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- Historical variation and trends in storminess along the Portuguese South CoastPublication . Almeida, L. P.; Ferreira, Óscar; Vousdoukas, Michalis; Dodet, G.This work investigates historical variation and trends in storm climate for the South Portugal region, using data from wave buoy measurements and from modelling, for the period 1952 to 2009. Several storm parameters (annual number of storms; annual number of days with storms; annual maximum and mean individual storm duration and annual 99.8th percentile of significant wave height) were used to analyse: (1) historical storminess trends; (2) storm parameter variability and relationships; and (3) historical storminess and its relationship to the North Atlantic Oscillation (NAO). No statistically significant linear increase or decrease was found in any of the storm parameters over the period of interest. The main pattern of storm characteristics and extreme wave heights is an oscillatory variability with intensity peaks every 7–8 yr, and the magnitude of recent variations is comparable with that of variations observed in the earlier parts of the record. In addition, the results reveal that the NAO index is able to explain only a small percentage of the variation in storm wave height, suggesting that more local factors may be of importance in controlling storminess in this region.
- Beach erosion and recovery during consecutive storms at a steep-sloping, meso-tidal beachPublication . Vousdoukas, Michalis; Almeida, Luis Pedro; Ferreira, ÓscarThis study analyses beach morphological change during six consecutive storms acting on the meso-tidal Faro Beach (south Portugal) between 15 December 2009 and 7 January 2010. Morphological change of the sub-aerial beach profile was monitored through frequent topographic surveys across 11 transects. Measurements of the surf/swash zone dimensions, nearshore bar dynamics, and wave run-up were extracted from time averaged and timestack coastal images, and wave and tidal data were obtained from offshore stations. All the information combined suggests that during consecutive storm events, the antecedent morphological state can initially be the dominant controlling factor of beach response; while the hydrodynamic forcing, and especially the tide and surge levels, become more important during the later stages of a storm period. The dataset also reveals the dynamic nature of steep-sloping beaches, since sub-aerial beach volume reductions up to 30m3/m were followed by intertidal area recovery (–2
- Performance of intertidal topography video monitoring of a meso-tidal reflective beach in South PortugalPublication . Vousdoukas, Michalis; Ferreira, P. M.; Almeida, Luis Pedro; Dodet, Guillaume; Psaros, Fotis; Andriolo, Umberto; Taborda, Rui; Silva, Ana Nobre; Ruano, Antonio; Ferreira, ÓscarThis study discusses site-specific system optimization efforts related to the capability of a coastal video station to monitor intertidal topography. The system consists of two video cameras connected to a PC, and is operating at the meso-tidal, reflective Faro Beach (Algarve coast, S. Portugal). Measurements from the period February 4, 2009 to May 30, 2010 are discussed in this study. Shoreline detection was based on the processing of variance images, considering pixel intensity thresholds for feature extraction, provided by a specially trained artificial neural network (ANN). The obtained shoreline data return rate was 83%, with an average horizontal cross-shore root mean square error (RMSE) of 1.06 m. Several empirical parameterizations and ANN models were tested to estimate the elevations of shoreline contours, using wave and tidal data. Using a manually validated shoreline set, the lowest RMSE (0.18 m) for the vertical elevation was obtained using an ANN while empirical parameterizations based on the tidal elevation and wave run-up height resulted in an RMSE of 0.26 m. These errors were reduced to 0.22 m after applying 3-D data filtering and interpolation of the topographic information generated for each tidal cycle. Average beach-face slope tan(β) RMSE were around 0.02. Tests for a 5-month period of fully automated operation applying the ANN model resulted in an optimal, average, vertical elevation RMSE of 0.22 m, obtained using a one tidal cycle time window and a time-varying beach-face slope. The findings indicate that the use of an ANN in such systems has considerable potential, especially for sites where long-term field data allow efficient training.
- Thresholds for storm impacts on an exposed sandy coastal area in southern PortugalPublication . Almeida, L. P.; Vousdoukas, Michalis; Ferreira, Óscar; Rodrigues, B.; Matias, AnaStorms are one of the most important phenomena responsible for coastal erosion. Their destructive power presents major challenges for coastal management, and knowledge of their characteristics and associated consequences is therefore of paramount importance. In this study, thresholds for storm impacts are defined for a sandy coast in southern Portugal (Faro beach, Ria Formosa) using two different approaches: i) hydrodynamic conditions associated with historical storm impacts (i.e., infrastructural damage); and (ii) computed maximum wave run-up values (RHIGH) compared with beach morphology. Damage thresholds are defined as limits above which the action of a storm exceeds beach front response capability and starts to act directly on human infrastructures. According to method (i), four different thresholds were defined. For individual storms directed from the SW the threshold is a significant wave height of 4.7 m with a storm duration of 2 days, and for storms directed from the SE is a significant wave height of 6 m with a storm duration of at least 2 days. Regarding storm groups, for those directed from the SW the threshold is 2 storms lasting at least 2 days each with significant wave heights greater than 3.5 m, whilst for those directed from the SE the threshold is 3 short storms (lasting 1 day each) with significant wave height over 3.9 m. The return period for the SW storm group threshold is just 1.7 years. For method (ii), beach morphological parameters (DHIGH — highest elevation of the frontal dune; DLOW — elevation of the dune base; and tanβf — foreshore slope) were determined for five chosen cross-shore profiles along the study area, and RHIGH computed for different hydrodynamic conditions. Results were validated through field observations of storm impacts. The lowest thresholds for overwash are along the central part of the study area where a dune crest is absent due to human occupation. The highest thresholds for overwash are on those profiles where a frontal dune is well developed or a foredune ridge is present (western and eastern parts of the study area). This study presents a methodological strategy to compute realistic thresholds for storm impacts along Faro beach based on historical datasets of hydrodynamics and storm impacts information. The approach could be implemented in other coastal areas to provide a comprehensive assessment of the storm impact, requiring only simple information like offshore storm hydrodynamics characteristics, beach morphology and reports of coastal infrastructure damage.
