Maria Amélia V.C. Araújo

A multi-criteria meshing method applied to a shallow water model

ABSTRACT Mazzolari, A., Trigo-Teixeira, A., and Araújo, M.A.V.C., 2013. A multicriteria meshing method applied to a shallow water model. In: Conley, D.C., Masselink, G., Russell, P.E. and OHare, T.J. (eds.) In this study a multi-criteria meshing method for a 2D shallow water model is tested for a domain encompassing several spatial and hydrodynamic scales, from the Western Atlantic Ocean shelf in front of the Iberian Peninsula to the Lima River (Portugal) and its estuary. For the mesh design, a series of scalar meshing criteria are identified from known physical factors of the relevant processes being studied, such as the bathymetry, the topographical length scales and the expected characteristics of the flow motion. Computational constraints are applied as well. Each criterion is expressed in terms of a node spacing function. The final node spacing function driving the meshing process is obtained by merging the requirements of the previously defined criteria. An advancing front mesh generator performs the discretization in a single, unstructured and well graded mesh, having element side lengths varying over three orders of magnitude (from tens of km to meters). A shallow water model application tests the generated mesh, where tide elevations and phases are imposed at the ocean boundary and freshwater inflow at the upstream boundary. The model validation shows a general good agreement between computed and observed water elevations, currents and the asymmetry of the tidal signal, with an underestimation of the high water levels for the most upstream station. The wet and dry process is reproduced realistically, as the position of the wetted front matches closely the submerged areas of time-referenced aerial images.

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.

Physical Characterization of Estuarine Sediments in the Northern Coast of Portugal

Abstract The physical characteristics of sediments were measured from transects in the Minho, Lima, Cávado, and Ave estuaries in the northern part of Portugal. The variables investigated were: chemical composition, particle size, water content, salinity, temperature, and pH. In a miniflume used to measure erodibility, erosion tests were filmed with a high speed camera and the rheological characteristics of overlaying fluid layer were measured using a rheometer. Because the physical sediment characteristics are related to its erosion, the physical characterization and the erodability were correlated. The influence of the season and sampling site on the sediment properties were assessed for the Cávado estuary. Large differences were found in the physical characterization between the four estuaries, namely in particle size, water content, and chemical composition, and these were important in determining the stability of sediments. Short-term changes in weather conditions prior to sampling and site-to-site variability were found to have more significant effects on the sediment physical properties than seasonal changes.

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.

Flow Structure Over a Simulated Bed for Costal Cohesive Sediment Erosion Studies

This paper reports an experimental assessment of various alternatives to construct an artificial bed appropriate to duplicate, in laboratory, the conditions relevant to marine sediment erosion. The main goal of the study is to find a simulated bed that approaches the behavior of natural sediments, based on the velocity and turbulence profiles and on the interface shear stress values. For that purpose, experiments on natural sediments collected in two estuaries and on a variety of artificial beds were carried out in a circular mini flume, 131 mm in diameter. Velocity profiles and turbulence data were taken by means of a two-component LDA system at a Reynolds number of 20,000. For artificial sediments both rigid and deformable (either fluid or grain) surfaces were investigated. The results show that bed deformation plays an important role in controlling shear stress. The use of two liquid layers simulated bed does not appear to be an appropriate option due to the very high deformation at the interface of the layers. Rigid surfaces that replicate sediment morphology may be appropriate for the simulation of highly cohesive sediments. Turbulence structure proved to be a wall driven phenomenon, except for very dense dispersed flows.Copyright