Date Published: December 2, 2013
Publisher: Public Library of Science
Author(s): João Miguel Dias, Juliana Marques Valentim, Magda Catarina Sousa, Inés Álvarez.
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, through the interpretation of amplitude and phase patterns of the main tidal constituents, tidal asymmetry, tidal ellipses, form factor and tidal dissipation. Results show that Tagus estuary tidal dynamics is extremely dependent on an estuarine resonance mode for the semi-diurnal constituents that induce important tidal characteristics. Besides, the estuarine coastline features and topography determines the changes in tidal propagation along the estuary, which therefore result essentially from a balance between convergence/divergence and friction and advection effects, besides the resonance effects.
The water movements and the turbulent mixture that result from the tidal forcing express problems and interesting challenges in the hydrodynamic field. Knowledge of tidal heights and tidal currents structure is therefore essential to understand problems such as dispersion rate of pollutants, sediment transport and erosion processes in coastal areas . Moreover, tidal asymmetries strongly influence nutrient balances, sediment loads, particles and pollutants transportations, etc. . Thus, the understanding of the central processes lined by the tidal wave propagation seems to be crucial to obtain an overview concerning to the different uses of the coastal systems and in this particular study, of the Tagus estuary (Fig. 1), which dynamics is mainly ruled by the tidal forcing .
Covering an area of approximately 320 km2 and with a mean volume of 188 km3, the Tagus estuary is one of the largest estuaries of Europe and it is the most extensive wetland area of the Portuguese territory ( and ). Its width varies between 400 m at its head and 15 km in the central bay and has an average depth of 5.1 m .
Previous hydrodynamic modelling implementations for the Tagus estuary in general have considered barotropic models, since the estuary is well mixed or partially mixed, except in flood scenarios . Therefore, in this work, a modified version of SIMSYS2D (,  and ), based on the shallow water equations is used, expressing the mass and momentum conservation, and assuming incompressibility and hydrostatic pressure. In order to describe the relevant areas of tidal flats in this estuary, the model grid cells are allowed to dry/flood in a complex process described in . This model was recently used with success in the hydrodynamic study of several coastal areas, as: Ria de Aveiro lagoon (Portugal) , , , ,  and ; Lima estuary (Portugal)  and ; Tagus estuary (Portugal) , Patos lagoon (Brazil)  and Maputo bay (Mozambique) .
Due to the small dimensions and shallowness of Tagus estuary, its tidal properties are essentially driven by the oceanic tidal characteristics at its entrance and by changes and co-oscillation that results from tidal propagation along the estuary. Consequently, the analysis of changes in astronomical constituents along the estuary becomes essential to understand the factors ruling its tidal dynamics.
A two dimensional numerical model has been used to examine the tidal dynamics of Tagus estuary. The first step of this study revealed that the model accurately reproduces the tidal wave propagation in this estuary, so it can be considered successfully calibrated. Therefore, the tidal dynamics of Tagus estuary has been successfully resolved through the application of the SIMSYS2D model. It should be pointed out that the use of a 2D model assures a good reproduction of the estuarine tidal dynamics, even neglecting the 3D baroclinic effects. Model results indicate that estuary bathymetry and coastline features, namely the differences in morphology and depth from the lower to the upper estuary, play a key role in setting the circulation patterns found in the system. This influence is observed in major and shallow water tidal constituents, tidal asymmetry, tidal dissipation and tidal ellipses patterns.