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At the basin scale, neglecting localized deviations caused by geological constraints like knick-points, alluvial rivers commonly exhibit a concave profile and a progressive sediment fining in the downstream direction. Although this configuration is, perforce, not in equilibrium, yet it usually shows a quasi-stationary behaviour at the very long (historical and geological) time-scale. A zero-dimensional, two-reach, two-grainsize hydro-morphological model is presented and applied to a schematic river. The description of the processes involved is done assuming several reasonable and verified simplifications, giving reason of the extremely slow evolution of many alluvial rivers and providing a quantitative approach to evaluate their response time. Different from previous analytical formulations, the response time appears here to be affected, among others, by the granulometry of the sediment input. Applications of the model to different geometries demonstrate that the present riverine morphology at the basin scale will persist practically stationary for extremely long periods of time (centuries and even millennia), if the climatic forcing remain unchanged and no anthropogenic perturbations are introduced in the system.

The Lower Zambezi River is influenced by the presence of two very large hydropower impoundments which have modified the natural seasonal flows, the sediment balance, the morphology of the river and the pattern of the riparian vegetation. Downstream of these large impoundments, appreciable local effects are reported to take place, such as scour, bank collapse and shoreline degradation. In order to quantify the sediment transport along the Lower Zambezi River and to predict the effects of the dams on morphology we have developed a simplified 1-D hydro-morphodynamic model capable to deal with the scarcity of available data. Besides the systematic flow records at the dam gauging station and few occasional measurements of turbidity and grain size of the river bed, only the Digital Elevation Model (DEM) and some recent satellite images of the river have been used as the input data of the model. The results confirm the expected qualitative response of the river to the constructions of dams: the reduction of water flow has an immediate effect downstream of the reservoirs by initially fostering the sediment deposition; afterwards, the total interception of sediment by the dams slowly takes over and inverts this tendency; a smaller aggradation rate with respect to the natural conditions without dams, seems to represent the dominant effect of damming in the long-term evolution of the river. The morphological effects of damming over the transversal cross-section of the river have been studied by a simplified transversal quasi 2-D sub-model, coupled to a 1-D longitudinal model. Besides the evolution of the bottom longitudinal profile and grain size composition, the model simulates also the evolution of the cross-section shape and the growth of the riparian vegetation. A first application confirms the qualitative observations reported in the literature on the recent planimetric evolution of the Lower Zambezi. © 2013 Copyright International Association for Hydro-Environment Engineering and Research.

One-dimensional (1D) models can be applied, for engineering purposes, to long-term and large-scale morphodynamic simulations of entire river systems only if appropriate simplifications are introduced. This paper proposes an improvement of an existing simplified 1D model based on the local uniform flow hypothesis, coupled with a synthetic description of the transverse profile, which provides the active river width by analyzing the total and the vegetated widths of a watercourse, with assumed variables of the water flow. The overall density of the riparian vegetation, expressed in terms of biological carrying capacity, is predicted as a function of the local climate and some stresses due to interaction between hydrology, morphology, and biology. The constitutive equations have been deduced and the relevant parameters have been calibrated and validated against various hydrological and geometrical data, taken by satellite imagery covering two large watercourses located in tropical and subtropical areas (Parana and Zambezi rivers) and small streams located in temperate zones. The work confirms the opportunity to deal with hydro-morpho-biodynamic river modeling at large spatial and temporal scale through a mixed 1D + quasi-2D approach. The 1D submodel permits long-term computations at watershed scale of the river longitudinal evolution, while the quasi-2D submodel provides the transversal description of the watercourse in terms of vegetated and nonvegetated widths and vegetation density, which in its turn reflects on the 1D computations.

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.