Cahora Bassa

Infobox_lake
lake_name = Cahora Bassa
image_lake = STS51B-51-14- Lake Cahora Bassa.jpg
caption_lake = A partial view of Cahora Bassa from space.
image_bathymetry =
caption_bathymetry =
coords = coord|15|40|S|31|50|E|type:waterbody_region:MZ|display=inline,title
type = Reservoir
inflow = Zambezi River
outflow = Zambezi River
catchment = 56,927 km²
basin_countries = Mozambique
length = 292 km
width = 38 km
area = 2,739 km²
depth = 20.9 m
max-depth = 157 m
volume = 55.8 km³
shore =
elevation = 314 m
islands =
cities =

The Cahora Bassa lake is Africa's second-largest artificial lake, situated in the Tete Province in Mozambique. The name Cabora Bassa is an earlier misspelling of the name. In Africa, only Egypt's Aswan dam is bigger in terms of surface water. The Cahora Bassa Dam system is the largest hydroelectric scheme in southern Africa with the powerhouse containing 5 x 415MW turbines. It is one of the three major dams on the Zambezi river system, the others being Kariba and Itezhi-Tezhi. However, Itezhi-Tezhi is not on the main stream of the Zambezi, but on its tributary the Kafue River. The dam began to fill in December 1974 after construction was commenced in 1969 by the Portuguese colonial government, and is 171 m (560 ft) high by 303 m (994 ft) wide at the crest. Built in the first half of the 1970s when Mozambique was still a Portuguese territory, long stretches of the power transmission lines were sabotaged during 16 years of Mozambican Civil War which ended in 1992. The lake has reached a maximum length and width of approximately 250 km and 38 km respectively, flooding an area of 2,700 km² with an average depth of 20.9 m.

The system includes two converter stations, one at Songo in Mozambique and the other at Apollo in South Africa. There are two parallel lines between these two stations, covering 1,400 km, of which 900 km is in Mozambican territory. These HVDC lines work at 533 kV and in Mozambique territory only have about 4,200 towers.

History

Portuguese period

The Cahora Bassa System started as a project of the Portuguese in the late 1960s. South African Governments were also involved in an agreement stating that Portugal would build and operate a hydroelectric generating station at Cahora Bassa together with the high-voltage direct current (HVDC) transmission system required to bring electricity to the border of South Africa. South Africa, on the other hand, undertook to build and operate the Apollo converter station and part of the transmission system required to bring the electricity from the South African/Mozambican border to the Apollo converter station near Midrand. South Africa was then obliged to buy electricity that Portugal was obliged to supply.

During its construction, the dam site was repeatedly attacked without success by Frelimo guerrilla insurgents in an attempt to sabotage the project. Portugal increased popular support in Mozambique with this and other development works (see Mozambican War of Independence). The dam began to fill in December 1974.

Until 2007 the dam was operated by Hidroeléctrica de Cahora Bassa and jointly owned by Mozambique, with an 18% equity stake, and Portugal, which held the remaining 82% equity. On November 27 2007 Mozambique assumed control of the dam from Portugal. [ [http://www.iol.co.za/index.php?art_id=nw20071125231516168C318774 Mozambique assumes control of Cahora Bassa] IOL ] In 2007, Portugal sold to Mozambique most of its 82 percent stake in the Cahora Bassa hydroelectric power facility in the Southeast African country. Finance Minister Fernando Teixeira dos Santos said Portugal would collect US$950 million (€750 million) from the sale of its part of southern Africa's largest hydropower project. Portugal keep a 15 percent stake in Cahora Bassa, though it planned to sell off another 10 percent at a later stage to an investor that would be proposed by the Mozambican government. Portugal's Prime Minister José Sócrates signed the agreement with the Mozambican government, during an official visit to Maputo. The agreement ended decades of dispute between Portugal and its former territory in East Africa over the company, called Hidroelectrica Cahora Bassa. The central disagreement was over the handling of the company's estimated US$2.2 billion (€1.7 billion) debts to Portugal. Mozambican authorities argued they had not guaranteed the debt and therefore should not be liable for the payments.