- Coastal vulnerability assessment based on video wave run-up observations at a mesotidal, steep-sloped beachPublication . Vousdoukas, Michalis; Wziatek, Dagmara; Almeida, Luís PedroCoastal imagery obtained from a coastal video monitoring station installed at Faro Beach, S. Portugal, was combined with topographic data from 40 surveys to generate a total of 456 timestack images. The timestack images were processed in an open-access, freely available graphical user interface (GUI) software, developed to extract and process time series of the cross-shore position of the swash extrema. The generated dataset of 2% wave run-up exceedence values R 2 was used to form empirical formulas, using as input typical hydrodynamic and coastal morphological parameters, generating a best-fit case RMS error of 0.39 m. The R 2 prediction capacity was improved when the shore-normal wind speed component and/or the tidal elevation η tide were included in the parameterizations, further reducing the RMS errors to 0.364 m. Introducing the tidal level appeared to allow a more accurate representation of the increased wave energy dissipation during low tides, while the negative trend between R 2 and the shore-normal wind speed component is probably related to the wind effect on wave breaking. The ratio of the infragravity-to-incident frequency energy contributions to the total swash spectra was in general lower than the ones reported in the literature E infra/E inci > 0.8, since low-frequency contributions at the steep, reflective Faro Beach become more significant mainly during storm conditions. An additional parameterization for the total run-up elevation was derived considering only 222 measurements for which η total,2 exceeded 2 m above MSL and the best-fit case resulted in RMS error of 0.41 m. The equation was applied to predict overwash along Faro Beach for four extreme storm scenarios and the predicted overwash beach sections, corresponded to a percentage of the total length ranging from 36% to 75%.
- Correlating wave hindcast and buoy data with artificial neural networksPublication . Almeida, Luis Pedro; Vousdoukas, Michalis; Ferreira, P. M.; Ruano, AntonioThis work presents results from the use of Artificial Neural Networks (ANN) to improve wave models hindcasting capacity off the South coast of Portugal. Comparison of the original model results with field measurements showed significant non linear deviations. To compensate for such deviations, a three-layer Multilayer Perceptron (MLP – a type of an ANN) was trained, using the Levenberg-Marquardt method, to improve the fit between the hindcast (generated by WW3) and Faro buoy data in an effort to reconstruct missing data from the wave buoy time series. The results obtained so far are very positive; with the training with annual datasets showing better results than the training with the entire dataset, while both improved significantly the fitting of the raw model results. Further improvements are expected by trying different ANN types, by searching for optimised ANN input-output structure, and by performing sub-set selection on the data sets.
- Joint use of data and modeling in coastal wave transformationPublication . Fortes, Conceicao Juana; Ferreira, Oscar; Silva, Paulo A.; Moura, Theo; Capitao, Rui; Amante, Luis; Vousdoukas, Michalis; Bezerra, MariaIn the framework of a research project entitled "BRISA-BReaking waves and Induced SAnd transport", a methodology was devised to characterize the waves joining together in-situ measurements and numerical wave propagation models. With this goal in mind, a number of in-situ measurements were made, for selected positions in front of Praia de Faro (South Portugal), during four days (25th to 28th March, 2009) by using different types of equipments (e.g., resistive wave gauges, pressure sensors, currentmeters and a new prototype pore pressure sensor using optical fibre). Wave records were obtained simultaneously offshore (at a water depth of 11.7 m below mean sea level, MSL) and at the surf and swash zones. The data processing and analysis were made by applying classical time domain techniques. Numerical simulations of the wave propagation between offshore and inshore for the measurement period were performed with two numerical models, a 1D model based on linear theory and a nonlinear Boussinesq-type model, COULWAVE, both forced by the measured offshore wave conditions of 27th March 2009. Comparisons between numerical results and field data for the pressure sensors placed in the surf and swash zones were made and discussed. This approach enables to evaluate the performance of those models to simulate those specific conditions, but also to validate the models by gaining confidence on their use in other conditions.