Sediment Budget of Unsurveyed Rivers at Watershed Scale: the Case of Lower Zambezi

The issue of sustainable management of natural resources, such as water and land, is rising to the attention of the technical and scientific community as a crucial theme of global relevance that asks for a global response both in terms of improved knowledge, better means and specific actions. Earth's erosion and sedimentation processes are of particular interest because they are directly related to human activities, in a bilateral way. The main constraint is often represented, especially in Developing Countries, by the lack of data and of economic means to collect them. The objective of the present study is trying to integrate the few available data with appropriate and innovative models of sediment transport for simulating the long-term profile evolution of a river and assess at the same time the necessary terms of a sediment balance at watershed scale. The method has been applied to the lower Zambezi river. In Chapter 1, an overview of recent developments in sediment management and research is presented, underlining the differences in regional approaches, depending upon the respective social and geographical settings. The three basic forms of sediment motion (surface, mass and linear movement, mainly responsible for river processes) and the time- and space-scales of sedimentary systems are considered, underlying the ample variety of features encountered moving along the river from the divide to the coast. A number of morphological models (one, two- and three-dimensional) developed at different time- and space scales and with various degrees of detail and approximation consent to describes these processes. Soil and water conservation is one of the most critical environmental issues facing many countries, especially in Developing Countries (DC) where the strong impact of climate change, urbanization, deforestation, land degradation, droughts and desertification is increasing conflicts for the use of natural resources. In the various Sections of Chapter 1 a review is made about the present state of research in the field of soil erosion, sedimentation and morphodynamics. The solution of all the related problems, however, require the monitoring of several natural and human induced phenomena. Unfortunately, the capability to collect and manage water and sediment resources-related information remains inadequate in many parts of the world: the African case is particularly dramatic due to the chronic lack of available data, not only on solid transport but also on the bathymetric and topographic river configuration. An innovative methodology to better integrate the scanty and sometimes unreliable bathymetric data is presented in Chapter 2. The waterflow and sediment transport equations have been linearized and analytically solved under the hypothesis of quasi-equilibrium conditions. This simplification permits to reconstruct the river bathymetry from planimetric data, the only ones available from satellite images for most of the large rivers of the world, and from averaged altimetric data, usually provided by the available DEM's. The linearized quasi-equilibrium solution provides a criterion to evaluate the accuracy of the approximate (uniform-flow) model, compared to the regular (steady-flow) model, also for non-linear equations in non-equilibrium conditions. The approximate solution presents many advantages which become crucial for long-term morphological computations at watershed scale. The accuracy of the approximate solution appears to improve when the river is schematized with a coarse computational grid although, of course, with a corresponding loss of spatial resolution. A detailed comparative analysis of the accuracy and resolution of both models has been carried out, with an application to the lower Zambezi river in Mozambique. Finally, with the methodology previously developed, in Chapter 3 we investigated the effects of damming on the morphological evolution of lower Zambezi river. In fact, the few, coarse and non simultaneous data have been integrated with the help of the same simplified model utilized for the morphological analysis. The Zambezi river is the fourth largest river in Africa (after Congo, Nile and Niger) and it is the largest African river flowing into the Indian Ocean. The lower Zambezi in Mozambique is strongly 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, non negligible effects are taking place, such as local scour, bank collapse and shore-line progressive erosion, together with economic and ecologic consequences on shrimp production and biodiversity alteration. In order to assess and possibly mitigate those effects, a quantitative and qualitative analysis of the erosion/sedimentation/sediment transport phenomena along the lower Zambezi is urgently needed. As already mentioned, the main constraint is represented by the scanty and unreliable data available: the Mozambican hydrometric monitoring network is very scarce and no bathymetric survey of the river has been made. Besides the systematic flow records at the dam sites and few occasional measurements of turbidity and bottom granulometry, only the Digital Elevation Model (DEM) is available. Therefore, the objective of Chapter 3 is investigating the effects of the presence of Kariba and Cahora Bassa dams on the downstream morphology, integrating the few, coarse and non simultaneous data with a simplified model. The results of simulations substantially agree with the celebrated scale of Lane, (quite often invoked to explain the effects of river damming), on the condition that the time- and space-propagation of the disturbances is taken into account. 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 larger degradation (or smaller aggradation) with respect to the natural conditions (no dams) seems to represent the eventual dominant effect of damming in the long term evolution of the river.