António Trigo-Teixeira

An object oriented mesh generator: application to flooding in the Douro estuary

ABSTRACT Araújo, M.A.V.C., Mazzolari, A. and Trigo-Teixeira, A., 2013. An object oriented mesh generator: application to flooding in the Douro estuary. This paper presents the modification and improvement of an existing 2D unstructured finite element mesh generator, based on the advancing front concept. The original version of this mesh generator (MESHGR) was developed in Fortran language and, in this study, it is ported to C++ and improved in terms of robustness, efficiency and speed. The C++ code is object oriented and uses classes to describe each object. Besides generating the mesh, the code also enables some post-processing operations: mesh smoothing, edge swap and edge collapse algorithms are implemented based on triangle quality criteria. A R-tree algorithm is used to speed up interpolation and increase robustness. Various mesh generation strategies for the finite element analysis of shallow water flow are presented. The mesh generation is based on a series of criteria or size functions defining the mesh node density, which take into account the hydrodynamic, physical and geometrical features of the domain, such as the bathymetry and topographical length scales. The implemented algorithms create efficient automated unstructured meshes in which the triangular elements do not require further manual processing. The methodology is tested in a real complex domain, in the simulation of an extreme flood event in the Douro estuary, using the ADCIRC shallow water model, and proves to be successful. Computed results are in agreement with the historical records of water level for this estuary.

Impact of detached breakwaters on shoreline evolution: a case study on the Portuguese West Coast

ABSTRACT Araújo, M.A.V.C., Di Bona, S., Trigo-Teixeira, A., 2014. Impact of detached breakwaters on shoreline evolution: a case study on the Portuguese west coast. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 041–046, ISSN 0749-0208. The Portuguese West Coast is facing severe erosion problems that threaten both the population andthe immediate seaside properties. Some locations, such as the Vagueira region, are especially vulnerable. In an attempt to reverse the present erosional trend, a coastal defense scheme involving two detached breakwaters was introduced in a phased manner. In this work, the impact of those two detached breakwaters on the down-drift coast is investigated. The study area extends from south of the Costa Nova village southwards to Mira. The shoreline evolution is predicted, for various decades, using the GENESIS model. The present time shoreline position, from which future situations are predicted, was obtained using LIDAR data acquired in 2011. Two different future scenarios are investigated: the ‘do-nothing’ scenario, which assumes that no further engineering interventions will be performed on the coast; and the ‘detached breakwaters protected’ scenario, with the first breakwater introduced in 2011 and the second in 2026. Depending on sediment supply, two types of simulations are carried out: calibrated simulations, which assume that the amount of sediments in the future will be equal to those in the present situation; and extreme simulations, which assume an acute shortage in sediment supply. Results show that if no further interventions are executed at the Vagueira region, the entire coastal stretch will continue to erode, which could lead to extreme consequences in some locations, depending on sediment supply. If two detached breakwaters are introduced at different intervals, a new accreting trend is predicted at the Vagueira region, reversing the present erosional phase. However, the erosional trend will continue southwards, unless a 3rd detached breakwater is introduced in Mira.

Improved Advancing Front Mesh Algorithm with Pseudoislands as Internal Fronts

AbstractThis paper presents an alternative implementation of the advancing front method for generating two-dimensional triangular unstructured meshes for finite-element applications. After initializing the front along the boundaries, internal fronts, called pseudoislands, are created inside the domain where a node-spacing function condition is met. When the element creation starts, several disjoint fronts are made active, one for each pseudoisland, and the temporal order of creation of the elements follows a smaller to bigger size sequence in an inside-to-outside propagation of the front. Postprocessing routines are available for enhancing the mesh quality. A test case for an idealized geometry shows the effectiveness of the method with high node-spacing function gradients. Several discretizations of a light detection and ranging (LIDAR) derived digital elevation model are compared for demonstrating the improvement of the new advancing front version with respect to the standard version in real application...

