Nature’s way15 March 2000
With the construction of the Kapichira hydro power project, Malawi has maximised power generation while minimising social and environmental impacts. Eric Cole and Nikola Muzic report
The decades-old goal of using the Kapichira Falls for hydro power development has become a reality. The Electricity Supply Corporation of Malawi (ESCOM) has commissioned the first two of four planned units at the Kapichira hydro power project.
Historically Kapichira Falls, the last major falls on the Shire river, protected Malawi from intruders but also impeded the country’s development. Later, as its scenic vista became a tourist attraction, there was a growing awareness that this stretch of river had the potential to generate significant hydro power.
Lake Malawi acts as a natural reservoir by regulating the flows in the Shire river which is the only outlet to the lake. This, along with the steep stretch of river in the Middle Shire, provides ideal conditions for the development of hydro power. The dominating presence of Lake Malawi with an area of 29,603km2, means that large dams and reservoirs are not required. It also minimises the social and environmental impact of hydro power development.
The first hydro power development on the Shire river was the Nkula Falls scheme, with 3x8MW units which came into operation in 1966. At the time it was the largest of Malawi generator ESCOM’s plants. The system grew rapidly, focusing on the hydro power potential of the Shire river. Four 10MW units at the Tedzani project were commissioned between 1973 and 1977. Five 20MW units were added at Nkula between 1980 and 1992 and, most recently, two additional 25MW units were added at Tedzani. Now, ESCOM has commissioned the first two of four planned 32MW units at Kapichira.
Early planning studies by ESCOM had established that about 500MW of additional capacity would be needed by the year 2010 in Malawi. Generation planning studies by TAMS Consultants of the US in 1985 identified the Kapichira project, located downstream of the existing Nkula and Tedzani projects, as the most advantageous next step. With World Bank funding, ESCOM awarded an engineering contract in 1992 to the joint venture of TAMS Consultants and Knight Piesold and Partners, in association with Merz & McLellan, for the design and supervision of construction of the Kapichira project. The first objective was to optimise the design, with particular emphasis on minimising its environmental impact and improving the sediment-handling characteristics of the project.
The Kapichira project is located in a sensitive area which has historic and scenic value and is wedged between the Majete game and Blantyre fuelwood reserves. The visual impact of the project was reduced by adopting a power tunnel solution in preference to the power canal and surface penstock option originally selected for the site, and by locating the power house in a deep tailrace excavation. Access roads were planned to minimise the intrusion on the scenic Kapichira Falls area, and the large surface switchyard originally planned for the site was replaced by a compact indoor scheme using gas-insulated equipment housed in a separate building behind the power house. This is the first use of such equipment in Malawi.
The small run-of-river reservoirs on the Shire river have had problems with sedimentation. As a result, stringent measures were taken in the design of Kapichira to ensure as much as possible that sediment could be passed through the scheme without excessive accumulations in the reservoir and without endangering the power plant equipment. Hydraulic model studies and mathematical modelling were used to develop a scheme to ensure that a good portion of the reservoir will be self-flushing at moderate flood flows.
The solution that was devised makes use of the main cofferdam, which curves upstream from the spillway to the main river channel, to deflect flows towards the spillway. This involved extending and raising a portion of the cofferdam to provide a deflection dike. The model tests showed that the deflection dike offered an added advantage. By preventing flood flows moving directly to the dam and then flowing along the dam to the spillway, the dike improves the spillway flow patterns significantly, eliminating a strong eddy at the right abutment pier.
With the flood flows directed normally to the spillway, it could be demonstrated that at moderate floods the flow velocities would be high enough to flush accumulated sediments over the low spillway crest. The model tests also showed that the power intake should be located adjacent to the spillway and oriented to it at almost right angles. The tests also indicated that during the sediment flushing mode, the scheme should be operated at minimum pool to maximise flow velocities in the reservoir. This in turn, necessitated a wide intake structure that could abstract the required discharge over a few metres depth of water towards the top of the minimum pool level.
The project consists of:
•A 30m high, 1km long embankment dam located at the head of the rapids to the Kapichira Falls.
•A gated spillway at the left abutment with five radial gates designed to pass 8750m3/sec.
•A wide power intake structure located adjacent to the spillway designed to abstract up to 270m3/sec of water at the higher levels of the reservoir.
•A single concrete-lined power tunnel 8.8m in diameter.
•A surge chamber 25m in diameter.
•A steel-lined penstock tunnel and manifold.
•A surface power house set in a deep tailrace excavation, accommodating 4x32MW units, two of which will be installed initially.
Rock material from the large power house and tailrace excavations was designated for use in the dam, cofferdam and dike construction, giving almost an exact balance between excavation and fill.
The project also includes almost 20km of access road, a 100m span bridge over the Shire river, and permanent housing and infrastructure comprising 40 houses, a school, a clinic, offices and laboratories. The project is connected to the system by a new 64km, 132kV transmission line from Kapichira to Tedzani and by an existing 132kV line to Blantyre, the main load centre.
Construction of access roads and camps began in 1995. The major civil works contract was awarded to the Impregilo-Salini joint venture a year later, and a variety of equipment contracts were awarded in late 1996 and early 1997.
In 1996 an early start was made on the deep excavation for the power house, followed by construction of the power house and install-ation of equipment, all of which made up the critical path in the overall schedule. With completion of excavation work in 1997, concrete work on the power house was begun and completed in the
first quarter of 1999 allowing installation of the main equipment to begin in April 1999.
Although not on the critical path, the construction of the dam, spillway and power intake was carefully tailored to Malawi’s wet and dry season cycle. During the first two years of construction, the river flowed in its natural channel and work concentrated on the construction of the spillway, power intake and the left abutment of the dam.
By late 1998, progress on these elements was sufficient to allow the river to be closed just before the flood season, forcing the river to discharge through the spillway. Work then focused on completing the dam and appurtenant works to enable the impounding of the reservoir to begin in late 1999.
The construction of the power waterways was conducted on a parallel schedule. The power tunnel was driven from the downstream end by drill and blast techniques when the power house excavation had progressed to the tunnel level in mid-1997. The installation of the steel liner downstream of the surge shaft started in October and the concrete lining of the position of the tunnel upstream of the surge shaft began in mid-1998. The water system as a whole was substantially completed by the end of 1999.
In general, the rock conditions at the site were as expected. The gneiss bedrock formed a good foundation for the dam, and grout takes were low.
Shaft and tunnel excavations required minimum support. The same was true of the large open-cut excavations, except in areas associated with the prominent dolorite dike which intersected the power intake and power house excavations.
The power house civil works were substantially completed and the tailrace was flooded in December 1999. The power tunnel was pressurised in early January 2000, enabling the first two units to be tested and commissioned shortly afterwards. These units increased ESCOM’s capacity by 64MW and were a welcome addition to a system that had been experiencing load shedding since the latter part of 1999.
The final two units will be installed to meet the growing demands on the interconnected system. All the civil works, except for the unit concrete works, have been completed for the future units including the waterways up to and including the butterfly valves.
With the project now in operation, the ESCOM system has begun receiving a supply of power that is expected to be adequate to meet Malawi’s needs into 2000. The goal that was set more than 30 years ago has largely been achieved with the taming of the Kapichira Falls.