This work concerns the investigation of the meandering phenomena, focusing on the hydrodynamic (Part I) and on the morphodynamic (Part II) of meandering rivers.

In the first part, we study the influence of the water depth on secondary flow and turbulence structures in sharp meandering open-channel flows at a laboratory-scale Re number, using large-eddy simulations (LES) for two different curved channels. The turbulent flow in curved channels has been extensively studied in the past because of its own importance for applications in geophysical contexts and, specifically, in river hydraulics. A number of investigations was carried out numerically, solving some reduced-order forms of the Navier-Stokes equations. Among the others, LES has been carried out mainly for curved single-bend open-channel flow [1]. The laboratory data of Blanckaert [2] are used for validation purposes. The laboratory flume consists of a straight inflow section, a curved section of 193° and a straight outflow section, over a flat bed, representative of the early phase of bed erosion. LES-COAST model is used to perform LES of incompressible fully developed turbulent flows [3]. It solves the curvilinear form of the Navier Stokes equations under the Boussinesq approximation (in case of stratified flow) on a structured non-staggered grid. The principal features of a meandering open channel flow are the origin of internal shear layers and the establishment of a centrifugal secondary flow that deeply influence the flow behaviour. The relevance of this curvature-induced secondary flow in the flow domain depends on the turbulence. It plays a dominant role especially in the flow regions near the banks. At the inner bank, the model applied is able to resolve accurately the boundary layer detachment and the formation of an internal shear layer. Furthermore, it adequately reproduces the outer-bank cell of secondary flow that gives rise in the region near the outer bank, followed by a local increase of turbulent kinetic energy. The comparison of the two flow fields indicates the significant influence of the water depth on the secondary flow distribution and the turbulence structures. The secondary flow and turbulence stresses increase when the water depth is further increased. This increase in turbulence is a key aspect, in fact it affects sediment transport, the bathymetry, mixing and spreading of pollutants and suspended matters.

In the second part, a mathematical model for meandering rivers with spatial width variations is developed. The mathematical modeling of the long-term evolution of meandering rivers needs an efficient computation of the flow field. Therefore, the development of a mathematical model based on the complete response of a meandering river to spatially varying channel axis curvature and width is necessary. For this purpose, we elaborate a morphodynamic model able to predict the spatial distribution of the flow field and the equilibrium bed configuration of an alluvial river characterized by arbitrary distributions of both the channel axis curvature and the channel width, starting from the model proposed by Frascati and Lanzoni [4]. Owing to analytical character of the model, it provides a computationally efficient tool that can be easily incorporated in long-term river planform evolution models. Furthermore, it can be used to rapidly evaluate the morphological tendencies of an alluvial river in response to variations in planform geometry or hydrodynamic forcing. The model is tested by comparison with the bed topography observed in a typical reach of the Po River, showing that in presence of wide, mildly curved and long bend and weak width variations, the river topography is described with a good accuracy.

[1] van Balen, W., Uijttewaal, W. S.J. and Blanckaert, K., 2010. Large-eddy simulation of a curved open-channel flow over topography. Physics of Fluids, 22, 075108.

[2] Blanckaert, K., 2009. Saturation of curvature-induced secondary flow, energy losses, and turbulence in sharp open-channel bends: Laboratory experiments, analysis, and modelling, Journal of Geophysical Research, 114, F03015.

[3] Petronio, A., Roman, F., Nasello, C. and , V., 2013. Large eddy simulation model for wind-driven sea circulation in coastal areas. Non-linear Processes in Geophysics, 20, 1095-1112.

[4] Frascati, A., and Lanzoni, S., 2013. A mathematical model for meandering rivers with varying width, J. Geophys. Res. Earth Surf., 118, 1-17.