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- Field measurements and hydrodynamic modelling to evaluate the importance of factors controlling overwashPublication . Matias, Ana; Carrasco, A.R.; Loureiro, Carlos; Masselink, Gerd; Andriolo, Umberto; McCall, Robert; Ferreira, Oscar; Plomaritis, Theocharis; Pacheco, André; Guerreiro, MarthaOverwash 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.
- Turbine design dependency to turbulence: an experimental study of three scaled tidal turbinesPublication . Slama, Myriam; Pinon, Grégory; El Hadi, Charifa; Togneri, Michael; Gaurier, Benoît; Germain, Grégory; Facq, Jean-Valéry; Nuño, José; Mansilla, Pablo; Nicolas, Erwann; Marcille, Julie; Pacheco, AndréIn this paper the turbulence effects are studied for three rotors mounted on the same instrumented hub. Two scaled models of industrial turbines and one open-geometry turbine are considered. The turbulence characteristics are obtained from 2D Laser Doppler Velocimeter measurements and the turbine behaviour is analysed from thrust and torque measurements. Three turbulence intensities and a large range of tip speed ratios and flow velocities are considered. The results are anonymised in order to ensure confidentiality. The rotors have different blade profiles, blade numbers and solidity. The rotor design largely modifies the mean power and thrust coefficients. The turbulence intensity only slightly changes these results but has a larger influence on the fluctuating loads than the different rotor designs. The spectral analysis of the rotor torque and thrust shows that, at low frequencies the load variations are correlated to those of the flow velocity with some differences due to the turbulence intensity levels. The coherences between the loads and the velocity seem to be not affected by the rotor type. At high frequencies, the load variations are correlated to the speed control unit of the scaled model and the rotor design has an impact on the rotational speed and loads coherences.
- Retrieval of nearshore bathymetry from Landsat 8 images: a tool for coastal monitoring in shallow watersPublication . Pacheco, André; Horta, João; Loureiro, Carlos; Ferreira, OscarNearshore 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.
- Hydrodynamic and sediment fluxes through the inlets of the Ria FormosaPublication . Pacheco, André; Ferreira, Óscar; Williams, JonathanResearch into the consequences of engineering works on the long-term behaviour of inlet systems has been hindered by the absence of suitable datasets, the shortcomings of existing formulae when applied to different inlets, and the difficulties particular to multi-inlet situations. Cross-sectional channel areas adjust to hydrodynamics and sediment transport conditions. The impacts of a new inlet on the adjacent shoreline can be unpredictable and are difficult to quantify. The coupling of morphology and hydrodynamics analysis should therefore be extended to all inlets in order to infer the stability of the overall system based on the distribution of the tidal prism through time and the patterns of inlet circulation and sediment transport. Within this thesis both historical and present-day hydrodynamic data collected using high frequency equipment are combined to analyse the recent evolution and equilibrium of a multiple-inlet system. Methods for calculating sediment budgets, analysing the evolution of inlet parameters and determining present hydrodynamic and sediment transport patterns are coupled together, with the aim of improving understanding and prediction of tidal inlet morphodynamics. The results augment existing knowledge about multiple-inlet systems and improve understanding of their short- to medium-term stability. The findings indicate that combining sediment budget calculation and inlet parameter analysis is useful for understanding historical sediment pathways and magnitudes, as well as for analysing the evolution of an inlet, or multiple-inlet system, towards equilibrium. Existing formulae for inferring inlet stability that relate crosssectional area to tidal prism should be revised with a view to including other external variables (e.g. stratigraphic controls) and making their application more flexible to cope with the range of different inlet conditions. Although it is generally acknowledged that most multiple inlets are unstable and cannot co-exist, detailed measurements of hydrodynamic variables obtained over complete spring and neap tidal cycles in a multiple-inlet system in Southern Portugal (Ria Formosa) indicate that the two main inter-connected inlets servicing an embayment (Faro-Olhão and Arrmona inlets) can coexist, at least over a time scale of several decades. Their coexistence cannot be explained simply using empirical equilibrium relations or inlet hydraulics. Residual flow between the inlets appears to play an important role in enhancing their stability. The morphology of the