An analysis of the Cahora Bassa Dam Water Balance and Reservoir Operations and their Flooding Impact on Upstream Settlements

Abstract

The Lower Middle Zambezi catchment is sandwiched between three major dams; Kariba, Kafue (Itezhi-tezhi) and Cahora Bassa. The upstream dams have an impact on the inflows in the downstream Cahora Bassa Dam and, also, on the area inundated upstream of the Cahora Bassa. This study aimed at estimating the water balance of the Cahora Bassa using available meteorological and hydrological data sets such as rainfall, temperature and runoff for the period 1980 to 2010. The flow data analysed includes the discharge from the Kariba, Kafue and Cahora Bassa dams and river flows for Luangwa, Chongwe, Musengezi and Manyame. Missing flow data was generated using regression and mean value infilling methods. GIS was used to calculate the isohyets for the average annual rainfall in the lake drainage basin using the nearest neighbour interpolation technique. GIS was further used to process the Digital Elevation Model (DEM) of the study area through the DEM hydroprocessing algorithm in ILWIS thus establishing the ungauged catchment area of the lake. Two gauging stations were installed at Manyame Mapomha and Angwa Bridge on the Manyame and Angwa rivers, respectively, as part of the project’s effort of moving from ungauged to gauged flow. The results from these stations were however too short to give any meaningful indications as to the discharge from the associated catchments. As a result, a hydrological model, the HEC-HMS, was then used to simulate runoff from the ungauged catchments which showed that these areas contribute about 12% of the total estimated inflows into the Cahora Bassa Dam. The average flow for one hydrological year revealed a total inflow of 71.73 x 109 m3/year, total outflows averaged 52.25 x 109 m3/year and a residual storage of 20 x 109 m3/year. While the high flood level for the Cahora Bassa Dam lies at the 329 masl contour line, which translates to an inundated area of 3,246 km2, the actual maximum water level for the period considered (1980 - 2010) reached only 328.18 m contour thus giving an inundated area of 2,920 km2. Analysis of the river flow hydrographs and the associated lake water level indicates that there is a close link between reservoir operations and flood occurrence in Luangwa, Kanyemba and Muzarabani areas. The flooding effect of the reservoirs is only felt when the water level in the Cahora Bassa Dam is above 324. The study concludes that the ungauged lower middle Zambezi contributes a significant amount which averages 8.5 x 109 m3/year, which is about 12 %of the total inflows into Cahora Bassa while the gauged flows from the Kariba, Kafue, Luangwa, Chongwe, Manyame and Musengezi contribute about 85 % of the total flows into the Cahora Bassa. The remaining 2.5 % comes from direct rainfall on the lake surface. Furthermore, the study showed that flooding is most likely when high water levels are accompanied by high flows in the associated rivers such as the Luangwa, the Mwanza Mtanda and the Musengezi.