The Palmiet Pumped Storage Scheme is an essential part of the Palmiet River Government Water Scheme, the objective of which is to make the Palmiet River, one of the last large conventional water sources in the south-western Cape, available for urban water supply to the Cape Town metropolitan region. The first phase of the scheme, descriptionbed in White Paper K-82, entails a pumped storage hydro-electric scheme with its lower reservoir on the Palmiet River, an upper reservoir on the watershed between the Palmiet and Steenbras Rivers, and a canal in which water can be conveyed from the upper reservoir to the existing upper reservoir of the Steenbras Pumped Storage Scheme (see Fig 1). The pumping capacity of the hydro-electric station is used for a small proportion of the available time to pump water, additional to that required for peak-time generation of electricity, to the upper reservoir for subsequent release to the Steenbras Reservoir. In this way the dependable yield of the Steenbras Reservoir can be increased initially by some 30 million m3 per annum and ultimately by a figure of 140 million m3 per annum.
The initial feasibility study for the Palmiet Scheme carried out by Eskom1, which was completed in June 1979, was based on a layout dictated by the hydraulic requirements at the scheme, fitted into the topography of the area in the best way possible. This feasibility study was undertaken to obtain a first estimate of the viability of the scheme, in both technical and cost terms. The importance of the geology of the area was recognized before this initial study had even been completed, and the initial geological investigations were undertaken by M J Mountain and Associates between October 1978 and March 19792, This work involved the mapping of the geology from aerial photographs, ground surveys and the examination of borehole logs.
Eskom and the Department of Water Affairs, both widely regarded as environmentally enlightened organizations, recognized as early as 1979 the need for environmental consideration of the Palmiet Pumped Storage Scheme. It should be noted that, at that time, the scheme was still in its early stages of planning. With the Pumped Storage Scheme now complete and in operation, it can be used as an example of the effective integrated environmental management of a large construction project, particularly when consideration is given to the environmental sensitivity of the area in which it has been developed. A preliminary environmental study was carried out in the first instance by the Department of Water Affairs. This study was aimed largely at assessing the validity of the environmental concerns that had been publicly expressed, and agreed that indeed there was an environmental case and particular care was warranted.
The Palmiet project is located in rocks of the Table Mountain Group. These are the distinctive mountain building rocks prominent throughout the Western Cape and are characterized lithologically by quartzite and quartzitic sandstone. Apart from the distinctive shale band of the Cedarberg Formation there are other minor shale, siltstone and argillaceous horizons throughout the succession. The excavations for the work at Palmiet are entirely within the Nardouw formation, which has been extensively folded in a series of anticlines and synclines with axes generally in a northeast-southwest direction, i.e. more or less perpendicular to the line of the hydraulic conduit from the upper reservoir at Rockview to the power station. Numerous small scale folds and fold reversals are developed throughout this anticlinorium. The structure is characterized by the very distinct bedding planes delineating the folding and varying from thickly to very thickly bedded (300 mm to more than 1 000 m spacing).
The waterhammer - defined as the change in pressure, above or below normal pressure - is caused by sudden changes in the rate of flow of water in closed conduits. Any manoeuvre of the pump turbine or the valve introduces sudden changes in the demand for water in all points of the system, to a greater extent in the conduit between the machine and the surge lank, and to a lesser extent in the conduit between the upper reservoir and the surge lank. The waterhammer caused by emergency manoeuvres of the pump turbine is usually severe and the system should be designed to withstand positive and sometimes negative pressure caused by these operations.
Physical model tests were conducted on a variety of proposed flow component designs for the Palmiet Pumped Storage Scheme. The purpose of the tests was to evaluate these designs and propose modifications to overcome any shortcomings found and to retest if necessary. The tests were undertaken by Eskom's Engineering Investigation Division in close co-operation with the Eskom design team at Megawatt Park and the civil consultants (SVE) to ensure that the final designs were validated and available for incorporation into the construction schedule.
Rockview Dam is the upper storage unit of the Palmist Pumped Storage Scheme and its full supply level is at RL 531 m, i.e. 285 m above that of Kogelberg Dam. The dam basin consists of two natural depressions, with the larger one located on the southern side. The two depressions are linked by an excavated channel (Fig 1). The main embankment, which is approximately 1 300 m long and has a maximum height of 48 m, has been constructed along mainly the southern perimeter of the basin. The secondary embankment, which is about 670 m in length and has a maximum height of 33 m, forms the northern perimeter of the dam basin. The natural topography borders the remainder of the basin.
This tower is a 67 m high, reinforced and prestressed concrete structure with an oval cross-section measuring 17,5 m by 14,5 m. It stands in the upper reservoir straddling the waterway and is linked to the crest of the main Rockview embankment by a vehicular prestressed concrete access bridge. The large screen protected bellmouth at its base has been shaped with the aid of a hydraulic model to prevent flow separation and air entrainment into the waterway, an aspect descriptionbed in more detail in the paper by Kimpton and Gibson.
Internal design pressures were determined by the surge and waterhammer analysis, descriptionbed in the paper by J R Kawiak. External design pressures from the waterway empty condition were taken to be 1,5 MPa for the tunnel and shaft portions and 0,5 MPa for the headrace and penstock. These were based on the maximum ground water or grouting pressures. Optimization of waterway diameters and the selection of steel types ranging from mild steel (250 MPa yield strength) to Roqtuf (up to 690 MPa yield strength) are also referred to in the paper on the development of the design by Pyzikowski, Heidstra and Hillyard. The allowable steel stress for internal pressure design varied from 50 per cent (for the bifurcation) to 60 per cent (for straight sections) of the guaranteed minimum yield point. A weld strength factor of 0,9 was assumed, except at stress-relieved sections, where 1,0 was used.
The design of the machine shafts and the foundations to the pump turbines and generator motors was undertaken by SVE consultants and ESKOM design engineers. The Electrowatt Engineering Services group of the consulting consortium had been involved in the design and construction of many such schemes throughout the world and therefore had ideal experience for this project. In the design of the machine foundations a very close interface exists between the civil engineering consultant and the mechanical and electrical contractors.
The power station proper, leaving aside the various small ancillary buildings (Fig 1), comprises three major subdivisions, namely the machine hall, the switchgear annexe and the administration annexe. Whilst being interrelated, each subdivision is distinctive in terms of its function and this is reflected in the structural and architectural form of the building (Figs 2 and 3). The basic planning of the building. including dimensions and levels of the various parts, and the architectural design were undertaken by Eskom, while the structural design was carried out by SVE Consultants.
The dual-purpose Palmiet Pumped Storage Scheme near Cape Town came into full operation in mid-1988, the third such scheme in South Africa after the Steenbras and Drakensberg stations. Palmiet contributes 400 MW of peaking power to the national electricity grid and has the capacity to transfer 140 million m3 of water annually to the adjacent Steenbras water supply scheme for the Cape Metropolitan Area. The scheme was developed jointly by Eskom and the Department of Water Affairs (DWA) over a period of seven and a half years and involved the usual elements of dam construction (both concrete and rockfill), underground works pertaining to the water transfer and power generation systems and surface structures for the control, operation and administration of the scheme.