Existing hydropower development

The Zambezi River Basin currently has approximately 5,000 MW of installed hydropower generation capacity (Table 2). Major dams include Kariba and Cahora Bassa dams on the mainstem Zambezi River, Itezhi-Tezhi and Kafue Gorge Upper dams on the Kafue River, and the Kamuzu Barrage that partially regulates Lake Malawi water levels for downstream Shire River hydropower production in association with Nkula Falls, Tedzani, and Kapichira Stage I hydropower dams. A review of these hydropower projects, and their impact on hydrological variability and uncertainty, is provided below.

Table 2. Existing hydropower projects and reservoirs in the Zambezi River Basin

Kariba and Cahora Bassa dams

Two large hydropower dams operate on the mainstem Zambezi River. Kariba Dam spans the border between Zambia and Zimbabwe, 397 km downstream of Victoria Falls. Cahora Bassa Dam occurs entirely within Mozambique, some 240 km downstream of the Zambia– Zimbabwe border. Kariba and Cahora Bassa share many similarities in their design and operation, as well as some important differences (Table 3). Kariba Dam has the largest reservoir by volume in the world (more than 180,000 Mm3 at full supply level) and fourth largest reservoir with respect to surface area (5,577 km2).

Kariba and Cahora Bassa dams are operated to maximize hydropower production, with a secondary flood control function. As described in the previous section, the operation of Kariba and Cahora Bassa dams has profoundly changed hydrological conditions in the Zambezi River, altering the timing, magnitude, duration, and frequency of natural flows. Evaporative water loss from the vast Kariba reservoir is approximately 16% of total inflows, and Cahora Bassa reservoir loses an additional 6% of inflows to evaporation.

Neither Kariba nor Cahora Bassa dams has sufficient discharge capacity to pass the maximum probable incoming flood. Both dams use a design flood rule curve1 to set their maximum end-of-month reservoir water levels. Water storage in the reservoirs is balanced between maintaining water levels close to the maximum permissible elevation (to maximize hydraulic head on the turbines) and releasing water from the reservoir before each rainy season (to accommodate and store incoming floodwaters without breaching the dam wall).

Table 3. Characteristics of Kariba and Cahora Bassa hydropower dams

*720 MW north bank power station and 750 MW south bank power station.

The deep, narrow Cahora Bassa reservoir has a very high hydropower output per unit of reservoir area (1.4 MW/km2) relative to Kariba (0.3 MW/km2). The ratio of reservoir storage volume relative to mean annual runoff volume for Cahora Bassa Dam (0.69) is about half that of Kariba (1.4). This has important consequences for water-release patterns from both dams. Kariba reservoir is capable of storing the Zambezi's entire mean annual inflow volume. During the prolonged dry period from 1981-2001, Kariba reservoir released only turbine outflows without any excess spillage. As reservoir levels fell close to minimum operating levels in the mid-1980s and again in the early and mid-1990s, even relatively large runoff events in the Zambezi Basin (e.g., 1989, 1992, 1998) were completely absorbed by the reservoir. Cahora Bassa, however, does not have the capacity to store the mean annual runoff volume, and frequently spills water through sluice gates in addition to waters released through turbines for hydropower generation. The spillage of excess waters has important implications for downstream environmental flows, flood management, navigation, and other management concerns in the Lower Zambezi region. Environmental flow releases for social or environmental purposes are not stipulated for Cahora Bassa or Kariba Dam at present.

The flow series used to estimate Kariba firm power output, total energy generation, and the design flood was originally based on a 47-year record for flows at Victoria Falls (covering 1907-1954). The record included the extreme drought period of the early 1900s, followed by a relatively wetter period through 1950. Studies for SADC by Shawinigan-Lavalin and Hidrotécnica Portuguesa (1990) later confirmed the current firm power and total energy targets based on an extension of the flow series through 1990.

The flow series used to estimate Cahora Bassa firm power output, total energy generation, and the design flood was originally based on a 34-year record (1930-1964) for flows at Dona Ana (near the Shire-Zambezi confluence) in Mozambique (Hidrotecnica Portu-guesa 1965). This record reflects a relatively wet period in the historic record, without any prolonged drought. Modeling studies suggest that power production would have been curtailed for prolonged periods during the critical drought period of 1980-95, but the dam did not transmit energy during this entire period until its international transmission lines, sabotaged during the civil war, were restored (Beilfuss 2010).