inner channels connecting the inlets can play an important role in inlet stability by controlling the interconnections between the inlets. The capacity to exchange large portions of the tidal prism, while maintaining independent behaviour for the majority of the neap-spring tidal cycle, can contribute to the stability of multiple inlets by altering residual flow and, consequently, transport capacity. This can be particularly important for tidal conditions in which inlets can drain each sub-basin independently. However, it is the availability of sediment stored in the ebb-tidal deltas (and its capacity to be carried into the inlets during storm events) that ultimately dictates the overall equilibrium, independently of hydraulic flushing capacity. If littoral drift is strong, ebb shoals trap significant quantities of sand. During periods of increased wave activity, the tide may not have the capacity to transport material away from the inlet mouth area, and the inlet will accumulate sand due to the shoreward migration of the ebb shoal. Such movement of sediment leads to the obstruction of the inlet channels, thereby affecting the hydraulic efficiency and eventually leading to inlet closure over the long term. The long-term equilibrium of sediment storage in the ebb-tidal deltas must therefore be considered when analysing the possible equilibrium of multiple-inlet systems. Regarding the techniques used, a statistical evaluation of the best method for extrapolating velocity measurements to the unmeasured areas of a vertical profile is presented. To calculate cross-sectional discharge, mean velocities and bed-friction velocities, custom Acoustic Doppler Current Profiler (ADCP) software makes use of theoretical models (e.g. 1/6 power-law and logarithmic law) to reconstruct velocity profiles based on models calibrated and tested for fixed current-meters The aim of this work was to define the best method to extrapolate velocities when using boat-mounted ADCPs, allowing the accurate calculation of the tidal prism, as well as increasing the reliability of the sediment transport estimations. The results revealed the logarithmic law to be more robust across different velocities and channel morphologies. Quantification of sediment transport in tidal inlets remains a fundamental requirement for developing both conceptual and numerical modelling of tidal inlet function and evolution. Well-established empirical formulae were used to estimate bedload, suspended load and total sediment transport rates. The results compared favourably with direct and indirect field observations of sediment transport rates. Although subject to a range of errors, the methods adopted have helped to quantify net accretion/erosion and enabled evaluation of both the flushing and bypassing capacity of tidal inlets in the system. The work has increased knowledge of sediment dynamics in multiple inlet systems and identified appropriate approaches for the prediction of sediment transport in these environments. They may therefore assist in identifying evolutionary trends for a single tidal inlet or a multi-inlet system. The approach and methodology followed in this study could be applied to other multiple-inlet systems around the world in an attempt to “design with nature”, combining an understanding of sediment movement in a region, development projects and regional sediment-management actions.
- Cost-benefit analysis of tidal energy production in a coastal lagoon: the case of Ria Formosa – PortugalPublication . Rodrigues, Nuno; Pintassilgo, Pedro; Calhau, Francisco; G-Gorbeña, Eduardo; Pacheco, AndréThe energy that can be extracted from tidal currents is one of the most promising renewable energy sources due to its high density/predictability. Within this paper this energy source is evaluated economically respecting sustainability principles. This evaluation contrasts from previous studies due to the application of a cost-benefit analysis based on a hydro-morphodynamic model, and moving away from the classic proxy of wind energy. It further includes, via the Monte Carlo method, a probabilistic underpinning to the project. The hydro-economic model was applied to a tidal energy project using an Evopod 1:4th scale prototype, based on a real deployment of an Evopod 1:10th scale device in the Ria Formosa, Algarve. The results show that, under the current costs and benefits, the project is not economically viable. However, there are admissible parameter ranges that make the project viable such as significant reduction of investment costs, increased capacity factors and favourable energy prices. This novel methodology has potential to be applied to other tidal energy projects on estuarine systems worldwide, and consists of a comprehensive modelling approach, including the technical, environmental, and socio-economic dimensions of the project, not only in a deterministic setting but also in a probabilistic one.
