ABSTRACT Numerical simulations are presented on the initial formation and on the competition of t... more ABSTRACT Numerical simulations are presented on the initial formation and on the competition of tidal channels evolved within youthful salt marsh environments. We present a model that simulates the evolution of tidal channel cross sections, by coupling calculations of bottom shear stresses due to tidal motions with erosion, taking into account the deposition of cohesive and noncohesive sediments. The simulations characterize the morphodynamic evolution of a reference cross section composed of incipient channels and of marsh surface zones. The discharges that shape the section vary in time as a surrogate of the tidal level with a quasi-static model. Wetting and drying cycles are therefore considered. We analyze, at first, the driving factors which lead to the actual observed channel cross sections in tidal basins, starting from the initial channel formation to their long term morphodynamic evolution. In particular, it is shown that large temporal gradients of leading flow rates induce important temporal variabilities of channel geometry in time. The local and temporary evolution of tidal channels, dictated by tidal flow rates, allows the identification of the chief landscape forming discharges. In the second part of our study we simulate the competition of tidal channels evolving within an assigned tidal flat. In these simulations the calculation of the characteristic distance between adjacent forming channels makes it possible to study the drainage density of salt marshes.
Salt-Marshes in tidal environments are characterised by complex patterns both in their geomorphic... more Salt-Marshes in tidal environments are characterised by complex patterns both in their geomorphic and ecological features. Such patterns arise through the elaboration of a network structure driven by the tidal forcing and through the interaction between hydrodynamical, geophyisical and ecological components (e.g. microphytobentos and vegetation). The presentation will introduce experimental ad theoretical work aimed at the characterisation of network morphological features and vegetation patterns by use of direct survey, remote sensing and mathematical modelling. Tools for a complete description of network geometry will be introduced. In particular, a detailed description of meandering characteristics and branching properties by means of a simplified hydrodynamical model will be discussed. Techniques for the determination of the spatial organisation of vegetation will also be described and links to hydrodynamics and sediment circulation will be addressed.
New insights are provided on the geometry of meandering tidal channels and on the drainage densit... more New insights are provided on the geometry of meandering tidal channels and on the drainage density of tidal networks which develop across tidal basins characterized by different tidal, hydrodynamic, topographic, vegetational and ecological features. We first analyze the geometrical properties of tidal meanders, in terms of local curvature, typical meander wavelengths and relevant Fourier harmonics, channel width and depth, with indications on possible dynamic implications on their evolution. These features show important spatial variabilities, suggesting a locally adapted evolution which involves the morphological adjustment to the chief landforming events driven by local hydrodynamics. Theoretical and observational analyses of the drainage density of tidal networks are then presented. Similarity of forms from one location to another, suggested by the constancy, in space and time, of conventional measures of drainage density, appears to be quite in contrast with the apparent site-sp...
When a 2D model is used to compute the flood propagation over an initially dry area some difficul... more When a 2D model is used to compute the flood propagation over an initially dry area some difficulties arise due to the changes in the computational domain as large parts of it are flooded and dried during the phenomenon. A second problem concerns the representation of soil surface: usually it is described as a piecewise constant or a piecewise linearly varying surface hence neglecting the ground unevenness. For very small water depths, unevenness plays instead a very important role. In this paper, new dynamic and mass balance equations for 2D free surface flows are presented which, by means of a conceptual sub-model, account for the ground unevenness effects and allow a very realistic description of flooding and drying phenomena. It is also shown that for a very smooth ground surface or for relatively large water depths, the new equations reduce to the well known De S. Venant equations.
