A two-dimensional numerical model for the investigation of the effects of dams on the Zambezi River Delta

[摘要] ENGLISH ABSTRACT: The Zambezi River is the largest east-draining river in Africa. It captures runoff from 8different countries before draining into the Indian Ocean in Mozambique through theZambezi Delta which is recognised as a (Ramsar) Wetland of international importance. TheZambezi River flows are currently regulated by four large hydropower dams within itscatchment. Much attention has been given in recent literature to the detrimental effects of thealtered flow regime as a result of dams on the Zambezi River and the Delta in particular.Existing research relating to these negative effects includes many detailed ecological,hydrological and qualitative morphological studies but to date no detailed morphologicalmodelling studies have been conducted in this regard.In this thesis a two-dimensional coupled hydrodynamic and morphological numerical modelof the Zambezi Delta is created using topographical information obtained from a navigationalstudy (Rio Tinto, 2011). The model hydrodynamics are calibrated using recorded water levelsand flows at two gauging stations within the model domain. The bed load sediment transportis calibrated using field measurements (ASP, 2012b).The effects of dams on the Zambezi Delta are investigated by performing two 10 yearsimulations, one representing the current (post-dam) scenario and the other representing apre-dam scenario. These simulation results show a significant decrease in flooded areas andsediment movement on the floodplains as a result of dams. Additional effects on channelwidths and depth, on bed gradings, and on tidal water level variations are analysed.The model is then used to simulate a proposed environmental flood release scenario. Suchreleases have been recommended as a means to partially mitigate the negative impacts ofdams on the Zambezi River. In this case an annual flood release supplying a peak flow of8500 m3/s (slightly less than the pre-dam mean annual flood of 10 000 m3/s) was found to cause slightly more flooding of the close floodplains and to have small effects on the riverchannel width.The model predicts hydrodynamics and bed sediment transport of non-cohesive sedimentswith suitable accuracy but an issue with the suspended transport of cohesive sediments wasidentified. Recommendations are made for addressing the suspended sediment transportinaccuracy. The model, in its current form, can provide quantitative information regarding thehydrodynamics and course sediment transport of the general delta region on a coarse scale.With additional computational resources and accurate topographical information the modelcan be refined to give accurate predictions for localised areas within the delta. Suchinformation would be valuable to specialist studies addressing the environmental effects ofvarious proposed flooding scenarios or future dams.

[发布机构] Stellenbosch University