Wave-Forced Dynamics at Microtidal River Mouths

Microtidal river mouths are dynamic environments that evolve as a consequence of many forcing actions. Under the hydrodynamic viewpoint, river currents, sea waves and tides strongly interact, and their interplay determines specific sediment transport and morphological patterns. Beyond literature evidence, information comes from field observations made at the Misa River study site, a microtidal river along the Adriatic Sea (Italy), object of a long-going monitoring. The river runs for 48 km in a watershed of 383 km2, providing a discharge of about 400 m3/s for return periods of 100 years. The overall hydrodynamics, sediment transport and morphological evolution at the estuary are analyzed with particular attention to specific issues like: the generation of vortical flows at the river mouth, the influence of various wave modes (infragravity to tidal) propagating upriver, the role of sediment flocculation, the generation and evolution of bed features (river-mouth bars and longitudinal nearshore bars). Numerical simulations are also used to clarify specific mechanisms of interest

On the hydro-morphodynamics of river mouths: the role of waves

Nel presente lavoro di tesi viene proposta un’analisi numerica del ruolo delle forzanti ondose sui processi idrodinamici, morfodinamici e di generazione di vorticità in ambienti estuarini semplificati. Allo scopo è stato predisposto un apposito modello numerico per la risoluzione dell’idro-morfodinamica costiera ed estuarina in regime di acque basse. Il modello è basato su un solutore pre-esistente con struttura ai volumi finiti sviluppato da Brocchini et al. (2001), ed è stato completato da un modello di estuario, per poter contemplare l’inclusione di estuari semplificati in una porzione limitata del bordo del dominio di calcolo. L’input di estuario viene fornito al solutore sotto forma di serie temporale di elevazione del livello idrico e/o di velocità del fluido. È stato inoltre sviluppato un semplice e flessibile generatore di griglie di calcolo, il quale crea mesh a partire da dataset ottenuti da rilievi batimetrici reali, in modo da riprodurre fedelmente batimetrie reali per le successive analisi numeriche. Le performance del modello di estuario è stata valutata attraverso due set di analisi numeriche. Con il primo set di test sono stati riprodotti gli aspetti più rilevanti dell’interazione onda-corrente, dei pattern morfodinamici e della generazione di vorticità in un estuario semplificato, sulla base della campagna di esperimenti numerici intrapresi da Olabarrieta et al. (2014). Il secondo set di test ha invece riguardato la riproduzione dell’evoluzione morfologica dell’estuario del fiume Misa, a Senigallia. Il modello numerico si è rivelato globalmente in grado di rappresentare le caratteristiche salienti dei flussi in esame; tuttavia, la capacità del modello di descrivere i processi di trasporto dei sedimenti e la conseguente evoluzione morfodinamica di casi reali necessita di ulteriori sviluppi.In the thesis a numerical analysis of the role of wave forcing on the hydrodynamics, sediment transport and vorticity patterns in idealized inlets and estuary mouths is proposed. A novel solver for the hydro-morphodynamics of shallow flows in presence of an estuary is described. The solver builds upon a previous robust finite-volume model for shallow waters by Brocchini et al. (2001) and is complemented by a suitable routine for the inclusion of simplified estuarine inlets at a certain portion of the computational domain boundary. The estuarine input is imposed at all nodes of the numerical inlet as a prescribed time series of water elevation and/or fluid velocity. A simple and robust mesher for the generation of computational meshes from bathymetric surveys datasets has also been devised, in order to represent real-case bottom profiles for use in numerical simulations. The performance of the estuarine solver is tested against two sets of numerical trials. The first set of tests aims at reproducing the structure of wave-current interaction, morphodynamic patterns and vorticity at an idealized inlet configuration, in similarity of the numerical campaign performed by Olabarrieta et al. (2014). With the second type of tests the morphological evolution at the estuary of the Misa River, Senigallia, Italy is reproduced. The model is generally capable of describing the main features of the investigated flows, although the capability of modelling sediment transport and the related morphological variations of real case scenarios need some improvement

From Boussinesq-Type to Quasi-3D Models: A Comparative Analysis

By comparing model results from tests of steady and oscillatory flows over a submerged bar (of interest for both riverine and marine environments), we highlight similarities and differences between a state-of-the-art Boussinesq model and a recently developed non-hydrostatic, quasi-three-dimensional (3D) model. To make the comparison as clean as possible, the complexities due to breaking-induced turbulence are avoided by suitably tuning the flow strength. Although the main flow features are similarly described by the two models, the non-hydrostatic model predicts the occurrence of extra, small-scale stationary oscillations on top of a submerged obstacle in the case of a steady current. This is attributed to a pattern of alternating upward and downward vertical velocity over the sill top, presumably due to rapidly converging flows as they climb over the submerged obstacle. The non-hydrostatic model overall allows for a more complete representation of flow dynamics in the vertical direction, with respect to the Boussinesq-type solver. This becomes necessary when tackling fundamental and applicative problems characterized by intense vertical flows (e.g., interactions of fluids with structures)

