Dynamics of river meanders: bankfull geometry and planimetric configuration

Meandering rivers are ubiquitous on the surface of the Earth and are key elements of every ecosystem they are embedded in. They distribute water, sediments and nutrients all over the floodplain, they provide a mean of transportation and energy production, and they offer many other ecosystem services that are vital for the flora, the fauna and the communities living close to them. However, even in the most remote areas of the world, meandering rivers are impacted more and more by climate change and by human activities, such as damming, agriculture, and mining. For these reasons, multiple studies were focused on the analysis of the dynamic behaviour of meandering rivers. However, there has been little development of models aimed to simulate the effects of these forcings on the long term evolution of meandering rivers. The aim of my PhD project is to understand how these external stressors affect the bankfull geometry and the planimetric configuration of meandering rivers. To achieve this goal, in this PhD thesis I combined various modelling tools, with satellite data and field measurements. I first focused the analysis on the relationship between the bankfull geometry and the morphometric properties of meanders, by means of the semianalytical evolutionary model developed by Bogoni et al. (2017). The analysis revealed a strong influence of the bankfull geometry on the planimetric configuration of meanders. In particular, the distance from the resonant condition, which can be viewed as a combined metric depending on the bankfull geometry, showed the largest differences in the morphometric properties, such as the bend skewing angle, and the bend and total sinuosity, thus providing a suitable tool to predict the long-term behaviour of a meandering river for different combination of the external parameters (e.g. flow and sediment inputs, valley slope, grain size). To exploit it, I specifically developed a simple model that predicts the evolution of the bankfull geometry with changing sediment supply, distinguishing sand and gravel bed rivers. Conversely, comparing model results with satellite imagery data revealed that the simulations tend to overestimate the observed total sinuosity of meanders. Furthermore, it pointed out the inability of the model to represent the effects of changes in water and sediment regime, and consequently to properly model the effects of external stressors. To overcome the latter shortcoming, I developed a simplified multi-reach model based on the approach recently proposed by Monegaglia and Tubino (2019), aimed at a more physically based description of the adaptation of meander bankfull geometry to planform changes. The new approach introduces a local update of the bankfull geometry parameters based on the effects of the changing sediment supply on the river bed slope. The capability of modelling the effects of changes in the sediment supply provides a powerful tool to describe the effects of external stressors. In the framework of this analysis, during a two-month visiting period at the Louisiana State University in the US, I collected flow field and bathymetric data on the Sabine River in a field survey. The ultimate goal was to characterize a meandering river stretch that exhibits a distinctive change of bankfull properties between adjacent reaches. Field observations also highlighted the existence of flow recirculation zones in high curvature bends, which protect the river banks from erosion and therefore inhibit a further increase in channel sinuosity. Their effect is not accounted for in present models, which may explain why they tend to overestimate channel elongation. Therefore, a suitable quantification of their role could lead to a more accurate bank erosion predictor. Finally, I focused my attention on the particular features of river flowing in permafrost covered floodplains. I compared the morphometric properties of these rivers with rivers in a temperate and tropical settings using satellite imagery. I found a more pronounced width variability in permafrost rivers, which can be related to the erosion dynamics of the glaciated bank. This precarious equilibrium could be easily disrupted by the effects of climate change and so it is relevant that these environments are further investigated to understand the underlying processes. In conclusion, the results of my PhD project shed some light on the often overlooked connection between the bankfull geometry and the long-term planimetric configuration of meandering rivers, and provide insight into the effect of external stressors on their evolution. Moreover, the proposed approach lays the foundation for a novel set of models for the long term evolution of meandering rivers, able to reproduce the changing condition that these rivers are facing.