Independent Mozambique

Mozambique became independent from Portugal in 1975. Since closure, the Zambezi, which is the fourth largest floodplain river in Africa, has received a far more regulated flow rate, but disastrous natural floods still occur. The 1978 flood caused 45 deaths, 100,000 people to be displaced and $62 million worth of damage. According to engineering consultants, "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". [Rendel, Palmer, and Tritton Consulting and Designing Engineers: "Review of the hydrological network, and study of the design of a flood warning system for the Zambezi River". Supplementary report. London: Institute of Hydrology, 1980. Quoted in [http://www.gorongosa.net/research/research_documents/Patterns_Hydrological_Change_Zambezi_Delta.pdf Richard Beilfuss & David dos Santos: Patterns of Hydrological Change in the Zambezi Delta, Mozambique.] Working Paper No 2 Program for the Sustainable Management of Cahora Bassa Dam and The Lower Zambezi Valley (2001)] For further details of ecological problems caused by the dam, see the article on the Zambezi River.

During the Mozambican Civil War (1977-1992) the transmission lines were sabotaged to the extent that 1,895 towers needed to be replaced and 2,311 refurbished over a distance of 893 km on the Mozambican side of the line.

In the 1990s, after the end of the civil war, Hidroelectrica De Cahora Bassa (HCB) appointed South Africa's Trans-Africa Projects (TAP) to perform the construction management, quality assurance and design support service for the rehabilitation of the project. TAP assisted HCB in awarding the construction contract to a joint venture company comprising of Consorzio Italia 2000 and Enel, and a scheduled period of 24 months was set for the project. The lines in South Africa were damaged to a minor extent and only normal maintenance was required by Eskom to get these lines back in operation.

Work on the project started in August 1995. The line route in Mozambique passes through dense bush and difficult terrain from Songo to the South African border near Pafuri, with both servitudes infested with landmines from the Mozambican Civil War (1977-1992) that needed to be cleared before construction work could commence. Heavy, unseasonable rainfalls later affected the programme to such an extent that the first line could only be completed in August 1997 and the second in November that same year. During the refurbishment period, TAP developed and implemented various designs and construction methods to improve the overall programme schedules and project costs. In spite of the extreme conditions within which they had to refurbish and reconstruct these lines, the work was completed within schedule and with a limited budget. The lines have, since completion, been subject to numerous tests and energised to its full potential. About 1,100 people were employed during the peak periods of construction.

The rainfalls and severed flood during February 2000 in the Limpopo River valley again caused considerable damage to both lines to the extent that about 10 towers collapsed and need to be reconstructed within the shortest possible timeframe to restore the power supply to South Africa. TAP was again entrusted by HCB with the engineering, procurement and construction management services. TAP managed to temporarily restore power supply through the one line while a more permanent solution could be carried out on the other line. The reconstructed line is used to carry the full line capacity. TAP had to implement unconventional construction techniques to recover temporary supply. The suspension towers next to the river crossing posed a significant challenge for a temporary power solution to obtain the required clearances of the 711 metre level terrain span.

Related economic activities

Most of the electricity generated by Cahora Bassa, which is located on the Zambezi River in western Mozambique, is sold to nearby South Africa. In 2006, Cahora Bassa transmitted about 1,920 megawatts of power, but the infrastructure is capable of higher production levels and the company had plans to almost double its output by 2008. In 1994 the total installed capacity in Mozambique was 2,400 MW of which 91 % was hydroelectric.

A considerable kapenta fishery has developed in the reservoir. The kapenta is assumed to originate from Lake Kariba where it was introduced from Lake Tanganyika. Annual catch of kapenta in the Cahora Bassa dam in 2003 exceeded 10 thousand tons.

It is widely believed that there is a breeding colony of Zambezi Shark "trapped" inside the dam.As the Bull Shark is known to travel >100km upstream, this phenomenon does not conflict withexisting scientific and biological fact. In particular an ocean-dwelling species, bull sharks are perfectly capable of living in fresh water for their entire lifespan. Local tribes have indeed reported sightings (and attacks) by this isolated community of shark, although these have yet to be substantiated with hard evidence.

ee also

* 2007 Mozambican flood
* Cahora Bassa (HVDC) for details of the power scheme based on the dam.

References

General references

* [http://www.ilec.or.jp/database/afr/dafr14.html World Lake Database for facts and figures]
* [http://www.britannica.com/eb/article-9018454/Cahora-Bassa Cahora Bassa] , "Encyclopedia Britannica".
* Allen Isaacman and Chris Sneddon, " [http://www.kyle.aem.cornell.edu/lusopaps/isaac_sned_M.Cornell.doc Portuguese Colonial Intervention, Regional Conflict and Post-Colonial Amnesia: Cahora Bassa Dam, Mozambique 1965–2002] ," "Conference on Lusophone Africa: Intersections between the Social Sciences" [http://www.kyle.aem.cornell.edu/Lusoconferencepage.htm] , Cornell Institute for African Development (May 2003).