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  • Field measurements and hydrodynamic modelling to evaluate the importance of factors controlling overwash
    Publication . Matias, Ana; Carrasco, A.R.; Loureiro, Carlos; Masselink, Gerd; Andriolo, Umberto; McCall, Robert; Ferreira, Oscar; Plomaritis, Theocharis; Pacheco, André; Guerreiro, Martha
    Overwash hydrodynamic datasets are mixed in quality and scope, being difficult to obtain due to fieldwork experimental limitations. Nevertheless, these measurements are crucial to develop reliable models to predict overwash. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring it to model overwash on a low-lying barrier island. Fieldwork was undertaken on Barreta Island (Portugal) in December 2013, during neap tides and under energetic conditions, with significant wave height reaching 2.6 m. During approximately 4 h, more than 120 shallow overwash events were measured with a video-camera, a pressure transducer and a current-meter. This high-frequency fieldwork dataset includes runup, overwash number, depth and velocity. Fieldwork data along with information from literature were used to implement XBeach model in non-hydrostatic mode (wave-resolving). The baseline model was tested for six verification cases; and the model was able to predict overwash in five. Based in performance metrics and the verification cases, it was considered that the Barreta baseline overwash model is a reliable tool for the prediction of overwash hydrodynamics. The baseline model was then forced to simulate overwash under different hydrodynamic conditions (waves and lagoon water level) and morpho-sedimentary settings (nearshore topography and beach grain-size), within the characteristic range of values for the study area. According to the results, the order of importance of factors controlling overwash predictability in the study area are: 1st) wave height (more than wave period) can promote overwash 3–4 times more intense than the one recorded during fieldwork; 2nd) nearshore bathymetry, particularly shallow submerged bars, can promote an average decrease of about 30% in overwash; 3rd) grain-size, finer sediment produced an 11% increase in overwash due to reduced infiltration; and 4th) lagoon water level, only negligible differences were evidenced by changes in the lagoon level. This implies that for model predictions to be reliable, accurate wave forecasts are necessary and topo-bathymetric configuration needs to be monitored frequently.
  • Retrieval of nearshore bathymetry from Landsat 8 images: a tool for coastal monitoring in shallow waters
    Publication . Pacheco, André; Horta, João; Loureiro, Carlos; Ferreira, Oscar
    Nearshore bathymetry is likely to be the coastal variable that most limits the investigation of coastal processes and the accuracy of numerical models in coastal areas, as acquiring medium spatial resolution data in the nearshore is highly demanding and costly. As such, the ability to derive bathymetry using remote sensing techniques is a topic of increasing interest in coastalmonitoring and research. This contribution focuses on the application of the linear transform algorithm to obtain satellite-derived bathymetry (SDB) maps of the nearshore, at medium resolution (30 m), from freely available and easily accessible Landsat 8 imagery. The algorithm was tuned with available bathymetric Light Detection and Ranging (LiDAR) data for a 60-km-long nearshore stretch of a highly complex coastal system that includes barrier islands, exposed sandy beaches, and tidal inlets (Ria Formosa, Portugal). A comparison of the retrieved depths is presented, enabling the configuration of nearshore profiles and extracted isobaths to be explored and compared with traditional topographic/bathymetric techniques (e.g., high- and medium-resolution LiDAR data and survey-grade echo-sounding combined with high-precision positioning systems). The results demonstrate that the linear algorithm is efficient for retrieving bathymetry frommulti-spectral satellite data for shallowwater depths (0 to 12 m), showing amean bias of−0.2m, a median difference of −0.1 m, and a root mean square error of 0.89 m. Accuracy is shown to be depth dependent, an inherent limitation of passive optical detection systems. Accuracy further decreases in areas where turbidity is likely to be higher, such as locations adjacent to tidal inlets. The SDB maps provide reliable estimations of the shoreline position and of nearshore isobaths for different cases along the complex coastline analysed. The use of freely available satellite imagery proved to be a quick and reliable method for acquiring updated mediumresolution, high-frequency (days and weeks), low-cost bathymetric information for large areas and depths of up to 12 m in clear waters without wave breaking, allowing almost constant monitoring of the submerged beach and the shoreface.
  • Estimating the optimum size of a tidal array at a multi-inlet system considering environmental and performance constraints
    Publication . González-Gorbeña Eisenmann, Eduardo; Pacheco, André; Plomaritis, Theocharis; Ferreira, Oscar; Sequeira, Claudia
    This paper investigates the optimum tidal energy converter array density at a tidal inlet by applying surrogate-based optimisation. The SBO procedure comprises problem formulation, design of experiments, numerical simulations, surrogate model construction and constrained optimisation. This study presents an example for the Faro-Olhão Inlet in the Ria Formosa (Portugal), a potential site for tidal in-stream energy extraction. A 35 kW Evopod™ floating tidal energy converter from Oceanflow Energy Ltd. has been used for array size calculations considering two design variables: (1) number of array rows, and (2) number of tidal energy converter per row. Arrays up to 13 rows with 6–11 tidal energy converters each are studied to assess their impacts on array performance, inlets discharges and bathymetry changes. The analysis identified the positive/negative feedbacks between the two design variables in real case complex flow fields under variable bathymetry and channel morphology. The non-uniformity of tidal currents along the array region causes the variability of the resource in each row, as well as makes it difficult to predict the resultant array configuration interactions. Four different multi-objective optimisation models are formulated subject to a set of performance and environmental constraints. Results from the optimisation models imply that the largest array size that meets the environmental constraints is made of 5 rows with 6 tidal energy converter each and an overall capacity factor of 11.6% resulting in an energy production of 1.01 GWh year−1. On the other hand, a higher energy production (1.20 GWh year−1) is achieved by an optimum array configuration, made of 3 rows with 10 tidal energy converters per row, which maximises power output satisfying environmental and performance restrictions. This optimal configuration permits a good level of energy extraction while having a reduced effect on the hydrodynamic functioning of the multi-inlet system. These results prove the suitability and the potential wide use of the surrogate-based optimisation method to define array characteristics that enhance power production and at the same time respect the environmental surrounding conditions.
  • Surrogate-based optimization of tidal turbine arrays: a case study for the Faro-Olhão inlet
    Publication . G-Gorbeña, Eduardo; Pacheco, André; Plomaritis, Theocharis; Ferreira, Oscar; Sequeira, Claudia; Moura, Theo
    This paper presents a study for estimating the size of a tidal turbine array for the Faro-Olhão Inlet (Potugal) using a surrogate optimization approach. The method compromises problem formulation, hydro-morphodynamic modelling, surrogate construction and validation, and constraint optimization. A total of 26 surrogates were built using linear RBFs as a function of two design variables: number of rows in the array and Tidal Energy Converters (TECs) per row. Surrogates describe array performance and environmental effects associated with hydrodynamic and morphological aspects of the multi inlet lagoon. After validation, surrogate models were used to formulate a constraint optimization model. Results evidence that the largest array size that satisfies performance and environmental constraints is made of 3 rows and 10 TECs per row.