Wave-current interactions and infragravity waves at the Tagus Estuary mouth (Portugal) (original) (raw)
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A numerical study of local variations in tidal regime of Tagus estuary, Portugal
PloS one, 2013
Tidal dynamics of shallow estuaries and lagoons is a complex matter that has attracted the attention of a large number of researchers over the last few decades. The main purpose of the present work is to study the intricate tidal dynamics of the Tagus estuary, which states as the largest estuary of the Iberian Peninsula and one of the most important wetlands in Portugal and Europe. Tagus has large areas of low depth and a remarkable geomorphology, both determining the complex propagation of tidal waves along the estuary of unknown manner. A non-linear two-dimensional vertically integrated hydrodynamic model was considered to be adequate to simulate its hydrodynamics and an application developed from the SIMSYS2D model was applied to study the tidal propagation along the estuary. The implementation and calibration of this model revealed its accuracy to predict tidal properties along the entire system. Several model runs enabled the analysis of the local variations in tidal dynamics, ...
Wave–current interactions in an idealized tidal estuary
Journal of Geophysical Research, 2003
1] We extended a recently developed model for wave-current interactions by Huang [1996a, 1996b] to simulate the impact of topography, currents, and slanting coastlines on sea state estimates. Our formulation uses the action conservation equation and the nonlinear dispersion relation, resulting in additional flux terms, @(c w A)/@w, A@c gl / @l, A@(c gf cos f)/@f, and A@c gq /@q, compared to ''standard'' wave models such as WAM, where A is spectral action density, c w is phase velocity, and c gl , c gf , and c gq are group velocities. For large-scale motions in deep water without varying currents, these effects may be neglected. However, for shallow estuary waters with varying currents, we show that these effects can cause as much as 25% variation in wave height estimates during moderate wind conditions. This phenomenon is consistent with observations and theory, for example, regarding isolated topography such as seamounts.
Infragravity-wave dynamics in a barred coastal region, a numerical study
Journal of Geophysical Research: Oceans, 2015
This paper presents a comprehensive numerical study into the infragravity-wave dynamics at a field site, characterised by a gently-sloping barred beach. The non-hydrostatic wave-flow model SWASH was used to simulate the local wave field for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyse the infragravity-wave dynamics at spatial scales not often covered before. Overall, the model predicted a wave field that was representative of the natural conditions, supporting the model application to analyse the wave dynamics. The infragravity-wave field was typically dominated by leaky waves, except near the outer bar where bar-trapped edge waves were observed. Relative contributions of bar-trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity wave growth was partly explained by nonlinear energy transfers from short-waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short-waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravitywave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the model's potential to study wave dynamics at field scales not easily covered by in-situ observations. Second, infragravity waves can break and lose most of their energy in a region close to the shore [van Dongeren et al., 2007; de Bakker et al., 2014 de Bakker et al., , 2015. Third, infragravity waves can lose energy due to bottom friction, although this mechanism is mainly significant in the case of extensive shallow regions such as coral reefs Van Dongeren et al., 2013].
Modelling of the wave-current interactions in the Tagus Estuary
A study of the wave propagation and of the consequences of the wave current interactions in the Tagus estuary is performed in the present work. Thus a couple of low tide and high tide subsequent situations, corresponding to an extreme energetic case, were considered. A Cartesian high-resolution SWAN grid was coupled to the wave prediction system developed in spherical coordinates. Using the ocean forcing this system provides the corresponding coastal transformation of the wave field. By introducing the Cartesian simulations, as a final extent of the system, some local processes as wave induced setup and diffraction can be also studied, and their relevance for this particular area is discussed. A balance between the accuracy of the results and the computational effectiveness was made by assessing the opportunity of using non stationary versus stationary simulations. However, the most important factors affecting the incoming waves are local currents and wind and their influences were evaluated and discussed.
Relevance of infragravity waves in a wave-dominated inlet
Journal of Geophysical Research: Oceans, 2016
This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave-dominated inlet located on the Western Coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low-frequency oscillations (i.e. 25 to 300 s) in water levels and current velocities. While these fluctuations were present over the ebbtidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to two hours after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m.s-1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100 % of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions. KEYWORDS Infragravity waves, wave-dominated inlet, low frequency waves, wave blocking. Keypoints Measurements revealed energetic IG waves in a wave-dominated inlet. Breakpoint generation and bound-wave shoaling both contributed significantly to IG wave generation. IG waves were blocked by opposing ebb-currents. 1. INTRODUCTION Tidal inlets are transition zones between the ocean and back-barrier lagoons or estuaries where constant exchange of water, sediments, nutrients and larvae occurs. Hydrodynamics and morphological changes in tidal inlets have strong ecological and socioeconomic repercussions, since they affect navigation safety, flooding extent, erosion of adjacent beaches, water renewal and material exchange between the lagoon (orestuary) and the open ocean. Due to the complex interactions between tides, wind waves, river outflows, sediments transport and morphology, understanding the resulting hydrodynamics and sediment transport patterns is still a challenge.
