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Effetti dell'eterogeneità della piana alluvionale sulla migrazione di meandri fluviali
Full text linkLe piane alluvionali e i relativi fiumi costituiscono complessi sistemi dinamici che coinvolgono processi idrodinamici, morfodinamici, geologici, geomorfologici, chimici e biologici agenti su un vasto intervallo di scale temporali (dai giorni ai secoli), con reciproche interazioni. La morfologia superficiale e la stratigrafia di una piana alluvionale sono modellate dalla mutua interazione di portata liquida, portata solida, processi di erosione e di deposito (Howard, 1996). L’effetto a lungo termine è controllato dalla configurazione morfologica del fiume (meandriforme, intrecciato, anastomizzato), dal regime fluviale e dalle peculiarità del trasporto solido. Confinamenti della piana alluvionale, la meccanica delle faglie e la distribuzione di biomassa possono inoltre assumere un ruolo apprezzabile nel determinare l’evoluzione della piana alluvionale.
L’obiettivo del presente lavoro è analizzare un particolare aspetto della dinamica fiume-piana alluvionale: il ruolo dell’eterogeneità della superficie alluvionale nel determinare la configurazione spaziale di un fiume meandriforme la cui dinamica, a sua volta, contribuisce a formare la struttura sedimentologica della piana alluvionale stessa. Questa interazione mutua è strettamente correlata alla formazione di oxbow lakes e scroll bars (Figura 1), i quali introducono modificazioni nella superficie della piana alluvionale e, pertanto, influiscono sull’evoluzione morfologica dei meandri (Lazarus & Constantine, 2013). Gli oxbow lakes rappresentano recipienti naturali per l’immagazzinamento di sedimenti e producono singolarità morfologiche, ecosistemi locali, serbatoi di idrocarburi ed inquinanti (Lewin and Ashworth, 2014). D’altra parte, la continua formazione di cutoffs, con la conseguente rimozione dei lobi di meandri pienamente sviluppati e la creazione di nuovi oxbow lakes, introduce un limite nello sviluppo della complessità geometrica del fiume e, di conseguenza, assicura una condizione di evoluzione stabile dal punto di vista statistico (Camporeale et al, 2005; Frascati & Lanzoni, 2010).
Le unità geomorfologiche formate da scroll bars e oxbow lakes assumono quindi un ruolo di primaria importanza sia nel passato del fiume, rappresentando un archivio della sua storia morfodinamica (Schwenk et al., 2015), sia per prevenire la sua evoluzione futura, operando come filtro sullo sviluppo planimetrico a breve e lungo termine
Long-term evolution of meandering rivers flowing above heterogeneous floodplains
Full text linkFloodplains, and rivers therein, constitute complex systems whose simulation involves modelling of hydrodynamic, morphodynamic, chemical, and biological processes which act, affecting each other, over a wide range of time scales (from days to centuries). Floodplain morphology and stratigraphy are shaped by the interplay of water flow, erosion, and deposition consequent to repeated flood events, as the river dissecting the floodplain evolves through feedbacks between bars, channels, vegetation and sediment characteristics shaping the floodplain itself.
The first topic of this thesis concerns the mutual interactions that leads to self-formed floodplains, produced by the sedimentary processes associated with the migration of river bends and the formation of abandoned oxbow lakes consequent to the cutoff of mature meanders.
The second topic addresses the presence of internal boundary conditions able to affect the main flow field and thus the curvature-driven flow that drives bend migration.
Point bar deposits and oxbow lakes are the products of lateral bend migration and meander cutoffs. The sediment deposits characterizing these geomorphic units link together the long term evolution of an alluvial river and the surrounding floodplain, altering the soil composition and, hence, bank strength controlling the rate of channel meandering.
On the other hand, a localized forcing internal to the main flow field (e.g., a variation in bed slope or in flow discharge) propagates either upstream or downstream, affecting the river dynamics.