Wave‐ and Tide‐Induced Infragravity Dynamics at an Intermediate‐To‐Dissipative Microtidal Beach

Numerical simulations of a range of wave climates and tide conditions made with the model XBeach are exploited to study propagation, evolution, dissipation, and reflection patterns of infragravity waves (IGW) at the intermediate-to-dissipative beach of Sabaudia (Tyrrhenian sea, Italy). On the basis of a novel calibration process performed using field swash properties collected by a nearby monitoring station, the model reproduces average run-up characteristics with good skill (Willmott index of agreement of 0.61 for R2%). IGW at Sabaudia beach evolves exclusively as bound long waves growing across the shoaling region for both mild and intense wave climates. Furthermore, values of the dimensionless bed slope βH at which transition from steep-to mild-slope behavior occurs are higher than the threshold commonly assumed in the literature. Finally, although the small tide (0.4 m between mean high water spring and mean high water spring; tide range/breaking wave amplitude ratios between 0.14 and 1.1) does not alter bulk IGW reflection significantly, low tide is effective in reducing onshore IGW fluxes and, ultimately, reflection coefficients R2 selectively for intermediate IG frequencies. Our study, for the first time, gives clear evidence that the tide has a role in determining frequency-dependent IGW dynamics and altering the dissipative state of a mild sloping beach also in a microtidal environment

Waves and Currents at a River Mouth: The Role of Macrovortices, Sub-Grid Turbulence and Seabed Friction

Numerical experiments of wave-current interaction have been performed to investigate the evolution and dissipation of horizontal large-scale vortical structures generated by differential wave breaking and current shearing at river mouths. Specific focus is on the role played by turbulence of scales smaller than the water depth and by seabed friction on the dissipation of the mentioned macrovortices. The analysis reveals two regions of turbulence generation: at the river mouth, and along the curved seaward boundary of the shoal. In the latter zone, macrovortices are formed due to differential wave breaking induced by the sudden variation in water depth and enhanced by opposing currents which favour wave steepening. Such vortices are then advected towards the shore. Among the dissipative mechanisms, dissipation induced by seabed friction is deemed dominant, in particular in the most shallow waters of the shoal. Sub-grid turbulence dissipation, conversely, is more efficient offshore, although exerting some effect also over the shoal when supported by the continuous action of waves

ANALYSIS OF MIXING AT RIVER MOUTHS: THE ROLE OF MACROVORTICES AND SUB-GRID TURBULENCE

INTRODUCTION A good understanding of flow mixing is important for properly describing the morphodynamics of the nearshore and river mouths. In recent years the modeling of flow mixing in natural shallow water regions has seen improvement thanks to the increasing computational power and the development of reliable and feasible models for large eddies and sub-grid turbulence. We here investigate the interplay between macrovortices, i.e. large-scale vortices with vertical axes, typical of shallow water flows, and the sub-grid turbulence characterizing river mouth flows, where a river current interacts with incoming sea waves. Generation of macrovortices is ensured by differential breaking at an estuarine shoal, whose presence also allows for inspection of the role of complex bathymetries (Olabarrieta et al., 2014). To this end, numerical simulations of wave-current interaction are performed by means of a solver for wave propagation in shallow water by Brocchini et al. (2001), integrated with a simple Horizontal Large Eddy Simulation (HLES) model proposed by Grosso et al. (2007). THE NUMERICAL MODEL The numerical model used for the tests (Brocchini et al., 2001) is based on the Nonlinear Shallow Water Equations (NSWE) and implements the finite-volume Weighted Averaged Flux method. The model is modified to give account of the dynamics induced by a river discharging at sea, and allows for computation of the morphological evolution of the bottom through the weak hydro-morphodynamic coupling proposed by Postacchini et al. (2012). The model also implements a simple HLES model to consider, in an approximate fashion, the dissipative terms due to sub-grid turbulence, which would, otherwise, be neglected by a classic approach of the NSWE (Grosso et al., 2007). THE TESTS Similarly to the wave-current interaction tests performed in the work of Olabarrieta et al. (2014), we reproduce the interaction of a steady river jet with weak-to-strong waves over a bathymetry characterized by a large river mouth shoal, and the generation, organization and migration of macrovortices as function of the sub-grid turbulence (HLES eddy viscosity varied between 0 and 0.1). THE RESULTS The numerical simulations reveal a significant generation and offshore advection of macrovortices at the seaward edge of the mouth shoal. Important turbulent structures are also observed, during the initial stages of the motion, at the corners of the inlet mouth, where the canalized, intense river current spreads over the shoal and interacts with the quiescent sea waters; these transient vortices dissipate fairly rapidly. The activation of the HLES model does not seem to have significant effects on the transient inlet vortices, causing only a limited decrease in their intensity as the sub-grid turbulence increases. Conversely, the shoal macrovortices are more sensible to changes in sub-grid turbulence: their intensity sensibly decreasing as the eddy viscosity increases. Notably, use of eddy viscosity parameters of the order 10-1 suppresses the generation of macrovortices in the shoal zone. Further considerations on these mechanisms will be illustrated at the conference