Evolution of the hydrodynamics of the Tagus estuary (Portugal) in the 21 st century
The ongoing rise in sea level affects tidal propagation and circulation in estuaries, and these changes can have far reaching consequences on the sediment dynamics, water quality and extreme water levels. This study aims at anticipating the evolution of the tidal dynamics in the Tagus (Portugal) in the 21 st century, in particular due to sea level rise (SLR). The existence of a resonance mode of about 8 hours in this estuary, that selectively amplifies both semi-diurnal and quarter-diurnal tidal constituents, makes the response of the Tagus estuary to SLR unique. The study was conducted with a shallow water model, forced by present and future conditions, namely higher mean sea levels and an extrapolated bathymetry based on present sedimentation rates. Model results showed that SLR will significantly affect tidal asymmetry, in particular because the intertidal area can decrease by up to 40% by the end of the 21 st century. As a result, the strong ebb-dominance of this estuary will decrease significantly. This evolution of tidal asymmetry will be counteracted by the effect of sedimentation of the salt-marsh areas. Also, SLR will enhance the resonance in the Tagus estuary. As a consequence, extreme water levels will be higher than the sum of present levels with the SLR.
Journal of Marine Science and Engineering, 2021
Barred macrotidal beaches are affected by continuous horizontal displacements of different hydrodynamic zones associated with wave transformation (shoaling, breaker and surf zones) due to significant tide-induced water level changes. A series of wave and current meters, complemented by a video imagery system, were deployed on a barred beach of northern France during a 6-day experiment in order to characterize the spatial and temporal variability of wave-induced processes across the beach. Wave and current spectral analyses and analyses of cross-shore current direction and asymmetry resulted in the identification of distinct hydrodynamic processes, including the development of infragravity waves and offshore-directed flows in the breaker and surf zones. Our results revealed a high spatial variability in the hydrodynamic processes across the beach, related to the bar-trough topography, as well as significant variations in the directions and intensity of cross-shore currents at fixed l...
Modelling the influence of currents on wave propagation at the entrance of the Tagus estuary
Ocean Engineering, 2011
A study of the wave propagation and of the consequences of the influence of currents on waves in the Tagus estuary is performed in the present work. For this purpose a high-resolution SWAN domain was coupled to a wave prediction system based on the two state of the art phase averaged wave models, WAM for wave generation and SWAN for nearshore wave transformation. The most important factors affecting the incoming waves are the local currents and the wind. These influences were evaluated by performing SWAN simulations in the target area with and respectively without considering the tide level and tide induced currents. The model results were compared with wave measurements, validating in this way the results of the wave prediction system developed herewith.
Deep Sea Research Part I: Oceanographic Research Papers, 2007
From ADCP measurements in the thermocline over the continental slope of the Bay of Biscay the vertical variation of the contribution of the inertio-gravity waveband to the kinetic energy and variance of the current shear was analysed. The semi-diurnal tides together with near inertial waves appeared to provide over 70% of the high-frequency kinetic energy (> 1 / 3 cpd). Over the vertical range of the ADCP bins, about 400 M, the phase of the M2 tide changed up to 155°, showing the importance of the contribution of internal waves to the observed tidal motion. Both semi-diurnal internal tidal waves and near-inertial waves were organized in wave beams with a limited vertical extent, probably about 50 to 60% of the vertical wavelength. The relatively large shear in the inertio-gravity wave band, supported an annual mean gradient Richardson number well below 1, and was probably capable of maintaining turbulent mixing for a large part of the time.
Journal of Marine Systems, 2021
The alongshore subtidal water circulation along the South Portugal inner shelf is characterized by the temporal alternation of equatorward (i.e., broadly eastward) flows related to coastal upwelling processes and poleward (i. e., broadly westward) Coastal Counter Currents (CCCs). The objective of this study is to get insights about the main drivers of CCCs based on kinematic parameters describing the structure of the flow at the moment it changes direction. The parameters are derived from an extensive bottom-mounted ADCP dataset (16 deployments; 34,121 hourly records) collected at a single mooring (23 m water depth). Results show that the socalled incipient flows present contrasted general patterns whether they turn from equatorward to poleward or the opposite. Complementary observations at a nearby station indicate that these characteristics are spatially consistent along the studied area. Although 70% of CCCs are generated under favourable wind conditions (Levanter), these flows generally develop through the bed layer, in particular in summer. Hence, the Levanter wind-expected to promote flow setup through the surface layer-is not the main driver of CCCs in most cases. The general structure of incipient CCCs strongly suggests that the dominant force competing with the wind stress is an alongshore pressure gradient (APG). Furthermore, the maximum equatorward flow magnitude before CCCs setup is significantly correlated with the following (poleward) acceleration of incipient CCCs near the bed. Such relation is consistent with the development of CCCs due to the unbalance of an APG (produced during active upwelling) when wind relaxes. This process is further supported by an analysis of the depth-averaged momentum equation which suggests that the coastal circulation is mainly driven by linear dynamics in the region.