Multivariate statistical and spectral tools may disclose the complexity of the resulting planform geometries, either simulated or natural, ensuring an objective comparison
Elementi speciali nella modellazione ai volumi finiti di correnti di densità: applicazione numerica al Fiume Adige.
No full textFinite volume modelling of a stratified flow with the presence of submerged weirs
No full textThe saltwater intrusion in the estuary of the Adige River has been investigated by a two-dimensional finite volume shock-capturing model. Owing to the relative small tide range characterizing the river mouth, a sharply stratified salt wedge tends to form during low discharge periods (e.g. in summer). Suitable hydraulic relations have been introduced to model the action of a submerged barrage, located close to the estuary mouth and built to hinder seawater intrusion. Field measurements of salinity profiles have been used to calibrate the model. The numerical results suggest that, as a consequence of increased water withdrawal that occurred in the last years, the barrage does not prevent efficiently the intrusion of the salt wedge any more
Modeling the meander morphodynamics with internal boundary conditions given by a localized variation in the flow field
No full textModeling of long-term evolution of meander planforms is usually applied to river reaches characterized by a uniform flow perturbed by the effects of the curvature and width distributions. However, in nature meandering rivers may be characterized by localized variations due to external conditions, e.g. changes in floodplain slope (geologic variation), confluence of a tributary into the main river (hydrologic variation), or backwater effects (hydrodynamic variation). As a consequence, the hypothesis of a sufficiently long reach having constant forcing characteristic could limit the reliability of the numerical simulations. We developed a mathematical extension of a well-known fully coupled two-dimensional morphodynamic model (i.e., the ZS model) able to manage an internally localized boundary condition which affects the characteristic of the main flow. The resulting modular model computes the flow field in the two meandering sub-reaches determined by the presence of a section entailing prescribed changes in external conditions, and simulates the long-term lateral migration above the floodplain surface due to erosion and deposition processes at the banks, and the possible occurrence of neck cutoffs. Calibration runs and simulations based on real test cases show that internal variations in the parameters controlling the flow field might strongly affect the morphodynamic behavior of the migrating planforms. Future research shall provide an extension of this approach in order to manage multiple internal boundary conditions within the investigated river reach. The aim is to relax the common hypothesis of a unique formative uniform flow, exploiting the less restrictive assumption of a sequence of uniform flows to describe the flow field that establishes in the river and controls its morphodynamic behaviour
Modeling the long-term planform evolution of meandering rivers in confined alluvial valleys: Etsch-Adige River, NE Italy
No full textWe combine the use of a morphodynamic model for river meander planform evolution with a geological dataset to investigate the influence of external confinements on the long-term evolution of a meandering river flowing in an Alpine valley. The analysis focuses on a 100 km reach of the Adige River, NE Italy, which had several sinuous/meandering sections before being extensively channelized in the 1800s. Geological surveys and historical maps revealed that many sections of the study reach impinge on the borders of the valley during its evolution. Moreover, a marked spatial heterogeneity in floodplain vertical accretion rates likely reflects preferential positions of the river channel in the floodplain. Valley confinements are represented by bedrock outcrops and by alluvial fans created by lateral tributaries, and were extracted from the geological and historical maps to build the computational domain for the meander morphodynamic model. The model predicts the long-term planform evolution of a meandering river based on a linear solution of the 2D De St Venant-Exner differential system and can manage changes in floodplain erodibility. Model applications allow to isolate the effects of valley bedrock and of alluvial fans in constraining the lateral channel migration. Modeled river channel persistence maps are compared with the available geological information. The present work allows further insights into the role of external confinements to river meander belts, which have been conducted so far mostly assuming the channel to evolve in unconfined floodplains. Future research shall incorporate model components for floodplain vertical accretion rates and for the advancement of alluvial fans occurring at the same time scale considered for meander evolution
Modeling meander morphodynamics over self-formed heterogeneous floodplains
Full text linkThis work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain
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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