Browsing by Author "Baptista, Paulo"
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- Analysis of Nonlinear Wave Parameters on Ofir Sandy Beach (NW Portugal)Publication . Abreu, Tiago; Silva, Paulo A.; Baptista, Paulo; Pais-Barbosa, Joaquim; Fernández-Fernández, Sandra; Ferreira, Caroline; Matos, JoãoThe characterization of wave transformation processes in the nearshore is of paramount importance when it comes to assessing storm and flooding impacts, sediment transportation and deposition, harbors safety or design of coastal protective structures. This study analyzes nonlinear wave parameters on Ofir sandy beach. This beach is located along the northwest Portuguese coast which is a highly energetic coast exposed to waves generated far away in the Atlantic Ocean. Despite the existence of rocky outcrops in the nearshore and intertidal zones at the study site, reducing the wave energy that reaches the beach, the study site exhibits pronounced erosive processes. Field observations of six near-bottom pressure records collected at the intertidal zone help to characterize the evolution of wave nonlinearities which are directly associated with sediment transport mechanisms. Data results show that there is an interrelation between the characteristics of the waves and the local morphology. It is also possible to ascertain, more clearly, the level of asymmetry present in the waves propagated at different depths, contributing to a better understanding of the local morpho-hydrodynamics.
- Longshore Transport Estimation on Ofir Beach in Northwest Portugal: Sand-Tracer ExperimentPublication . Fernández-Fernández, Sandra; Baptista, Paulo; Martins, Virgínia A.; Silva, Paulo A.; Abreu, Tiago; Pais-Barbosa, Joaquim; Bernardes, Cristina; Miranda, Paulo; Rocha, Mariana V. L.; Santos, Fábio; Bernabeu, Ana; Rey, DanielThis work aims to shed some light on longshore sediment transport (LST) in the highly energetic northwest coast of Portugal. Data achieved through a sand-tracer experiment are compared with data obtained from the original and the new re-evaluated longshore sediment transport formulas (USACE Waterways Experiment Station’s Coastal Engineering and Research Center, Kamphuis, and Bayram bulk formulas) to assess their performance. The field experiment with dyed sand was held at Ofir Beach during one tidal cycle under medium wave-energy conditions. Local hydrodynamic conditions and beach topography were recorded. The tracer was driven southward in response to the local swell and wind- and wave-induced currents (Hsb=0.75mHsb=0.75m, Tp=11.5sTp=11.5s, θb=8−12°θb=8−12°). The LST was estimated by using a linear sediment transport flux approach. The obtained value (2.3×10−3m3⋅s−12.3×10−3m3⋅s−1) approached the estimation provided by the original Bayram formula (2.5×10−3m3⋅s−12.5×10−3m3⋅s−1). The other formulas overestimated the transport, but the estimations resulting from the new re-evaluated formulas also yield approximate results. Therefore, the results of this work indicated that the Bayram formula may give satisfactory results for predicting the longshore sediment transport on Ofir Beach.
- Nearshore Bathymetry Retrieval from Wave-Based Inversion for Video ImageryPublication . Santos, Diogo; Abreu, Tiago; Silva, Paulo A.; Santos, Fábio; Baptista, PauloA wavelet-based method for bathymetry retrieval using a sequence of static images of the surface wave field, as obtained from video imagery, is proposed. Synthetic images of the water surface are generated from a numerical Boussinesq type model simulating the propagation of irregular waves. The spectral analysis is used to retrieve both wave periods and wavelengths by evaluating the spectral peaks in the time and spatial domains, respectively. The water depths are estimated using the linear dispersion relation and the results are validated with the model’s bathymetry. To verify the proposed methodology, 2D and 3D simulations considering effects of wave shoaling and refraction were performed for different sea conditions over different seafloors. The method’s ability to reproduce the original bathymetry is shown to be robust in intermediate and shallow waters, being also validated with a real case with images obtained with a shore-based video station. The main improvements of the new method compared to the consideration of a single image, as often used in Satellite Derived Bathymetry, is that the use of successive images enables the consideration of different wave periods, improving depth estimations and not requiring the use of subdomains or filters. This image processing methodology shows very positive results to provide bathymetry maps for shallow marine environments and can be useful to monitor the nearshore with high time- and space-resolution at low cost.
- Retrieval of nearshore bathymetry from Sentinel-1 SAR data in high energetic wave coasts: The Portuguese case studyPublication . Santos, Diogo; Fernández-Fernández, Sandra; Abreu, Tiago; Silva, Paulo A.; Baptista, PauloThe ability to derive bathymetry using remote sensing techniques enables rapid and cost-efficient mapping of large coastal areas. This contribution focuses on the application of both fast Fourier transform (FFT) and wavelet spectral analysis to obtain satellite-derived bathymetry maps of the nearshore, from freely available and easily accessible Sentinel-1 synthetic aperture radar (SAR) data with 10 m pixel resolution. For this purpose, an extension of 220 km of the Portuguese west coast is analyzed using six satellite images obtained during the years 2018, 2019 and 2020. This extension allows to assess the applicability to coastal sectors with distinct geomorphological constraints. The peak wave periods corresponding to the acquisition of these images approximately range between 11 and 16 s. The spectral analysis is carried to estimate the water depths in near-shore water regions from the observed wavelength patterns. This signature of the sea surface, reflected in the variations of the wavelengths, is captured by the satellite images, making it possible to infer the underlying bathymetry. The bathymetric estimates obtained from both methodologies are compared with data extracted from the Coastal Nautical Charts provided by the Portuguese Hydrographic Institute. Wavelet image processing methodology shows very positive results, particularly extending the depth inversion limits of the FFT methodology, allowing to obtain bathymetric data for the entire shoaling zone where the wavelength patterns are visible. The achieved results also highlight that both FFT and wavelet methodologies are dependent from the seabed slope. For gentle slopes, the inferred depths from 2018 SAR images lead to relative errors between 2.5 and 20% when compared with the observed isobaths (10, 20 and 30 m). For steeper slopes, the errors are generally greater than 20% and increase with depth. The capabilities of the wavelet methodology to map shallow marine environments for high energetic coasts seems promising, regarding research purposes and management interventions.