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Morphodynamic regime and long-term modelling of meandering rivers
No full textNatural rivers are self-formed features whose shapes are the result of interaction between
erosion, deposition and transport of sediments. The study of their morphodynamics and
the characterization of related sedimentary processes are of great interest not only to environmental
engineers but also to hydrologist and geologist, contributing to the interpretation
of stratigraphic records. In particular, this thesis deals with the morphodynamics
and the long term behaviour of meandering rivers, a very common pattern in nature. This
class of dynamical systems, which occurs at the spatial scale of the channel width, are
driven by the coexistence of various intrinsically nonlinear mechanisms which determine
the possible occurrence of two different morphodynamic regimes: the sub-resonant and
the super-resonant regime. On the short term time scale, the formation of meandering
patterns can be suitably explained as an instability process, driven by bank erosion (bend
instability). The planar development of the river is described by a non-linear integrodifferential
bend evolution equation, complemented with a suitable model for flow and
bed topography in sinuous channels with cohesionless bed. On the long-term timescale,
a further highly non-linear process must be accounted for, namely channel shortening via
cutoff processes. Depending on the description adopted for the flow field, various mathematical
models allowing the description of the temporal evolution of the channel axis can
be developed. Here, we develop a simulation model for river meandering employing two
physics-based linear models characterized by a different degree of approximation. The
fully nonlinear computational approach permits exploration of the long term dynamics of
meandering rivers. Investigating the full range of morphodynamic conditions, we objectively
compare the morphologic characteristics exhibited by synthetically generated and observed planimetric patterns. The analysis is carried out examining, through principal
component analysis, a suitable set of morphological variables. We show that, even in
the presence of the strong filtering action exerted by cutoff processes, a closer, although
not yet complete, similarity with natural meandering planforms can be achieved only by
adopting a flow field model which accounts for the full range of morphodynamic regimes.
We also introduce a new morphodynamic length scale, associated with spatially oscillating
disturbances. Once normalized with this length scale, the relevant morphologic
features of the simulated long-term patterns (e.g., the pdf of local curvature and the
geometric characteristics of oxbow lakes) tend to collapse on two distinct behaviours, depending
on the dominant morphologic regime. Finally, the signatures of possibly chaotic
dynamics or self-organized criticality triggered by repeated cutoff events are investigated.
To these aims, some robust nonlinearity tests are applied to both the spatial series of local
curvatures and the time series of long term tortuosity. Temporal distribution of cutoff
inter-arrivals are also investigated. The results are coherent and show that no evidence of
chaotic determinism or self-organized criticality are detectable in meandering dynamics.La descrizione dei processi che caratterizzano gli ambienti sedimentari fluviali costituisce
una tematica di grande interesse non solo in ambito ingegneristico ambientale, ma anche
in quello idrogeologico. Gli alvei naturali presentano un'estrema eterogeneità e la loro
forma va necessariamente vista come il risultato dell’azione combinata dei processi di erosione,
trasporto e deposito dei sedimenti. Le cause che inducono un alveo ad assumere
naturalmente una determinata configurazione vanno quindi ricercate nell’interazione esistente
tra i flussi d'acqua e di sedimenti in ingresso al bacino di drenaggio e tutti quei
fattori che concorrono alla formazione del paesaggio, ovvero: la topografia e le caratteristiche
idrogeologiche della piana alluvionale, il tipo di sedimenti al fondo, il materiale che
costituisce le sponde ed il tipo di vegetazione che su di esse cresce. Nella presente tesi
focalizzeremo la nostra attenzione sugli alvei ad andamento meandriforme il cui sviluppo
planimetrico è strettamente associato all'erodibilità delle sponde e alle caratteristiche del
campo di moto. In particolare, verrà sviluppato un modello numerico in grado di simulare
lo sviluppo planimetrico di tali corsi d’acqua, le cui caratteristiche morfologiche
sono strettamente legate al carattere sub o super-risonante del regime morfodinamico
dominante. Obiettivo del presente lavoro è quello di valutare, sul lungo periodo, le
caratteristiche morfologiche degli alvei meandriformi determinate dalla struttura non lineare
dell'equazione integro-differenziale che descrive levoluzione planimetrica dell’asse del
canale e dalla natura fortemente non lineare del fenomeno del cosiddetto taglio di meandro
(comunemente noto in letteratura come cutoff). La corrispondenza tra le caratteristiche
morfologiche delle configurazioni planimetriche calcolate numericamente utilizzando due
diversi modelli idrodinamici linearizzati e quelle osservate in natura viene analizzata utilizzando un'analisi delle componenti proncipali (PCA). Mostreremo come il contributo
dinamico delle forti non linearità indotte dai processi di cutoff non sia tale da eliminare
completamente le caratteristiche morfologiche associate alla struttura del campo di moto.