Field data and mathematical modeling have demonstrated that the morphological evolution of shallo... more Field data and mathematical modeling have demonstrated that the morphological evolution of shallow tidal basins is the result of the combined effect of tidal currents and wind waves. Tidal currents, in particular, drive the morphological evolution of shallow tidal systems in proximity of the inlets and within the channel network, whereas in shallow areas tidal current mainly acts enhancing the bottom shear stress due to wind waves and redistributing sediments within the basin. In this study we present a mathematical model for sediment entrainment, transport and deposition due to the combined effect of tidal currents and wind waves. The model is coupled with a hydrodynamic module based on the shallow water equations and with a module for the generation and propagation of wind waves. The sediment transport model describes the sediments by the way of a bi-granular mixtures composed by both cohesive and non-cohesive sediments thus considering the contemporary presence of clay, silt and ...
In this study we present a mathematical model for sediment entrainment, transport and deposition ... more In this study we present a mathematical model for sediment entrainment, transport and deposition caused by the combined action of tidal currents and wind waves in shallow micro-tidal basins. The model uses a bi-granular mixture made up of cohesive and non-cohesive sediments thus considering clay, silt and sand, all of which commonly characterize the sediment bed composition of estuaries and tidal basins. The model also describes the evolution of bed elevation and evaluates the variation of bed sediment composition distinguishing cohesive from non-cohesive behavior.A stochastic approach is proposed to evaluate sediment entrainment close to incipient sediment motion. Particular attention is also given to the problem of reconstructing a reliable initial bed composition as this has a significant impact on sediment entrainment.The model was applied to the test case of the Venice lagoon (Italy) and good agreement was found when comparing model results to a series of turbidity measurements...
An array of vertical cylinders in an otherwise uniform open channel flow can generate a seiche in... more An array of vertical cylinders in an otherwise uniform open channel flow can generate a seiche in the transverse direction due to the lift associated with vortex shedding behind each cylinder. Seiche amplitude attains a maximum at resonant conditions, i.e., when the vortex shedding frequency is close to the natural standing wave frequency. In this work we propose a model to predict maximum seiche amplitude based on the analytical solution to the linearized shallow water equations. The model prediction compares favorably with available experimental data from the literature outperforming pre-existing models. The model is also used to highlight that even small disturbances in the flow largely affect the dynamics and can dramatically reduce the wave amplitude.
SOMMARIO Il presente lavoro trae origine dalla formalizzazione di un criterio generale per la def... more SOMMARIO Il presente lavoro trae origine dalla formalizzazione di un criterio generale per la definizione dell'orientazione dei fronti d'onda. L'applicazione del criterio permette una migliore comprensione dei fenomeni di riflessione di fronti d'onda stazionari, in particolare per le riflessioni di von Neumann, di Guderley e di Vasilev. Si dimostra come, per i suddetti tipi di riflessione, sia possibile ricostruire agevolmente il campo di moto stazionario e la configurazione dei fronti d'onda in prossimità del punto triplo, per via numerica o sperimentale, avvalendosi del criterio suggerito per l'orientazione dei fronti. La forte analogia che lega i moti supersonici alle correnti supercritiche a superficie libera amplia inoltre gli orizzonti applicativi del presente lavoro, sviluppato con riferimento a correnti bidimensionali a superficie libera, all'ambito della gasdinamica.
Il disastroso evento di piena che ha colpito il Veneto il primo novembre 2010, particolarmente dr... more Il disastroso evento di piena che ha colpito il Veneto il primo novembre 2010, particolarmente drammatico per la città di Vicenza, ha reso evidente la necessità di disporre di metodi di previsione, soprattutto a breve termine, che siano efficaci strumenti di supporto alle decisioni. Infatti, nelle fasi concitate di un’alluvione che colpisce un’area urbana densamente popolata, è assai difficile mantenere una visione chiara dell’evoluzione dell’evento e, di conseguenza, stabilire e coordinare un piano per la gestione delle fasi dell’emergenza. Questa nota presenta un modello di previsione che adotta un metodo di tipo statistico basato sull’acquisizione in tempo reale dei dati misurati dalle stazioni idro-meteorologiche del bacino del Bacchiglione e delle previsioni di modelli meteorologici.
