xitizap # 32
Vida versus GWh
a HCB e a Lei
Cahora Bassa - montante
Cahora Bassa - jusante
Zambeze - outras cheias
o Dia dos Generais
Hidrodinâmicas no Zambeze
Monografia da bacia Zambeze a montante de Cahora Bassa
Kariba Dam and Reservoir
Some 385 km downstream of Victoria Falls at the eastern extent of the Gwembe Valley, the Kariba Reservoir was created by the damming of Kariba Gorge, an intrusion of granitic gneiss into which the Zambezi has incised a deep gorge (Balon and Coche 1974) (Figure 2-2). The 131 m high, 633 m long double-curved concrete arch dam was closed in December 1958. The reservoir, extending 280 km from Devils Gorge to Kariba Gorge and 30 km at its widest point, is the third largest in Africa and inundates 5250 km2 of the Gwembe Valley floor. Kariba Reservoir has a total capacity of 64,798 x 106 m3, with 54 x 106 m3 of dead storage (Olivier 1977).
Kariba Reservoir operates to regulate the Zambezi flow regime for hydropower production. The total generating capacity of Kariba hydropower station is 1275 MW, including the 600 MW North Bank power station and 675 MW South Bank power station (Figure 2-9). Currently, Zambia and Zimbabwe use almost all the electricity generated at Kariba. Kariba Dam has six spillgates that can discharge up to 9515 m3/s in combination with turbine outflow at maximum flood level. This discharge capacity is not sufficient to pass the 1:10,000 year design flood of 19,600 m3/s (Batoka Joint Venture Consultants 1993b), so adequate volume must be set aside in the reservoir before each flood season to store part of the inflows. A design flood rule curve (see Working Paper #4) is used to set maximum end-of-month water levels for the reservoir, resulting in periodic drawdowns prior to each flood season.
The Kafue River Basin
Below Kariba Reservoir, the Zambezi briefly flows due north and captures runoff from several small, seasonal rivers including the Lusito. As the Zambezi bends again to the east, it flows through a series of deep gorges and is fed by two major tributaries draining the Central Africa Plateau, the Kafue River and the Luangwa River. Although the catchments of these two river systems are similar in size, they differ significantly in geomorphology and yield very different runoff patterns. Hydrological processes in the Kafue River basin, with extensive floodplain systems and two large dams, are particularly complex.
The Kafue River basin (154,200 km2) drains most of the northern portion of the Middle Zambezi catchment9 (Figure 2-2). The Kafue River headwaters rise on the plateau of the South Equatorial Dividein the Copperbelt region of Zambia. The Upper Kafue basin (50,480 km2) includes the Munyonshi and Luswishi tributaries and is largely mountainous and forested with headwater dambos (Balek and Perry 1973). The Kafue River is deeply incised in this region with fairly steep gradients and rapid runoff (FAO 1968). Annual rainfall averages 1100-1200 mm (Table 2-11).
The Luangwa River Basin and other tributaries of the Middle Zambezi
Below the confluence with the Kafue River, the Zambezi flows gently through the Central African plateau for 180 km to its confluence with the Luangwa River just upstream of Cahora Bassa Reservoir.
Over much of this distance, Lower Zambezi National Park flanks the Zambezi on the north bank and Mana Pools National Park on the south bank. Both parks feature narrow zones of floodplains that were annually inundated by floodwaters prior to Kariba Reservoir regulation. The Luangwa River basin (151,400 km2) rises on the South Equatorial Divide west of Lake Malawi (Figure 2-2). The Luangwa generally follows the base of the Luangwa Rift Valley that forms an extension of the East African rift system and links with the Gwembe Rift Valley (Mhango 1977). Much of the Luangwa River is fed by short, steeply falling tributaries draining from the rift escarpment, most notably the Luwumbu, Lundazi, Lukusuzi, and Lutembwe Rivers. In its lower reaches, the Luangwa captures runoff from the vast Lunsemfwa River catchment that drains the Muchinga Escarpment in central Zambia (44,000 km2)(Balek 1971b). The Luangwa River discharges to the Zambezi in the vicinity of Feira at the western end of Cahora Bassa Reservoir. For the last few kilometers it forms the international boundary between Zambia and Mozambique.
Cahora Bassa Dam and Reservoir
Some 240 km below the its confluence with the Luangwa River, the Zambezi passes through a major gorge incised into large intrusive rock masses (Figure 2-2). The eastern end of the gorge is dammed to form Cahora Bassa Reservoir. Cahora Bassa is a concrete arch dam, 163 m high and 303 m wide. The dam crest altitude, 331 m, is designed to provide the maximum possible storage without backing up water into Zambia and Zimbabwe. Cahora Bassa Reservoir has a dead storage of 12.5 x 109 m3, a live storage of 51.7 x 109 m3 up to the maximum operating level of 326 m amsl, and surge flood storage capacity of 8.0 x109 m3 up to the maximum crest height. Total surface area is 2700 km2 at 326 m (Olivier 1977). The total catchment area of Cahora Bassa is 1,050,000 km2 (86% of the total Zambezi catchment).