In particolare si constaterà come, seppure molti degli aspetti fondamentali delle configurazioni
planimetriche naturali siano ben riprodotti anche utilizzando per il campo di
moto delle soluzioni linearizzate molto semplificate, tuttavia, una stringente corrispondenza
con gli alvei meandriformi reali sia resa possibile solo dall'adozione di soluzioni più
raffinate in grado di analizzare in modo pi`u completo i complessi meccanismi morfodinamici.
Inoltre, utilizzando un modello linearizzato di campo di moto in grado di analizzare
l'influenza morfodinamica nella sua interezza, sarà possibile lìindividuazione di una nuova
lunghezza scala caratteristica dei meandri, la quale, essendo in grado di sostenere anche
il regime super-risonante, si rivelerà una scelta decisamente migliore rispetto alle vecchie
scale spaziali utilizzate per i meandri. Infine, adottando una metodologia d'indagine consolidata
nell'ambito dell'analisi delle serie temporali non-lineari ed analizzando la serie
degli interarrivi dei cutoff, si metterà in evidenza come nelle dinamiche che regolano lo
sviluppo dei sistemi meandriformi non sia riscontrabile né l'esistenza di un chiaro determinismo
caotico, né tantomeno una loro tendenza ad auto-organizzarsi (self-organized
criticality)
Long river meandering as a part of chaotic dynamics? A contribution from mathematical modelling
No full textIn the present contribution we focus our attention on the possible signatures of a chaotic behaviour or a self-organized criticality state triggered in river meandering dynamics by repeated occurrence of cutoff processes. The analysis is carried out examining, through some robust nonlinear methodologies inferred from time series analysis, both the spatial series of local curvatures and the time series of long-term channel sinuosity. Temporal distribution of cutoff inter-arrivals is also investigated. The analyzed data have been obtained by using a suitable physics-based simulation model for river meandering able to reproduce reasonably the features of real rivers. The results are consistent and show that, at least from a modelling point of view, no evidence of chaotic determinism or self-organized criticality is detectable in the investigated meandering dynamics
Morphodynamic regime and long-term evolution of meandering rivers
No full textIn the present contribution we focus our attention on the long-term behavior of meandering rivers, a very common pattern in nature. This class of dynamical systems is driven by the coexistence of various intrinsically nonlinear mechanisms which determine the possible occurrence of two different morphodynamic regimes: the subresonant and the superresonant regimes. Investigating the full range of morphodynamic conditions, we objectively compare the morphologic characteristics exhibited by synthetically generated and observed planimetric patterns. The analysis is carried out examining, through principal component analysis, a suitable set of morphological variables. We show that even in the presence of the strong filtering action exerted by cutoff processes, a closer, although not yet complete, similarity with natural meandering planforms can be achieved only by adopting a flow field model which accounts for the full range of morphodynamic regimes. We also introduce a new morphodynamic length scale, L(m), associated with spatially oscillating disturbances. Once normalized with this length scale, the relevant morphologic features of the simulated long-term patterns (e. g., the probability density function of local curvature and the geometric characteristics of oxbow lakes) tend to collapse on two distinct behaviors, depending on the dominant morphologic regime
Three-dimensional Reduced-Complexity Simulation of Fluvio-Deltaic Clastic Stratigraphy
No full textA novel reduced-complexity approach to 3D forward modeling of siliciclastic stratigraphy is presented
for the simulation of erosion, transport, and sedimentation in continental, transitional, and marine depositional
domains. The numerical model is based on defining centerlines that connect sediment input points to the shoreline. For
each centerline, erosional and depositional surfaces bound depositional domains, and sand and mud proportions are
assigned to each domain. The position of each depositional surface follows a set of geologic rules and ensures mass
balance with sediment input. The numerical model is tested by simulating the basin-fill architecture of the XES02
laboratory experiment run at the University of Minnesota, which generates stratigraphy mimicking a passive-margin
basin fill. Automatic calibration is used to test multiple combinations of uncertain model input parameters to find
those that produce scenarios consistent with the experimental stratigraphy.
Calibrated models accurately reproduce shoreline and mass-balance centroid migration, marine sediment
proportions, and shoreline trajectories measured in the XES02 experiment. The models also provide reasonable
approximations of sand distribution. Of interest, falling shoreline trajectories in the experiment and the calibrated
models develop coeval to topset aggradation or topset incision depending on rate of base-level fall. The results
reported herein validate the numerical approach for simulating sand distribution in an experimental basin,
representing a first step towards the application of the numerical model in fluvio-deltaic settings. For field
applications, analogue data can be used to independently constrain the geometry of surfaces bounding depositional
domains and their composition. The simplicity of the numerical model then enables multiple realizations by quickly
varying uncertain boundary conditions, allowing probabilistic predictions in settings with limited data constraints
Typhoon-induced megarips as triggers of turbidity currents offshore tropical river deltas
No full textIntense rip currents caused by tropical cyclones can drive sediment-laden turbidity flows down submarine canyons, according to numerical simulations. Shoreline shape, bathymetry and incoming wave direction are key factors controlling this phenomenon
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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