment supply contracts 3 to 11 were let shortly after. The construction progress was in general good. There was a setback in the overall schedule by 195 days, but this was due to the shifting of the power house (as discussed above). Apart from a slower than expected advance rate in the tunnel excavation (40m per week, rather than 64m per week) as estimated by Contractor 2, and also a delayed shaft excavation, the other activities followed on schedule.

The suppliers of the power house equipment, the switchyards and the transmission lines performed exceptionally well. The commissioning and placing of unit 1 into commercial operation on 22 December 1999 was made 45 days ahead of schedule, earning a bonus for the involved contractors. The benefits of early completion were tangible to TANESCO as low rainfall and unscheduled shutdown of some of the thermal units had put a stress on the overall power system.