Cahora Bassa Reservoir operates to regulate the Zambezi flow regime for hydropower production. The total generating capacity of Cahora Bassa hydropower station is 2075 MW through 5 hydraulic turbines (Figure 2-9). The deep, narrow reservoir has a very high hydropower output per unit reservoir area, 1.4 MW/km2, relative to Kariba Reservoir (0.3 MW/km2). The storage ratio—reservoir capacity divided by mean annual inflows—is 0.86, compared to 3.5 for Kariba Reservoir. This has important implications for water release patterns from Cahora Bassa as discussed below.
Each turbine discharges about 452 m3/s at maximum output. Cahora Bassa Dam has eight spillgates, each 7.8 m x 6 m at their outlet, at a lower sill level of 231 m, together with a single weir emergency spillgate at the crest level of 331 m. The total discharge capacity is about 16,250 m3/s, which is not sufficient to pass the 1:10,000 year design flood of 30,226 m3/s (Li-EDF-KP Joint Venture Consultants 2000b). Because of the low storage ratio of Cahora Bassa Reservoir, water storage in the reservoir is balanced between maintaining water levels lose 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). A design flood rule curve is used to set maximum end-of-month water levels for the reservoir (Working Paper #4). Following yearsof above-average inflows, rule curve operation results in large drawdowns during the end of the dry season to ensure adequate storage volume before the next flood season. Water levels in the reservoir fluctuate by more than 18 m between wet and dry years (Figure 2-27). The theoretical range of water levels between full supply level and minimum operating level is 34 m.
Excertos retirados de
Patterns of Hydrological Change in the Zambeze Delta, Mozambique
by Richard Beilfuss e David dos Santos
a montante, a HCB subestimou as contribuições das redes zambianas de Kafue e Luangwa; umas sub-bacias que, não tão infrequentemente como isso, representam não só mais de metade dos usuais influxos Cahora Bassa, mas também pontas de cheia que, como na rede Luangwa, chegam a superar os 10,000 m3/s como no caso de 1989.
in xitizap # 31
ZRA, Zesa Holdings
Carry Out Inspection On Kariba Dam
THE Zambezi River Authority and Zesa Holdings yesterday conducted major checks on the Kariba Dam wall and key electricity generation infrastructure to assess structural maintenance needs.
This resulted in the shutdown of both Kariba North and South power stations feeding Zimbabwe and Zambia for up to six hours to allow consultants to assess the level of corrosion on the apron -- subsequently switching off most areas in the supply grid of electricity in the two neighbouring countries.
An apron is a concrete structure at the foot of a dam wall to protect it from the impact of falling water when floodgates are opened at first before it starts falling into the 81-metre-deep plunge pool.
This has been eroded over the years especially when the floodgates are opened.
The gates were last opened in 2001 and there are no immediate prospects of them being opened owing to the low levels of water in Lake Kariba which is at 30 percent capacity.
The last inspection was conducted in 1992 and the outcome of yesterday's inspection is expected to be almost the same as then when maintenance needs are minimal.
ZRA chief executive officer Dr Michael Tumbare said preliminary indications were that the main structure was intact while it should be established if there was need to resurface its coating.
"So far we have looked at the apron and it seems the main structure is intact although we now need to wait for the consultants to find out whether there is a need to resurface it or not," he said.
This means that no major maintenance work would be needed in the next five years that could warrant the shutting down of the two power stations for long periods.
"If there was need for major work on the apron it would have meant a shutdown of up to a week but the core of the structure is still intact," he said.
Dr Tumbare said the dam wall was intact and still had at least 90 more years of its projected 147-year lifespan.
ZRA board member from the Zambian side Mrs Emily Striedel said the process went on smoothly on the North station.
The gradual shutdown of the four generators on either side of the Kariba Dam wall began at 8am resulting in water levels dissipating along the Zambezi.
However, one generator each from the North and South power stations was left running at 25 megawatts to allow for a smooth switch-on of the other generators after the inspection.
This shutdown left some fish exposed owing to the lowering of the water levels playing into the hands of expectant people who were waiting to catch trapped fish.
People on either banks of the Zambezi River were able to catch some fish although officers from the Parks and Wildlife Management Authority of Zimbabwe were there to monitor the situation.
Security officers were deployed to monitor activity along the Zambezi as people tried to take advantage of the subsiding water levels to poach fish or smuggle goods in and out of the country.
"This time we could not catch as many fish as the last time when generators from the two power stations suddenly tripped and the water levels dropped almost instantly meaning fish did not have time to move to safety," said a man who had managed to catch some fish in the subsiding water.
Zesa also took the opportunity to inspect and repair all its four tailraces -- an outlet of water used to run the turbines for power generation into the Zambezi River -- for the smooth control of water flow.
The shutdown was a spectacle for tourists who were lucky to be in Kariba.
"It is spectacular because we can see the hippos and fish while they are in the water unlike when the volume of water is high," said Mrs Lyn Schwim of Bulawayo who is on holiday in the resort town with her family.