A UAV-Based Dye-Tracking Technique to Measure Surface Velocities over Tidal Channels and Salt Marshes

An accurate description of hydrodynamic processes in coastal wetlands is needed to improve their management and conservation. As a consequence, higher knowledge of the connected morphological and ecologic processes is achievable. However, it is very costly to collect spatially distributed values of flow velocities over tidal channels and intertidal areas by means of in situ sensors. Also, when deploying sensors, humans perturb the ecosystem, which takes time to recover. In this study, a new low-cost unmanned aerial vehicle (UAV)-based method to measure surface velocities is proposed and validated. The study area is a salt marsh system on the southeast coast of Little Sapelo Island, Georgia, USA. Two unmanned aerial vehicles were used in the survey. A first UAV released a fluorescent dye tracer on marshes and tidal creeks, while a second one tracked its movement by collecting RGB images. Flow direction and magnitude were calculated from the images using a newly developed algorithm. A comparison with velocities measured with an acoustic Doppler current profiler confirmed the effectiveness of the method. Our results indicate that the calculated flow field is affected by vegetation, marsh morphology, and marsh width. In particular, a non-negligible velocity component perpendicular to the creek axes is detected both in ebb and in flood. Our technique proves to be an effective, non-intrusive, low-cost way to survey the two-dimensional hydrodynamics on salt marsh environments at a km scale. Collected data would be beneficial for calibrating and validating numerical models with accurate water flux information

Dynamics of River Mouth Deposits

Bars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion

Numerical Modelling of Alluvial Rivers by Shock Capturing Methods

The problem of modelling both the unsteady hydrodynamics and the bed morphological variations in natural channels is generally performed by solving the De Saint Venant balance equations for the liquid phase together with the Exner continuity equation for the sediments carried as bed-load. This thesis focuses on the development of an high-order accurate centred scheme of the finite volume type for the numerical solution of the coupled De Saint Venant-Exner system. A new scheme, called PRICE-C, is proposed. It solves the system of equations in a non-conservative form, however it has the important characteristic of reducing automatically to a conservative scheme if the underlying PDE system is a conservation law. It is applied to the shallow water equations in the presence of either a fix or a movable bed. The scheme is first introduced in a one-dimensional framework, and it is then extended to the two-dimensional case. The extension is not straightforward in the case of an unstructured mesh, since averages over suitable edge-based control volumes have to be performed. The scheme is extended to high order of accuracy in space and time via the ADER-WENO and MUSCL technique respectively for the one- and twodimensional case. The well-balanced property of the scheme is proven, i.e. the ability to reach steady states also in the presence of discontinuous water surface or discontinuous bottom profile. The scheme can deal with subcritical and supercritical flows, as well as transcritical situations. Moreover the proposed approach leads to a correct estimate of the celerity of surface discontinuities as well sediment bores and small bottom perturbations. The main characteristic of the scheme is its simplicity: it is based on a simple centred approach, that means that the knowledge of the eigenvalues of the matrix of the system is not required. This is important since the interaction between sediment transport and water flow not always admits detailed knowledge of the eigenstructure. Hence this scheme can be useful to engineers since they need simple numerical tools that can be easily used without entering in the mathematical detail of the differential hyperbolic system under consideration. Moreover the centred strategy gives generality to the scheme: in fact, it can be applied without modification to any kind of hyperbolic equations with non-conservative terms.La modellazione dell’idrodinamica e delle variazioni orfologiche in canali naturali `e generalmente effettuata risolvendo numericamente le equazioni delle onde lunghe in acque basse, che regolano il moto della fase fluida, assieme all’equazione di Exner, che descrive l’evoluzione del fondo. L’argomento della presente tesi consiste nello sviluppo di un schema ai volumi finiti di tipo ”centrato” per la soluzione accoppiata di tale sistema di equazioni. Un nuovo schema, denominato PRICE-C, `e qui introdotto: esso risolve le equazioni in forma conconservativa, ma ha l’importante propriet`a di degenerare in uno schema conservativo se il sottostante sistema di equazioni ammette una forma conservativa. Lo schema `e applicato alle equazioni delle onde lunghe in acque basse sia nel caso di fondo fisso che di fondo mobile, dapprima in un ambito unidimensionale e successivamente in quello bidimensionale. L’estensione non `e immediata nel caso in cui il reticolo di calcolo sia non-strutturato, dal momento che le equazioni differenziali devono essere mediate su opportuni volumi di controllo. Lo schema `e poi esteso ad alti ordini di accuratezza nello spazio e nel tempo attraverso le procedure ADER-WENO e MUSCL rispettivamente per il caso unidimensionale e bidimensionale. Inoltre si dimostra come lo schema proposto verifichi la ”well-balanced property”, che consiste nella capacit`a di raggiungere soluzioni stazionarie, anche in presenza di discontinuit`a della superficie libera e del fondo. Condizioni di corrente lenta e rapida, come pure condizioni di tipo transcritico vengono correttamente risolte. Inoltre lo schema in grado di riprodurre le celerit`a di propagazione di discontinuit`a della superficie e fronti di sedimenti al fondo, cos`? come la celerit`a di propagazione di piccoli disturbi del fondo. Caratteristica principale dello schema `e la sua semplicit`a: `e basato su un semplice approccio di tipo centrato, cio`e non necessita la conoscenza degli autovalori della matrice del sistema. Questa `e un’importante caratteristica dal momento che non sempre autovalori e autovettori sono calcolabili analiticamente, in particolare nel caso di complesse formule di chiusura per il trasporto al fondo. Quindi questo schema pu`o rivelarsi utile per l’ingegnere che spesso necessita di un semplice strumento numerico che possa essere applicato ad un sistema di equazioni differenziali di tipo iperbolico senza dover entrare nel dettaglio delle propriet`a atematiche del sistema stesso. Data la sua generalit`a, infatti, lo schema pu`o essere applicato ad ogni tipo di sistema iperbolico contenente termini non-conservativi

