Numerical modeling of river migration incorporating erosional and depositional bank processes

Understanding the nature of morphologic changes of meandering rivers has long attracted the attention of the scientific community, in the fields of fluvial geomorphology, hydraulic engineering and river restoration. Several meander evolution models developed over the years have provided a deeper insight into meander dynamics. In almost all these models however, a river width that remains constant in space and in time is imposed as a user-specified parameter, without any consideration of the processes that establish this width. Consistent width variation patterns found in most meandering rivers are largely ignored by these models. Perhaps even more importantly, the questions as to how river width is selected and how width variation interacts with meander evolution remain unanswered. This dissertation implements a model framework for meander migration where both bank processes (erosion and deposition) are considered independently, interacting via the intervening channel. In this framework, bank erosion is modeled as erosion of purely non-cohesive bank material damped by natural armoring due to basal slump blocks and channel deposition is modeled as a function of vegetal encroachment. Since banks are allowed to move independently, channel width is allowed to vary locally as a result of differential bank migration. In this dissertation, a fully nonlinear depth-averaged iterative scheme with coupled flow and in-channel bed morphodynamics is developed and implemented so as to capture width variation. Both the migration model and the in-channel morphodynamics model have been coupled adopting a slow planform evolution approximation i.e. in-channel morphodynamics are assumed to be steady state at the time scale of planimetric evolution. A series of numerical studies have been conducted across a wide range of cases, from the simplified case of a straight channel, to constant curvature bend flow, and then to a freely meandering river up to and beyond cut-off. The results show the general tendency for a channel to arrive at an equilibrium/quasi-equilibrium width from any initial width configuration, and delineate phase-plane trajectories for bank interaction which include bank push and bar pull. Co-evolution of local curvature, width and bed slope is demonstrated for the case of a freely meandering river and general metrics for predicting width variation patterns in meandering river systems are outlined. Simulations have been done with realistic field numbers and results are in general agreement with observed natural trends