André B. Fortunato

On the effect of tidal flats on the hydrodynamics of the Tagus estuary

Abstract The effects of tidal flats on the hydrodynamics of the Tagus estuary are analysed using an accurate high-resolution shallow water model, supported by field data. Tidal flats act mainly as energy filters, rather than sinks, transferring energy from astronomic to non-linear frequencies. They also play a major role in determining the strong resonance mode that amplifies semi-diurnal constituents. We show that the upper estuary, with extensive tidal flats, has an increasing accretion rate. This “filling-up” results from a positive feedback between sediment deposition and the hydrodynamics: accretion in the upper estuary lengthens the duration of ebbs, decreasing sediment flushing and further increasing accretion.

Tidally generated shelf waves off the western Iberian coast

A high-resolution finite element model of barotropic tides in the Iberian Atlantic shelf is presented. The model accurately reproduces available tidal data, with average amplitude discrepancies at 12 Atlantic stations below 3% and 15% for semi-diurnal and diurnal tides, respectively. The accuracy of the model simulations is supported also by a qualitative comparison with velocity data. The model results are used to examine the generation of shelf waves by diurnal tides off the western Portuguese coast. Shelf waves in this area are responsible for large diurnal currents, which can exceed semi-diurnal currents by an order of magnitude. Numerical experiments show that these shelf waves can be generated by obstacles perpendicular to the coastline, and are enhanced in the presence of wide and shallow shelves and steep continental slopes. Along the Portuguese coast, the Tagus plateau is responsible for most of the shelf wave generation.

A three-dimensional model of tidal currents in the mouth of the Tagus estuary

Abstract Three-dimensional simulations of barotropic tides are used to investigate the complex circulation at the mouth of the Tagus estuary. Comparison with field data shows that elevations are well represented in the main area of interest, but velocities are slightly over-predicted due to a simplified treatment of intertidal flats. Model results show the existence of strong velocity phase lags (up to 1.75 hours for the semi-diurnal constituents) between the shallow areas and the deep channel. These phase lags are partially responsible for the generation by advective accelerations of a strong residual velocity field (velocities reach 0.5 m s −1 ) with well defined eddies. The interaction between the residual and tidal velocity fields in turn generates strong chaotic stirring. Localized sigma coordinates (LSC), a recently proposed method which allows the number of nodes per vertical to vary horizontally, are used for the first time in a three-dimensional application. A previously proposed criterion for the horizontal distribution of the total number of vertical nodes is shown to be inadequate when advective accelerations are important. However, with an alternative criterion, the use of LSC reduces maximum errors by a factor of two relative to traditional sigma coordinates.

A modeling system for tidally driven long-term morphodynamics

A modeling system to simulate tidally driven morphodynamics is developed and assessed. The system couples an existing shallow water model with a new morphological updating model. Four innovative features to improve the accuracy and efficiency of the system are implemented and tested. First, two alternatives are proposed to update the velocity field without running the hydrodynamic model. Both alternatives, based on an assumption on the behavior of friction, are more accurate than the traditional “continuity correction”. Second, hydrodynamic results are provided in the frequency domain. The harmonic synthesis of the velocity time series simplifies interpolation and extrapolation of the hydrodynamic model results and the use of different time steps for the various components of the modeling system. Third, the time-integration of the sediment fluxes is performed with a Runge-Kutta method so the time step adapts in space and time to the flow characteristics. Finally, a criterion to determine the need for a ne...

Numerical modeling of the impact of the Ancão Inlet relocation (Ria Formosa, Portugal)

This work describes the application of hydrodynamic (ELCIRC) and transport (VELA and VELApart) models to the Ria Formosa lagoon (Portugal) to study the impact of the relocation of the Ancao Inlet. Located in the south of Portugal, this lagoon is a mesotidal barrier island system that communicates with the sea through 6 inlets. The Old Ancao Inlet was artificially closed and the New Ancao Inlet was relocated into a westward position. This work investigates the hydrodynamic patterns and the potential pathways of tracers in Ria Formosa in two distinct configurations: before and after the Ancao Inlet relocation. The hydrodynamic model was successfully calibrated and validated against elevation, velocity and inlet discharges data, accurately reproducing the tidal propagation. The inlet relocation increases the magnitude of tidal currents, residual velocities and the tidal prism across the bar, suggesting a better stability. The tracers transport simulations suggest enhanced water exchanges through the Ancao Inlet and smaller residence times in the western part of Ria Formosa with the present configuration. Overall, it is concluded that the Ancao Inlet relocation had a positive contribution towards increasing the water renewal of the western part of the lagoon, thus decreasing its vulnerability to pollution.

