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

soltas

 

 

Extractos de

Monografia da bacia Zambeze a jusante de Cahora Bassa

 

 

 

 

The Lower Zambezi catchment below Cahora Bassa Reservoir

The Zambezi basin below Cahora Bassa covers an area of approximately 340,000 km2 from the upper Rift Valley highlands to the Zambezi Delta. The Lower Zambezi is a complex physical system with four river-floodplain zones comprising narrow gorges, mobile sand-braided reaches, anabranching reaches, and coastal distributaries (Davies et al. 2001). The course of the Zambezi reflects the extensive tectonic patterns of the region, following the E-W and then SE fault lines on its path to the Indian Ocean (Li-EDFKP Joint Venture Consultants 1999). The main tributaries in this region include the Luia and Revuboe of the Moravia-Angonia Plateaus, Luenha River of the Manica Plateau, and the Shire River of the lower Rift Valley. The Zangue River is also important as an historical hydrologic link between the Zambezi and Pungue River systems.

 

The Moravia-Angonia and Manica Plateau tributaries

Immediately below Cahora Bassa Dam, the Zambezi cuts east for 35 km through a deeply incised, steep-sided granite gorge of the Moravia-Angonia Plateaus (Figure 2-2). The drainage morphology of the plateau region is directly related to the geology of the region. The plateaus are underlain by pre-Cambrian formations composed of highly metamorphic granites and gneisses that form strongly dissected hills and mountains. Karoo and post-Karoo sediments associated with large outcrops of volcanic rock overlie these basement rocks. Drainage from these sediments is only weakly incised, with occasional river terraces.

Distinctive red lateritic soils cover the region (Li-EDF-KP Joint Venture Consultants 1999).

 

The Luia River, entering the Zambezi 30 km below Cahora Bassa Dam, is the first major tributary of the Lower Zambezi catchment (Figure 2-2). The Luia catchment (28,000 km2) includes the Capoche, Upper Luia, and Cherize Drift basins. The Capoche River rises on the Moravia Plateau at 1000-1500 m in eastern Zambia, and captures runoff from the Sadeza, Pivanhe, and Nhimbe Rivers over a total catchment area of 14,686 km2. The Upper Luia rises further to the east on the Angonia Plateau, also at 1000-1500 m, and captures the Vuboe, Muangadeze, and Luangua Rivers over an 11,031 km2 catchment. Some 23-km upstream of its confluence with the Zambezi, the Luia River captures runoff from the Cherize River catchment (1917 km2). Most of the Luia basin is covered in mopane woodland, with dry deciduous miombo at higher elevations. Downstream of the Luia confluence, the Zambezi bends 45 degrees to follow a SE trajectory towards its outlet at the Indian Ocean. Here, the valley-floor-trough widens to several kilometers, but gradients remain high and boundary sediments (mostly fine gravel and sand) are highly mobile, so a braided sandbed river dominates (Davies et al. 2001). Further downstream, the Zambezi discharges through the narrow Mepanda Uncua gorge and broadens to more than 1-km width. After capturing runoff from the Mavudezi River catchment (3154 km2) and several smaller tributaries, the Zambezi River reaches Tete, the first major city of the lower Zambezi Valley. The total catchment between Cahora Bassa Gorge and Tete is 40,000 km2.

 

From Tete to the Lupata Gorge, two major tributaries, the Revuboe and the Luenha (Figure 2-2) feed the Zambezi. The Revuboe catchment (15,540 km2) is very similar to the Luia catchment, rising on the north bank near Mount Domue’ of the Angonia Plateau, a region of high rainfall (1000-1400 mm annual), and capturing runoff from the Ponfi and Condedezi Rivers and several smaller tributaries. Rainfall and runoff characteristics in the Revuboe catchment are also very similar to those in the Luia catchment. The monthly distribution of rainfall from long-term gauging stations in the Revuboe catchment at Metengo (upper catchment) and Chingoze (lower catchment) are given in Table 2-18. Hydrographs of daily flows for wet, dry, and average rainfall years in the Revuboe catchment are given in Figure 2-36. Heavy rainfall events may generate flood peaks of more that 2000 m3/s during wet years. Flood flows are negligible during dry years.