Long-term evolution and morphodynamic equilibrium of a tidal channel flanked by tidal flats

This contribution investigates the long term morphodynamic equilibrium of a movable bed tidal channel flanked by tidal flats. The two dimensional shallow water equations, the bed evolution equation, and the advection-diffusion equation for suspended-load are solved numerically. As far as the channel evolves, starting from a given straight configuration, with spatially constant depth and width, it turns out to be always ebb dominated near the inlet and flood dominated in the landward part. As a consequence, the net sediment flux within a tidal cycle is directed seaward near the inlet and landward in the remaining part of the channel, leading to sediment scour in the outer part of the channel and deposition in the inner part, where a sediment front propagates landward. The system evolves asymptotically toward an equilibrium configuration characterized by an approximately symmetric suspended-load transport rate during the tidal cycle. Moreover a nearly constant value of the maximum flood/ebb speed along the channel is observed, that implies a important morphologic adjustment toward a constant erosion potential in all the sections of the channel. The width of the channel is found to decrease exponentially toward the land. The numerical results show that tidal flats are progressively carved by channels, with a spacing varying between 200 and 1000 m. The relationship between inlet cross section of these channels and their watershed is approximately linear

A mass-conservative centered finite volume model for solving two-dimensional two-layer shallow water equations for fluid mud propagation over varying topography and dry areas

In this paper we develop a finite volume model to solve the two-dimensional shallow water equations governing the propagation of two superimposed layers, with the upper water layer carrying a dilute sed- iment suspension, and the underlaying layer being a high concentration non-Newtonian fluid mud mix- ture. The model formulation contains non-conservative terms as well as source terms. We propose a scheme able to deal with varying topography and dry areas, providing well-balanced solutions when both water and fluid mud are quiescent. The model is tested against both exact solutions and numerical exam- ples. The results show the ability of the model to deal with wetting and drying of both water and fluid mud layers, providing mass-conservative solutions. Moreover, the model solves discontinuities and steep fronts, computing accurate and oscillation-free solutions

A UAV-Based Dye-Tracking Technique to Measure Surface Velocities over Tidal Channels and Salt Marshes

An accurate description of hydrodynamic processes in coastal wetlands is needed to improve their management and conservation. As a consequence, higher knowledge of the connected morphological and ecologic processes is achievable. However, it is very costly to collect spatially distributed values of flow velocities over tidal channels and intertidal areas by means of in situ sensors. Also, when deploying sensors, humans perturb the ecosystem, which takes time to recover. In this study, a new low-cost unmanned aerial vehicle (UAV)-based method to measure surface velocities is proposed and validated. The study area is a salt marsh system on the southeast coast of Little Sapelo Island, Georgia, USA. Two unmanned aerial vehicles were used in the survey. A first UAV released a fluorescent dye tracer on marshes and tidal creeks, while a second one tracked its movement by collecting RGB images. Flow direction and magnitude were calculated from the images using a newly developed algorithm. A comparison with velocities measured with an acoustic Doppler current profiler confirmed the effectiveness of the method. Our results indicate that the calculated flow field is affected by vegetation, marsh morphology, and marsh width. In particular, a non-negligible velocity component perpendicular to the creek axes is detected both in ebb and in flood. Our technique proves to be an effective, non-intrusive, low-cost way to survey the two-dimensional hydrodynamics on salt marsh environments at a km scale. Collected data would be beneficial for calibrating and validating numerical models with accurate water flux information

One-dimensional numerical modeling of the long-term morphodynamic evolution of a tidally-dominated estuary: The Lower Fly River (Papua New Guinea)

We use a one-dimensional morphodynamic model to analyze the long-term evolution of the lower reaches of the Fly River, Papua New Guinea, from the Everill Junction to the delta mouth. The model shows how the break in the exponential trend of river width triggers deposition, thus producing a tidal region characterized by a higher bed elevation with respect to the river-dominated one. Numerical simulations indicate that the river attains a dynamic equilibrium configuration in which the amount of sediment entering upstream is flushed seaward. A sensitivity analysis is performed, in which the effect of varying solid discharge, tidal har- monics, and initial conditions is discussed. The model shows that an equilibrium configuration results from a delicate balance between the aggrading effect associated with channel divergence (acting mainly during neap tide and at slack water) and the opposite effect of tidal flushing driven by residual water discharge. A physically meaningful morphodynamic equilibrium occurs only for a small range of values of sediment dis- charge prescribed at the upstream boundary. In particular, an increase in sediment discharge leads to aggra- dation, while a decrease triggers extensive scour and a deepening of the estuary

Finite volume modelling of a stratified flow with the presence of submerged weirs

The 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

Tidal hydrodynamics and erosional power in the Fly River delta, Papua New Guinea

A two\u2010dimensional numerical model is used to study tidal hydrodynamics and distribution of bed shear stresses in the Fly River delta, Papua New Guinea. The model describes the propagation of the tidal wave within the delta and along the river. Model results indicate that tidal discharge at the mouths of the distributary channels is between 10 and 30 times larger than the river discharge, and that the upstream part of the delta is flood\u2010dominated, whereas near the mouth, the delta is ebb\u2010dominated. Numerical simulations allow us to investigate the sensitivity of fluxes and bottom stresses with respect to the variations of sea level and the area of delta islands. The results suggest that a decrease in the total area of the delta occupied by islands increases the tidal prism and, therefore, the bed shear stresses. Similarly, an increase in sea level reduces the dissipation of the tidal signal and speeds up the propagation of the tidal wave within the delta, thus yielding higher discharges and increased bed shear stresses