Harbour Hydro-Morphodynamics and Freshwater Discharges: The La Spezia Arsenale Case

The hydrodynamics and sediment transport at the microtidal harbour of La Spezia Arsenale (Liguria, Italy) were studied through a numerical approach, with the objective of providing useful information for: (1) the understanding of the hydro-morphodynamics of microtidal harbour settings and (2) the operation management and planning for the Arsenale, the pivotal harbour for the Italian Navy. Three different scenarios were used to parametrically gain knowledge on the role of extreme (100-year return period) meteomarine forcing. FUNWAVE and Delft3D were used to simulate, respectively, the wave propagation from the open sea toward the Arsenale and the influence of two freshwater streams on the basin circulation. The first scenario was aimed at understanding the effect of wind waves and swell on the basin dynamics; the second scenario was set up to inspect the role of the rivers’ discharges on the Arsenale hydro-morphodynamics; the third scenario combined all of the above forcings. All the simulations also included the tidal forcing and were run under two different wind directions. We found that the hydrodynamics inside the Arsenale were mainly influenced by the tide and the wind; the former caused the water to enter/exit the basin during the flood/ebb phases, respectively, and the latter influenced the circulation cell, whose sense of rotation depended on the wind direction. In addition, the discharge of the Lagora stream, debouching into the sea close to the Arsenale entrance, partially entered the basin and created an eddy whenever its direction was opposite to that of the wind-forced circulation cell, while the Caporacca stream, flowing directly into the Arsenale, mainly fed the dominant circulation without altering it. On the other hand, the morphodynamics were strongly affected by the rivers’ discharges, which were solely responsible for the supply of sediment to the basin. Also, the major influence on the sediment transport was exerted by the rivers and the wind forcing. Small sedimentation rates were observed in the Arsenale close to the rivers’ mouths, particularly after the occurrence of the rivers’ discharges, while no siltation due to waves took place. This study evaluated for the first time the influence of freshwater streams flowing nearby/into the Arsenale, representative of semi-enclosed microtidal ports located in the vicinity of rivers. It was found that the contribution of the rivers to the hydro-morphodynamics of the Arsenale cannot be neglected; indeed, it represents one of the main forcings of the harbour dynamics and should, therefore, be considered from a management viewpoint

Steady streaming under a surface wave propagating over a rough bottom: A model of the bottom boundary layer

The steady streaming generated by nonlinear effects at the bottom of a propagating surface wave is determined when the bottom is characterized by a roughness, the size of which scales with the boundary layer thickness. Therefore, the cornerstone contribution by Longuet-Higgins, who considered a smooth bottom, is extended to sea waves and sandy bottoms characterized by a grain size that ranges from fine silt to fine gravel. For values of the grain size d* up to 0.05 delta* , delta* being the thickness of the viscous bottom boundary layer, the velocity profile is practically coincident with that predicted by Longuet-Higgins. If the grain size is further increased, the steady velocity component becomes larger and reaches a maximum value that is approximately 70% larger than that predicted by Longuet-Higgins. The maximum of the steady velocity component is attained for d* = 0.6 delta*. A further increase in d* leads to a decrease in the steady velocity component that, however, keeps always larger than that predicted for a smooth bottom. As the roughness size increases up to the values typical of medium sand, the steady velocity component increases. Then, a further increase in the roughness size leads to a decrease in the steady streaming even though, in the range of the roughness size presently investigated, the steady velocity component is always larger than that predicted for a smooth bottom

Monitoring for Coastal Resilience: A Project for Five Italian Beaches

This paper presents the COPEMAP project, fruit of a research collaboration between the Università Politecnica delle Marche and the Istituto Superiore per la Protezione e la Ricerca Ambientale. The project focuses on developing a methodology for the assessment of beach resilience as a function of both natural forcing (e.g. incoming wave fields) and natural protection (e.g. underwater sand bars). This is accomplished through simplified analyses of available video-monitoring data

Wave-Current Interactions and Infragravity Wave Propagation at a Microtidal Inlet

Recent studies have shown that wave blocking occurs at river mouths with strong currents typically preventing relatively short period sea and swell waves from propagating up the river. However, observations demonstrate that lower frequency waves, so-called infragravity waves, do pass through and propagate up the river, particularly during storm events. We present observations from the Misa River estuary of infragravity wave propagation up the river during storm conditions. A model of the complex nonlinear interactions that drive infragravity waves is presented. The results are discussed in the context of an observed river mouth bar formed in the lower reach of the Misa River