 

The Luenha River, the largest tributary of the plateau region, rises on the south bank of the Zambezi. The Luenha catchment (54,144 km2) includes three major tributaries. The upper Luenha channel rises in the Inyanga Mountains of the Manica Plateau at 1000-1500 m amsl, the Ruia River rises in the Mvuradona Mountains of Zimbabwe at 500-1000 m amsl, and the Mazoe River (catchment area 34,216 km2) rises also in the Zimbabwe Highlands near Harare. Throughout its lower course, the Luenha is a

braided sand-bed river with high sediment fluxes. Rainfall and runoff characteristics in the Luenha catchment differ from those in the Luia and Revuboe catchments. Annual rainfall is lower in the region south of the Zambezi, with maximum rainfall of 800-850 mm in the rocky Zimbabwe highlands and average rainfall of 500-600 mm per annum in the lower catchment. Rainfall in the southern region is also affected earlier by the movement of the ITCZ, and typically peaks from late December to early February,

Below the Luenha confluence, the Zambezi follows a well-defined channel between 800 and 1000 m wide. After passing through the narrow Lupata Gorge at 95 m amsl, river channel energy decreases and the valley opens up into the first significant floodplains of the lower Zambezi Valley, an anabranching channel system 3-5 km wide with ill-defined banks (Davies et al. 2001). The Zambezi may flow in several channels in the dry season that merge together into a single, swift-flowing body of water during the wet season. Several small, seasonal tributaries drain this region, including the Minjova River from the north bank and the Muira, Pompe, Sangadeze, and Mepuse Rivers from the south bank. The floodplain anabranch zone continues past the Zambezi confluence with the Shire River and Zangue River systems to the apex of the Zambezi Delta. Cumulative runoff patterns for the Zambezi catchment are measured at Muturara (Dona Ana gauging station), located immediately upstream of the confluence with the Shire and Zangue River systems at 40 m amsl (Figure 2-2). The total catchment area commanded by the gauge is 118,800 km2. Between 1930 and 1958, mean annual runoff volume at Muturara was approximately 102 x 109 m3 (3200 m3/s). Over the past 25 years, runoff is only about 70 x 109 m3 (2200 m3/s).

 

The importance of runoff from the Moravia-Angonia and Manica Plateaus to the hydrological regime

at Muturara cannot be under-estimated.

…...

 

Lake Malawi and the Shire River basin

 

Downstream of Mutarara, the Zambezi River bisects the Great Rift Valley as it traverses eastern Africa from the Red Sea to central Mozambique. The Shire River, the largest tributary in the lower Zambezi catchment, drains 154,000 km2 of the Rift Valley in southern Tanzania, Malawi, and Mozambique north of the Zambezi (Figure 2-2). The Shire River originates as outflow from Lake Malawi, the only large natural lake in the Zambezi basin. Lake Malawi is the third largest lake in Africa, spanning 580 km in length and 16-80 km in width with a surface area of 29,610 km2 (Pike 1968). The catchment area of Lake Malawi is 126,550 km2.

 

 

Excertos retirados de

 

Patterns of Hydrological Change in the Zambeze Delta, Mozambique

 

by Richard Beilfuss e David dos Santos

 

 

leia o texto integral (PDF)

…

 

a jusante, a HCB subestimou grosseiramente a importância das redes afluentes de Norte (Shire, Revuboe, etc) e de Sul (Moz/Zim). Um erro que, na zona de Mutarara / Caia, pode ter significado ignorar que, às suas já imensas descargas de 8,400 m3/s, havia que acrescentar pelo menos outro tanto destes agitados tributários.

in xitizap # 31