Aspects of riverine hydro-morpho-biodynamics at watershed scale

The integration of various scientific disciplines is becoming more and more important to define the complete evolution of an entire river system, both in time and in space. The aim of this thesis is to analyze the evolution of a river subjected to hydrological, morphological and biological interactions. This analysis is done at watershed scale, by using a numerical 1-D model plus a quasi 2-D sub-model. These models are able to integrate the available data, often scarce both qualitatively and quantitatively, especially in large unsurveyed rivers, typical of less developed countries. The models analysis allows to study the evolution of a river system at large space-scale and long-time scale, because the computational effort required by simplified models are quite low. The results of these models are indicative of a trend of the evolution of the river, in space and time, which can be useful in different studies, for example as input for other detailed models. In the first Chapter we analyze the principal mechanisms of formation and transport of sediments, making an overview of the various models that can be adopted to describe the morphological evolution of a river system. In addition to this description, in this chapter we study the different conformations of riverbeds, classified on the basis of their geometry and morphology. The lack of detailed data, from the point of view both of geometry and biology, implies the application of a non-detailed model, able to describe the hydrodynamic, morphodynamic and biological main processes of the river along its course. This model, essentially 1-D, is based on simplifications related to the hypothesis of the Local Uniform Flow. The 1-D model is associated to a quasi 2-D sub-model, able to provide a synthetic description of the river cross-section. This sub-model is useful for the analysis of large river systems, for which often we do not have bathymetric surveys. In the second Chapter and in the third Chapter is made a detailed analysis of these models, highlighting the advantages and disadvantages of the various simplifications. The growth of the riparian vegetation is strongly influenced by the forcing terms present in a river, related to hydrology and morphology. In Literature there are only a few site-specific studies or purely qualitative analysis, which study the interaction between the forcing terms and the riparian vegetation. The aim of this thesis is trying to model the influence of hydrology and morphology on the growth of the riparian vegetation, by a simplification of all the possible mechanisms involved. In this Chapter we have reported a detailed analysis of our studies, highlighting the limitations related to the lack of experimental data needed for a good calibration of our quasi 2-D sub-model. In the next two Chapters two different applications of the complete model are given: the first one about the Parana River, and the second one about the Zambezi River. The application to the Parana River has been made to highlight the goodness of the model for describing the evolution of the river, even compared to a commercial 1-D model as the Hec-Ras code. That application was also made to verify the general approach and the formulation used in the description of the synthetic river cross-section and the interaction between the river forcing terms and the growth of the riparian vegetation. The study of the Zambezi River is useful to see how the alteration of flow due to hydropower dams along the river strongly influences both morphology and biology of the river, with medium and long term effects. This analysis evaluates the planimetric and bathymetric changes subsequent to an alteration of the natural flow regime of a river due to the construction of hydroelectric reservoirs. In the last Chapter we have discussed the general results and the future developments, underlining, however, that the simplified models used in this work require further verifications, also against the analysis of more experimental data.