Morphological effects of damming on lower Zambezi River

The Zambezi River is the fourth largest river in Africa (after the Congo, Nile, and Niger), and it is the largest African river flowing into the Indian Ocean. The lower Zambezi in Mozambique is influenced by the presence of two very large reservoirs (Kariba dam and Cahora Bassa dam) that have modified the natural seasonal flows, as well as the sediment balance and morphology of the river. In particular, downstream of the Cahora Bassa reservoir down to the delta, no negligible effects are reported to take place, such as local scour, bank collapse, and shoreline progressive erosion (Davies et al., 2000; Beilfuss and Dos Santos, 2001). In order to quantify and possibly mitigate these effects, a simplified numerical model of the sediment transport and erosion/sedimentation phenomena along the lower Zambezi has been developed, capable to deal with the scanty and uncertain data available. Indeed, besides the systematic flow records at the dam sites and few occasional measurements of turbidity and bottom granulometry, only the Digital Elevation Model (DEM) of the river will be used for the model. The objective of this paper is to predict the present and future effects of the presence of the Kariba and Cahora Bassa dams on the downstream morphology, integrating the few coarse and nonsimultaneous data, somehow improving their overall quality. The model reproduces the time and space propagation of the disturbances, that confirm, with more generality, the qualitative response of the river to the constructions of dams, anticipated by various geomorphologists. In fact, the reduction of waterflow seems to have an immediate effect downstream by initially fostering the sediment deposition. Subsequently, the total interception of sediment by the dam slowly takes over and inverts this tendency. A slightly smaller aggradation (or slightly larger degradation) rate with respect to the natural conditions (no dams) seems to represent the dominant effect of damming in the long term evolution of the lower Zambezi. © 2009 Elsevier B.V. All rights reserved.