- Estimating the optimum size of a tidal array at a multi-inlet system considering environmental and performance constraintsPublication . González-Gorbeña Eisenmann, Eduardo; Pacheco, André; Plomaritis, Theocharis; Ferreira, Oscar; Sequeira, ClaudiaThis 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.
- Marine energy prototype testing at Ria FormosaPublication . Pacheco, André; Gorbeña, Eduardo; Sequeira, ClaudiaEconomic growth and increasing human demands are among the most important factors for growing world energy consumption. Energy is present in everything around us: it is a property of all objects and is essential to life. We find various forms of energy in the world around us. When plants grow, for example, they are converting sunlight energy into chemical energy in the form of carbohydrates and other compounds stored in your body (e.g. sugars). The form of energy that man uses most today is the chemical energy contained in fossil fuels, such as oil, coal and natural gas. About 80% of the energy we use comes from these sources. However, these sources are very polluting, since their use releases substances harmful to the environment and to public health. An example of this is the increased concentration of greenhouse gases such as carbon dioxide (CO2) in the atmosphere, which is a cause of climate change. In addition to pollutants, these sources of energy are non-renewable, which means that they are being consumed at a faster rate than is necessary for their production, so their availability is decreasing. Due to the increasing oil and natural gas prices, reduced fuel reserves and the requirement for reduced CO2 emissions to avert climate change, the use of alternative energy sources is both financially unavoidable and environmentally preferable (UN, 2015). Hence, generating renewable energy is nowadays one of the most relevant endeavours for research. Countries worldwide now recognise the need to incorporate renewable energy resources in their energy policy as an alternative to finite fossil fuel resources in order to achieve future energy security and to mitigate the effects of climate change induced by human activities. Today, renewable energy is now firmly entrenched as the world's fastest growing energy sector (IEA, 2017).
- Surrogate-based optimization of tidal turbine arrays: a case study for the Faro-Olhão inletPublication . G-Gorbeña, Eduardo; Pacheco, André; Plomaritis, Theocharis; Ferreira, Oscar; Sequeira, Claudia; Moura, TheoThis 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.
- Assessing the effects of Tidal Energy Converter array size on hydrodynamics of Ria Formosa (Portugal)Publication . G-Gorbeña, Eduardo; Pacheco, André; Plomaritis, Theocharis A.; Sequeira, ClaudiaThis paper investigates the effects of Tidal Energy Converter (TEC) array size at a tidal channel on flood/ebb discharges at multi-inlet coastal lagoon by applying numerical modelling. The paper presents a case study for the Faro-Olhão inlet in the Ria Formosa (Portugal), a potential site for tidal in-stream energy extraction. Arrays of up to 11 rows with 5 TECs each were studied to assess impacts on inlets discharges changes. For the particular cases assessed the results show that tidal energy extraction will have a greater impact on Ancão and Armona inlets discharges together with the Faro-Olhão inlet. Future work is directed to include impacts on sediment dynamics and optimise TEC array size as a function of multiple design variables subject to environmental constraints.
- Analysis of the efficiency of wind turbine gearboxes using the temperature variablePublication . Sequeira, Claudia; Pacheco, André; Galego, P.; González-Gorbeña Eisenmann, EduardoThe aim of this paper is to evaluate how lubricant selection affects gearbox efficiency and overall energy production by analysing real data from wind farms, monitored and controlled by a Supervisory Control and Data Acquisition (SCADA system). The turbines analysed worked with two or more oil types for the same amount of hours, which allowed to establish relations between the active power curves and wind velocity; oil temperature inside gearboxes and wind velocity; and oil temperature inside gearboxes and active power production. The results of this study evidenced a direct relation between oil characteristics and energy efficiency i.e. gearboxes working with mineral oil perform better then gearboxes working with synthetic oils. Those differences can be significant in terms of active power production. Also, it was observed oil degradation as function of temperature increase, with changes on viscosity, which reveals that temperature behaviour along the active power curve is strongly related to oil' characteristics. (C) 2018 Elsevier Ltd. All rights reserved.
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