Analysis and modelling of river meandering

Abstract

This thesis examines the morphological changes of non-tidal meandering rivers at the spatial scale of several meanders. With this purpose, a physics-based mathematical model, MIANDRAS, has been developed for the simulation of the medium-term to long-term evolution of meandering rivers. Application to several real rivers shows that MIANDRAS can properly simulate both equilibrium river bed topography and planimetric changes. Three models of different complexity can be obtained by applying different degrees of simplification to the equations. These models, along with experimental tests and field data, constitute the tools for several analyses. At conditions of initiation of meandering, it is found that river bends can migrate upstream and downstream. This depends on meander wave length and width-to-depth ratio, irrespective of whether the parameters are in the subresonant or the superresonant range. Varying lag distances between flow velocity and bed topography are found to offer an explanation why local channel migration rates reach a maximum at a certain bend sharpness, in addition to previous explanations based on flow separation. A new method has been developed to calculate the number of bars in a river channel with given width. It predicts successfully whether reducing or enlarging the river width would lead to meandering or braiding. Channel migration coefficients are demonstrated to depend not only on physical properties of the eroding bank, but also on physical properties of the accreting bank and the numerical scheme. Moreover, they need to account for overbank flows. Model results and a re-examination of experimental observations suggest that intrinsic initiation of meandering is not necessarily related to steady bars due to a permanent upstream disturbance. They may also be related to a steady bed deformation due to small quickly-varying periodic or random disturbances, for instance due to the presence of migrating alternate bars. The latter finding still requires further confirmation.