ABSTRACT Numerical simulations are presented on the initial formation and on the competition of t... more ABSTRACT Numerical simulations are presented on the initial formation and on the competition of tidal channels evolved within youthful salt marsh environments. We present a model that simulates the evolution of tidal channel cross sections, by coupling calculations of bottom shear stresses due to tidal motions with erosion, taking into account the deposition of cohesive and noncohesive sediments. The simulations characterize the morphodynamic evolution of a reference cross section composed of incipient channels and of marsh surface zones. The discharges that shape the section vary in time as a surrogate of the tidal level with a quasi-static model. Wetting and drying cycles are therefore considered. We analyze, at first, the driving factors which lead to the actual observed channel cross sections in tidal basins, starting from the initial channel formation to their long term morphodynamic evolution. In particular, it is shown that large temporal gradients of leading flow rates induce important temporal variabilities of channel geometry in time. The local and temporary evolution of tidal channels, dictated by tidal flow rates, allows the identification of the chief landscape forming discharges. In the second part of our study we simulate the competition of tidal channels evolving within an assigned tidal flat. In these simulations the calculation of the characteristic distance between adjacent forming channels makes it possible to study the drainage density of salt marshes.
Salt-Marshes in tidal environments are characterised by complex patterns both in their geomorphic... more Salt-Marshes in tidal environments are characterised by complex patterns both in their geomorphic and ecological features. Such patterns arise through the elaboration of a network structure driven by the tidal forcing and through the interaction between hydrodynamical, geophyisical and ecological components (e.g. microphytobentos and vegetation). The presentation will introduce experimental ad theoretical work aimed at the characterisation of network morphological features and vegetation patterns by use of direct survey, remote sensing and mathematical modelling. Tools for a complete description of network geometry will be introduced. In particular, a detailed description of meandering characteristics and branching properties by means of a simplified hydrodynamical model will be discussed. Techniques for the determination of the spatial organisation of vegetation will also be described and links to hydrodynamics and sediment circulation will be addressed.
New insights are provided on the geometry of meandering tidal channels and on the drainage densit... more New insights are provided on the geometry of meandering tidal channels and on the drainage density of tidal networks which develop across tidal basins characterized by different tidal, hydrodynamic, topographic, vegetational and ecological features. We first analyze the geometrical properties of tidal meanders, in terms of local curvature, typical meander wavelengths and relevant Fourier harmonics, channel width and depth, with indications on possible dynamic implications on their evolution. These features show important spatial variabilities, suggesting a locally adapted evolution which involves the morphological adjustment to the chief landforming events driven by local hydrodynamics. Theoretical and observational analyses of the drainage density of tidal networks are then presented. Similarity of forms from one location to another, suggested by the constancy, in space and time, of conventional measures of drainage density, appears to be quite in contrast with the apparent site-sp...
When a 2D model is used to compute the flood propagation over an initially dry area some difficul... more When a 2D model is used to compute the flood propagation over an initially dry area some difficulties arise due to the changes in the computational domain as large parts of it are flooded and dried during the phenomenon. A second problem concerns the representation of soil surface: usually it is described as a piecewise constant or a piecewise linearly varying surface hence neglecting the ground unevenness. For very small water depths, unevenness plays instead a very important role. In this paper, new dynamic and mass balance equations for 2D free surface flows are presented which, by means of a conceptual sub-model, account for the ground unevenness effects and allow a very realistic description of flooding and drying phenomena. It is also shown that for a very smooth ground surface or for relatively large water depths, the new equations reduce to the well known De S. Venant equations.
