Meandering evolution and width variation, a physics-statistical based modeling approach

Many models have been proposed to simulate and understand the long-term evolution of meandering rivers. These models analyze the hydraulics of the in-channel flow and the river bank movement (erosion \u2013 accretion) process in different ways, but some gap still remain, e.g. the stability of long-term simulations when width variations are accounted for. Here we proposed a physics-statistical based approach to simulate the river bank evolution, that erosion and deposition processes act independently, with a specific shear stress threshold for each of them. In addition, we link the width evolution with a parametric probability distribution (PPD) based on a mean characteristic channel width. We are thus able to obtaining stable long-term simulations with realistic and reasonable spatio-temporal distribution of the along channel width

Width variations in river meandering evolution and chute cutoff process

Many models have been proposed to simulate and understand the long-term evolution of meandering rivers. Nevertheless, some modeling problem still needs to be solved, e.g., the stability of long-term simulations when width variations are accounted for. The present thesis proposes a physics-statistical based approach to simulate the river bank evolution, such that erosion and deposition processes act independently, with a specific shear stress threshold for each of them. In addition, the width evolution is linked with a river-specific parametric probability distribution. The analysis of a representative sample of meandering configurations, extracted from Lidar images, indicate that Generalized Extreme Values (GEV) probability density function nicely describe the along channel cross-section width distribution. For a given river, the parameters of the distribution keep almost constant in time, with significant variations observed only as after cutoff events that significantly sharpen the length of the river. The constraint of the river width based on the assumption of a GEV probability distribution ensures as the river moves throughout the floodplain adapting its width, the stability of long-term simulations. The application of the model to a reach of the Ucayali river appears to satisfactorily reproduce the planform evolution of the river and yields realistic values of the cross-section widths. The second topic considered in the thesis is the formation of chute cutoffs, which produce substantial and non-local changes in the river planform, thereby affecting the morphological evolution. The occurrence of this type of cutoffs is one of the less predictable events in the evolution of rivers, as a multiplicity of control factors are involved in their formation and maintenance. Significant contributions have appeared in the literature in the recent years, which shed light on the complex mechanisms that first lead to the incision of chutes through the floodplain, and that eventually determines the fate of both the cutoff bend and the new channel. However, the subject is not yet settled, and a systematic physic-based framework is still missing. In this thesis, two different forcing factors leading to chute cutoffs are highlighted, the channelized flow inertia and the topographic and sedimentary heterogeneity of the floodplain. Using two hydrodynamic models, the general features of the processes leading to chute cutoffs are investigated by assessing a few representative case studies.Molti modelli sono stati proposti per simulare e comprendere l'evoluzione a lungo termine dei fiumi meandriformi. Ciò nonostante, alcuni problemi di modellazione ancora necessitano di essere risolti, come ad esempio la stabilità delle simulazioni a lungo termine quando sono tenute in considerazione le variazioni di larghezza. Questa tesi propone un approccio fisicamente-statisticamente basato per simulare l'evoluzione dell'argine del fiume, cosicché i processi di erosione e deposizione agiscano indipendentemente, con una soglia di sforzi tangenziali specifica per ciascuno di essi. Inoltre, l'evoluzione della larghezza è connessa con una distribuzione di probabilità specifica per ogni fiume. Le analisi di un campione rappresentativo di configurazioni meandriformi, estratte da immagini Lidar, indicano che la funzione di densità Generalizzata dei Valori Estremi (GEV) descrive in bene la distribuzione delle larghezze lungo la sezione trasversale del canale. Per un dato fiume, i parametri della della distribuzione si mantengono praticamente costanti nel tempo, con variazioni significative osservate solo nel caso in cui eventi di "taglio del meandro" allunghino significativamente la lunghezza del fiume. il vincolo della larghezza del fiume basato sull'assunzione della distribuzione di probabilità GEV assicura la stabilità delle simulazioni a lungo termine, mano a mano che il fiume si sposta attraverso la piana alluvionale adattando la sua larghezza. L'applicazione del modello a un ramo del fiume Ucayali sembra riprodurre in maniera soddisfacente l'evoluzione della pianura del fiume e produce valori realistici delle larghezze della sezione trasversale. Il secondo argomento considerato nella tesi è la formazione di chute cutoff, che producono cambiamenti sostanziali e non locali nella piana del fiume, influenzando in tal modo l'evoluzione morfologica. L'occorrenza di questo tipo di cutoff è uno degli eventi meno prevedibili nell'evoluzione dei fiumi, dal momento che un gran numero di fattori di controllo sono coinvolti nella loro formazione e mantenimento. Negli ultimi anni contributi significativi sono comparsi in letteratura, che chiariscono i complessi meccanismi che dapprima portano all'incisione dei chutes attraverso la piana alluvionale, e che alla fine determina il destino sia della curva tagliata e del nuovo canale. In ogni caso, l'argomento non è ancora risolto, e una struttura sistematica fisicamente basata è ancora mancante. In questa tesi, sono evidenziati due differenti fattori forzanti che portano alla formazione di chute cutoffs, l'inerzia del flusso canalizzato e l'eterogeneità topografica e dei sedimenti della pianura alluvionale. Usando due modelli idrodinamici, vengono studiate le caratteristiche generali dei processi che guidano i chute cutoffs, valutando alcuni casi studio rappresentativi

Chute cutoffs in meandering rivers: formative mechanisms and hydrodynamic forcing

Chute cutoffs are autogenic mechanisms typical of many meandering rivers with wide cross sections, large curvature bends, high discharges and high overbank flow gradients. The shortening of the original meander loop through a bypassing channel produces a greater water-surface gradient and, hence, increases the overall transport capacity of the reach, enhancing the downstream sediment delivery. As a consequence, the mean channel width, as well as the planform shape of the meandering bends adjacent to that bypassed by the chute tend to progressively readjust. The occurrence of this type of cutoff is one of the most fascinating and less predictable events in the evolution of rivers, as a multiplicity of control factors are involved in chute cutoff formation. In the last decade, various researchers tried to shed light on the complex mechanisms that lead to chute incision and eventually determine the fate of the bypassed bend and the new chute channel. However, the subject is not yet settled and a systematic physics-based framework is still missing. In this contribution, formative mechanisms are reviewed, and two different forcing factors leading to chute cutoffs are highlighted; the inertia and direction of the channelized flow, and the topographic heterogeneity of the floodplain. The general features of the involved processes are investigated using a hydrodynamic finite-element model for the two-dimensional flow in the channel and over the floodplain. A linearized two-dimensional hydro-morphodynamic model is used to estimate the channel bed topography in the absence of field data. Two representative case studies are specifically considered, occurred in the Sacramento River (California) and in the Cecina River (Italy). The first concerns a chute cutoff driven by in-channel flow. The second deals with a chute cutoff due to overbank flow and the particular topography of the point bar sediment deposits placed inside the meander loop