dc.description.abstract | During the past decades, the systematic investigation of the morphodynamics
of meandering rivers mostly involved the theoreticalanalytical
methodology. The development of analytical models enabled
the definition of equilibrium conditions, stability and evolution
of river meanders and to investigate the interaction between planform
and bedform processes and mechanisms. In recent years the
new branch of remote sensing applied to river morphodynamics has
been constantly developing simultaneously to the rapid increase of
computational and satellite resources. The remote sensing analysis
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is nowadays employed in a wide range fields in geophysics; for this
reason, the past years have seen the prolific development of numerous
algorithms for remote sensing analysis. However, remote sensing
of meandering river morphodynamics has not been consistently
integrated with morphodynamic modelling so far. There is a lack
of sophisticated algorithms for the extraction of extensive morphodynamic
information from the available remotely sensed data; this gap
prevented researchers from seeking systematic validation of analytical
models to define their range of applicability, and to exploit their potential
for improved insight on observations in real world meandering
rivers. The evolutionary dynamics of the channel width, at local and
bend scale, as well as the dynamics of bars in meandering rivers represent
two major unsettled issues in our present understanding of river
meandering dynamics.
In this thesis I first provide a systematic methodology for the automated
extraction of meandering river morphodynamic information
from multitemporal, multispectral remotely sensed data, coded in the
PyRIS software. Moreover, I develop an analytical model to investigate
the long-term planform evolution of periodic sequences of meander
bends incorporating spatio-temporal variations of channel curvature,
width and slope. A first model component predicts the temporal evolution
of the channel width and slope based on a novel treatment of
the sediment continuity at the reach scale. A second model component
is a fully analytical, evolutionary model of periodic meanders
with spatially and temporally oscillating width accounting for nonlinear
feedbacks in flow and sediment transport by means of a twoparameters
perturbation approach.
Application of the PyRIS software to several long reaches of freeflowing
meandering rivers allows me to develop a consistent set of
observations on the temporal and spatial evolution of channel width
and curvature with unprecedented level of detail. Furthermore, model
outcomes indicate that meander-averaged width and slope invariably
decrease during meander development, and that the temporal adjustment
of the hydraulic geometry is controlled by the ratio between
the evolutionary timescales of planform and riverbed, quantified from
the analyzed meandering rivers dataset. The nonlinear perturbation
model indicates that width and curvature co-evolve according to a hysteretic
behavior in time and predicts that the meander belt width dramatically
decreases when the meander resonance threshold is crossed.
The modelling approach predicts wider-at-bend meanders when the
bank pull is dominant with respect to bar push, which in turn promotes
meander bends that are wider at inflections.
Analytical modeling and remote sensing analysis are mostly integrated
through a statistical approach; bend-scale evolutionary analysis
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of planform descriptors such as channel width, width oscillations and
curvature in large pristine meandering rivers exhibit good agreement
with the outcomes of the proposed analytical models.
Finally, the integration between analytical modeling and remote
sensing analysis allows me to identify the key processes controlling
the interaction between migrating sediment bars and planform-driven
steady point bars. The conditions for the formation of migrating bars
in meandering rivers are mostly related to the production of sediment
supply by the basin, contrarily to the widespread idea that meandering
rivers exhibiting migrating bars typically display lower values of
the channel curvature. | en_US |