Storm-Surge Hindcast at Viana do Castelo: An Oceanic and Estuarine Domain Approach

ABSTRACT Marujo, N.R.C.S.; Araújo, M.A.V.C., Trigo-Teixeira, A.; Falcão, A.P., and Mazzolari, A., 2014. Storm-surge hindcast at Viana do Castelo: An oceanic and estuarine domain approach. In the present work a storm-surge event, which occurred in Viana do Castelo (Portugal) on 14–17 October 1987, is modelled using the advanced circulation model (ADCIRC) . The main purpose of the work is to investigate the ability of ADCIRC to model storm-surge events in the region. Modelling this type of phenomena is of the utmost importance to comply with the European Union directive 2007/60/CE, concerning the assessment and risk management of floods, and to create risk charts used in coastal zone management. During this process, a strong emphasis was given to model calibration before storm-surge modelling. Preceding model calibration a field survey, using global navigation satellite system techniques, was carried out in the Lima Estuary to acquire lacking information required to build the corresponding digital terrain model (DTM). While modelling storm surge, two different domains were considered: a large oceanic and a smaller estuarine. For each of these domains, the model was calibrated with astronomy forcing using, respectively, the 6 and 10 most important tidal constituents, which were determined by harmonic analysis of the Viana do Castelo tidal gauge record. The friction coefficient, wave continuity, and lateral viscosity parameters were changed with this aim. Although the friction coefficient proved to be the most important calibration parameter, the other parameters are also important to achieve stable simulations. For the oceanic domain, the obtained results were improved by considering a variable Coriolis factor. Once the model was calibrated, it was forced using astronomy and meteorology jointly and separately. Atmospheric pressure, wind, and, for the estuarine domain, also river flow were considered as meteorological agents. The storm event was satisfactorily reproduced by the model and it was concluded that meteorology and astronomical effects interact nonlinearly. This nonlinearity is more pronounced when considering the river flow due to the tidal wave–current interaction.

Using simplified bathymetry and SAR imagery in the validation of a hydraulic model for the Tagus River floodplain

ABSTRACT Araújo, M.A.V.C.; Pestana, R.; Matias, M.; Roque, D.; Trigo-Teixeira, A., and Heleno, S., 2016. Using simplified bathymetry and SAR imagery in the validation of a hydraulic model for the Tagus River floodplain. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 13 - 17. Coconut Creek (Florida), ISSN 0749-0208. This work presents several approaches in the validation of the hydrodynamic model Tuflow on the simulation of flood extents and water levels, based on satellite SAR imagery. A methodology that uses a simplified bathymetry in the river main course is employed, which proves to be reliable and accurate for high-flow events. This was made possible as the digital terrain model was acquired in a dry period, accounting for large dry areas in the river bed, avoiding in this way the need of expensive river bathymetry surveys. Also, two methods are applied to the SAR imagery to extract the flood boundaries: visual interpretation followed by manual delimitation and an object-based algorithm approach. The hydraulic model is tested on a reach of the Tagus River, Portugal, where the largest flood inundation areas occur, using a historical flood event to verify its robustness and reliability. The accuracy of model prediction is done through comparisons of water levels at a hydrometric station and the determination of commission and omission errors of flood extent, between the reference SAR image and the predicted inundation. It was concluded that the methodology followed in this work is well suited for the hydraulic model validation.

MODELLING OF LONG WAVES (SEICHES) IN CASCAIS BAY

This study is focused on the analysis and characterization of long waves (seiches) in Cascais Bay-Portugal, where the phenomenon is known to occur. For this purpose, time series records of water surface elevation (WSE) from the tidal gauge were analyzed in time and frequency domains. The periodicities of the long waves were found, firstly with a plotting of time series, where two waves appear immediately: the astronomic tide (T=12h 25min) and a long wave with period of approximately (T=12min), and secondly with the application of the spectral FFT analysis to obtain the periodogram. Before the application of the FFT, the WSE elevation time series was first filtered for the astronomic signal using harmonic analysis and then a pass-band of second order filter was used to removal the noise. The frequency analysis allows identification of hidden periodicities and conclude that most of the spectral energy was concentrated on a single frequency. Finally, some statistics were calculated, which indicated the presence of long waves in the tidal record for most of the period of analysis, without apparent relationship with the season of the year: maritime summer (May-Oct) or maritime winter (Nov-Apr). On a second phase the setup of a 2D finite element model to solve the mild-slope equation (MSE) was done to study the phenomenon of long wave propagation both into the bay and in a small craft harbour (Cascais Marina). The model used was CGWAVE [1] and it was forced with long waves of different frequencies on the boundary to investigate the appearance of seiches both in the bay and in the harbour. The model permits the identification of resonant periods and the location of nodal lines (where vertical WSE oscillation is nil and water velocity is at a maximum) and ventral lines (where just the opposite occurs). The hydrodynamic drag on boats moored on nodal lines is a cause of great trouble on the Marina operation. It might be concluded that seiches occur, not only in the marina basin, but in all water body of the bay, so it is a natural phenomenon on the area.