Influence of intertidal flats on tidal asymmetry

Abstract The generation of tidal asymmetries in estuaries is analyzed by examining the differences in celerity at high and low tide. An analytical expression for the difference between ebb and flood durations is introduced and applied to a cross section composed of a channel flanked by tidal flats. The expression depends on the cross-sectional average of the celerity, for which two approximations are proposed and verified with a numerical model. Results confirm that tidal flats enhance ebb dominance, whereas large tidal amplitudes promote flood dominance. Maximum ebb dominance occurs for tidal flats at or above mean water level, depending on tidal amplitude and the extent of the tidal flats. Finally, qualitative arguments indicate that friction also enhances flood dominance.

A cross-scale numerical modeling system for management support of oil spill accidents

A flexible 2D/3D oil spill modeling system addressing the distinct nature of the surface and water column fluids, major oil weathering and improved retention/reposition processes in coastal zones is presented. The system integrates hydrodynamic, transport and oil weathering modules, which can be combined to offer different-complexity descriptions as required by applications across the river-to-ocean continuum. Features include accounting for different composition and reology in the surface and water column mixtures, as well as spreading, evaporation, water-in-oil emulsification, shoreline retention, dispersion and dissolution. The use of unstructured grids provides flexibility and efficiency in handling spills in complex geometries and across scales. The use of high-order Eulerian-Lagrangian methods allows for computational efficiency and for handling key processes in ways consistent with their distinct mathematical nature and time scales. The modeling system is tested through a suite of synthetic, laboratory and realistic-domain benchmarks, which demonstrate robust handling of key processes and of 2D/3D couplings. The application of the modeling system to a spill scenario at the entrance of a port in a coastal lagoon illustrates the power of the approach to represent spills that occur in coastal regions with complex boundaries and bathymetry.

Evaluation of horizontal gradients in sigma-coordinate shallow water models

Abstract Evaluating horizontal gradients in three‐dimensional shallow water models that use bottom‐following sigma coordinates can lead to large errors near steep bathymetry. A technique that has been proposed to minimize this problem involves computing horizontal gradients in cartesian coordinates, while treating all other terms in a sigma coordinate framework. We study this technique through both truncation error analysis and numerical experimentation, and compare it to the direct application of sigma coordinates. While the Cartesian coordinate method has better convergence properties and generally smaller truncation errors when horizontal gradients are zero, the sigma coordinate method can be more accurate in other physically relevant situations. Also, the Cartesian coordinate method introduces significant numerical diffusion of variable sign near the bottom (where physical diffusion is particularly small), thus potentially leading to instabilities. Overall, we consider the sigma coordinates to be the ...

NUMERICAL MODELING OF THE AVEIRO INLET DYNAMICS

The dynamics of the Aveiro lagoon is assessed through a combination of field data and numerical modeling. An unstructured grid hydrodynamic model is set-up for this system for the first time and its results compare well with data and with a wellestablished model. The model and the data are then used to analyze the propagation of the tide in the lagoon and the variability of tidal asymmetry in the upper and lower lagoon. The lagoon shifts from mild ebb-dominance at the inlet to strong flooddominance in the upper lagoon. This variability may be responsible for the importing capacity suggested by the sediment particle model, estimated at 10% of the littoral drift. On the contrary, a preliminary application of the morphodynamics modeling system MORSYS2D suggests a net export capacity. These opposite results maybe due to the tidal excursion of sediments in the system, which is much larger than the scale of variability of the hydrodynamics.

Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios

ABSTRACT Rilo, A., Freire, P., Guerreiro, M., Fortunato, A.B., Taborda, R. 2013. Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios. Sea level rise (SLR) is increasing the risk of flooding in marginal low-lying estuarine areas. High water levels in estuaries are associated with the simultaneous occurrence of various phenomena ranging from high tidal levels to large fresh-water discharges. The intense and diverse human occupation of some of these marginal areas increases the flood vulnerability, endangering human lives and causing heavy economic and environmental losses. This issue becomes even more relevant since the European Floods Directive establishes mandatory guidelines to all member states regarding flood risk assessment, integrating flood maps with territorial characteristics. The present study aims to assess the vulnerability to floods of a specific area in Tagus estuary, by integrating hydrodynamic modelling results with land use cartography. Results suggest that about 16.1% of the estuarine marginal fringe could be flooded for the 2050 scenario, rising up to 23.7% for the 2100 scenario. Moreover, industrial and urban areas will be the most affected sites. Considering future development scenarios planned for that site, the results point to an increasing vulnerability of urban areas and therefore to the relevance of this knowledge in future planning and management guidelines as well as measures to adapt to SLR. The uncertainties and implications of the study are also discussed, and future research topics are indicated.