Beilfuss (2002) describes the management of extreme floods since the hydrological regulation of the Zambezi. Kariba and Cahora Bassa dams face significant long-term challenges in their management of extreme floods in the basin. Both dams are optimized for hydropower production, and seek to maintain maximum allowable water levels in their reservoirs, in accordance with the design flood rule curve. This strategy provides maximum head for turbine generation, but compromises the secondary (and subjugated) objective of flood control. Descriptions of major flooding events in the Lower Zambezi region dating back to 1830 are common in the oral histories of people in the delta region. Since the construction of large hydropower dams, however, rapid large flooding events had a severe social and economic toll. In 1978, flooding on the lower Zambezi caused an estimated $62 million worth of damage and necessitated flood relief operations costing an additional $40 million. Many of these costs can be attributed to the encroachment of people onto lowland areas of the Zambezi floodplains that had never been historically occupied before Kariba regulation. As noted by the engineering firm Rendel, Palmer and Tritton (1980), "this was the first flood since completion of Cahora Bassa, and destroyed the widely held belief that the dam would finally bring flooding under full control. "The flood resulted from a combination of emergency releases from Kariba and Cahora Bassa dams and heavy runoff from lower Zambezi tributaries. RPT (1980) showed that if the reservoir had released water in January and February, gradually stepping up the outflow to 7,000 m3/s, releases would have been significantly less than actually occurred with adequate time to evacuate the most flood-prone areas. Dam management during subsequent large flooding events in 1989, 1997, 2001, 2005, and 2008 also has been the subject of considerable public scrutiny (e.g., Hanlon 2001)

Itezhi-Tezhi and Upper Kafue Gorge dams

The Kafue River is the most regulated tributary of the Zambezi River. The first dam on the Kafue River was completed at the Upper Kafue Gorge site in 1972. Kafue Gorge Upper Dam is a gravity, earth-rockfill dam, with a crest height of 50 m at 981.5 masl and a total reservoir capacity of 885 Mm3 (SWECO 1971). Six turbines generate 900 MW at capacity2, with a maximum discharge of 252 m3/s.

Because high evaporation losses from the Kafue Flats reduce the water available for power generation at Kafue Gorge hydroelectric station, a second dam was designed to stabilize river flows below 250 m3/s, the discharge at which overbank flooding occurs (DHV 1980). Construction of Itezhi-Tezhi Dam commenced in 1973 and began impounding water in December 1976. The dam is a gravity earth-rockfill dam, with a crest height of 65 m and length of 1,800 m. Reservoir capacity is 5,700 Mm3. Itezhi-Tezhi Dam has a maximum outlet capacity of 6,000 m3/s. This spillway is inadequate to pass extreme floods, and a design flood rule curve is adopted to draw the reservoir down prior to flood seasons.

Kariba and Cahora Bassa dams face significant long-term challenges in their management of extreme floods in the basin.

Releases from Itezhi-Tezhi Dam are dictated by power generation needs at Kafue Gorge Dam (typically about 168 m3/s, well below the level of required to inundate the floodplain) except during periods of exceptional runoff from the upper catchment.3 As a result, the extent of flooding in the western portion of the Kafue Flats has been greatly reduced, while the eastern portion of the flats has been inundated by Kafue Gorge Upper reservoir. During March, an ecological freshet of 315 m3/s is stipulated for the Kafue Flats, but it is irregularly released, as described above.

Lake Malawi/Shire River dams

Kamuzu Barrage at the outlet of Lake Malawi is operated to maintain high dry season flows in the Shire River for run-of-river hydropower generation at Nkulu A&B, Tedzani, and Kapichira 1 stations. The Nkula Falls hydropower development, commissioned in 1966 and located downstream of Liwonde, consists of two powerhouses with a total capacity of 124 M W. The Tedzani hydropower development, located downstream of Nkula Falls, has a total capacity of 90 M W. Kapichira Phase I, recently completed and located downstream of Tedzani, can generate a total of 64 M W. Above 475.32 masl, Kamuzu Barrage has no flow control function. The head ponds of all three power plants are severely affected by siltation and thus require periodic dredging.

Other hydropower projects

Victoria Falls hydropower consists of three power plants that produce 105 M W. The oldest of the generating stations was constructed in 1937. The power stations are fed by a left-bank diversion at the level of the falls. The power plants do not run year round; production is curtailed during low flows to maintain discharge at the falls.

Three small hydropower stations are located in the Luangwa sub-basin. The Mulungushi power plant located on the Mulungushi River tributary of the Luangwa sub-basin has four turbines with a generating capacity of 16 M W. A small reservoir with 230 Mm3 storage capacity, located five kilometers upstream of the powerhouse, provides regulation. The Lunsemfwa powerhouse is located on the Lunsemfwa River, also a tributary of the Luangwa. Commissioned in 1945, its total capacity is 18 MW through three turbines. Flow regulation is provided by a reservoir (45 Mm3) located 30 kilometers upstream from the powerhouse. The Lusiwasi powerhouse, located on the Lusiwasi River tributary, has a capacity of 4 MW.

The 4.5 MW Wovwe run-of-river hydropower dam is located in northern Malawi on the Wovwe River tributary of Lake Malawi.

1. The Design Flood Rule Curve stipulates the maximum permissible end-of-month water levels in the reservoir to prevent over-topping of the dam. The curve is derived from the maximum discharge capacity of the dam relative to the maximum probably flood. The rule curves do not stipulate that the water must reach a particular level at a particular time, but only that it cannot exceed (or must remain above) a particular level at a particular time.

2. The Kafue Gorge Upper power plant is being upgraded from 900 to 990 MW.

3. Itezhi-Tezhi reservoir must release a minimum flow of 40 m3/s, which includes 25 m3/s for baseflow maintenance and 15 m3/s for various water abstractions.