Field data and mathematical modeling have demonstrated that the morphological evolution of shallo... more Field data and mathematical modeling have demonstrated that the morphological evolution of shallow tidal basins is the result of the combined effect of tidal currents and wind waves. Tidal currents, in particular, drive the morphological evolution of shallow tidal systems in proximity of the inlets and within the channel network, whereas in shallow areas tidal current mainly acts enhancing the bottom shear stress due to wind waves and redistributing sediments within the basin. In this study we present a mathematical model for sediment entrainment, transport and deposition due to the combined effect of tidal currents and wind waves. The model is coupled with a hydrodynamic module based on the shallow water equations and with a module for the generation and propagation of wind waves. The sediment transport model describes the sediments by the way of a bi-granular mixtures composed by both cohesive and non-cohesive sediments thus considering the contemporary presence of clay, silt and ...
In this study we present a mathematical model for sediment entrainment, transport and deposition ... more In this study we present a mathematical model for sediment entrainment, transport and deposition caused by the combined action of tidal currents and wind waves in shallow micro-tidal basins. The model uses a bi-granular mixture made up of cohesive and non-cohesive sediments thus considering clay, silt and sand, all of which commonly characterize the sediment bed composition of estuaries and tidal basins. The model also describes the evolution of bed elevation and evaluates the variation of bed sediment composition distinguishing cohesive from non-cohesive behavior.A stochastic approach is proposed to evaluate sediment entrainment close to incipient sediment motion. Particular attention is also given to the problem of reconstructing a reliable initial bed composition as this has a significant impact on sediment entrainment.The model was applied to the test case of the Venice lagoon (Italy) and good agreement was found when comparing model results to a series of turbidity measurements...
An array of vertical cylinders in an otherwise uniform open channel flow can generate a seiche in... more An array of vertical cylinders in an otherwise uniform open channel flow can generate a seiche in the transverse direction due to the lift associated with vortex shedding behind each cylinder. Seiche amplitude attains a maximum at resonant conditions, i.e., when the vortex shedding frequency is close to the natural standing wave frequency. In this work we propose a model to predict maximum seiche amplitude based on the analytical solution to the linearized shallow water equations. The model prediction compares favorably with available experimental data from the literature outperforming pre-existing models. The model is also used to highlight that even small disturbances in the flow largely affect the dynamics and can dramatically reduce the wave amplitude.
SOMMARIO Il presente lavoro trae origine dalla formalizzazione di un criterio generale per la def... more SOMMARIO Il presente lavoro trae origine dalla formalizzazione di un criterio generale per la definizione dell'orientazione dei fronti d'onda. L'applicazione del criterio permette una migliore comprensione dei fenomeni di riflessione di fronti d'onda stazionari, in particolare per le riflessioni di von Neumann, di Guderley e di Vasilev. Si dimostra come, per i suddetti tipi di riflessione, sia possibile ricostruire agevolmente il campo di moto stazionario e la configurazione dei fronti d'onda in prossimità del punto triplo, per via numerica o sperimentale, avvalendosi del criterio suggerito per l'orientazione dei fronti. La forte analogia che lega i moti supersonici alle correnti supercritiche a superficie libera amplia inoltre gli orizzonti applicativi del presente lavoro, sviluppato con riferimento a correnti bidimensionali a superficie libera, all'ambito della gasdinamica.
Il disastroso evento di piena che ha colpito il Veneto il primo novembre 2010, particolarmente dr... more Il disastroso evento di piena che ha colpito il Veneto il primo novembre 2010, particolarmente drammatico per la città di Vicenza, ha reso evidente la necessità di disporre di metodi di previsione, soprattutto a breve termine, che siano efficaci strumenti di supporto alle decisioni. Infatti, nelle fasi concitate di un’alluvione che colpisce un’area urbana densamente popolata, è assai difficile mantenere una visione chiara dell’evoluzione dell’evento e, di conseguenza, stabilire e coordinare un piano per la gestione delle fasi dell’emergenza. Questa nota presenta un modello di previsione che adotta un metodo di tipo statistico basato sull’acquisizione in tempo reale dei dati misurati dalle stazioni idro-meteorologiche del bacino del Bacchiglione e delle previsioni di modelli meteorologici.
Uploads
Papers by Andrea Defina