Percorrer por autor "Ferreira, Óscar"
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- Applicability of sediment transport models to evaluate medium term evolution of tidal inlet systemsPublication . Pacheco, A.; Williams, J. J.; Ferreira, Óscar; Garel, Erwan; Reynolds, S.This paper derives local formulae to estimate bed roughness and suspended transport and present a method to calculate net sediment transport at tidal inlet systems, combining field data and a range of well established empirical formulations. To accomplish this, measurements spanning a spring-tidal cycle of mean water levels, waves, near-bed flow turbulence and bed forms were obtained from the Ancão Inlet, Ria Formosa lagoon system, Portugal. High-resolution hydrodynamic data were gathered using acoustic equipments and by measuring sediment properties (grain-size diameter and bed form dimensions) under fair-weather conditions. The results compared favourably with available direct and indirect field observations of sediment transport rates. The approach appears to be robust and widely applicable and so can be applied to the same conditions in any tidal inlet system. This is of particular importance when attempting to understand sediment transport at inlet mouths, and has practical applications in a range of coastal engineering and coastal management areas concerned with navigation safety, coastal erosion, ecosystem health and water quality. The study discusses the applicability of the method on evaluating system flushing capacity, giving important insights on multiple inlet evolution, particularly with regard to their persistence through time. The methodological framework can be applied to assess the long-term stability of single- and multiple-inlet systems, provided that estimates of sediment storage at ebb-tidal deltas are available and sediment transport estimates during storm events are statistically considered.
- Applicability of the “Frame of Reference” approach for environmental monitoring of offshore renewable energy projectsPublication . Garel, Erwan; Rey, Cibran Camba; Ferreira, Óscar; van Koningsveld, MarkThis paper assesses the applicability of the Frame of Reference (FoR) approach for the environmental monitoring of large-scale offshore Marine Renewable Energy (MRE) projects. The focus is on projects harvesting energy from winds, waves and currents. Environmental concerns induced by MRE projects are reported based on a classification scheme identifying stressors, receptors, effects and impacts. Although the potential effects of stressors on most receptors are identified, there are large knowledge gaps regarding the corresponding (positive and negative) impacts. In that context, the development of offshore MRE requires the implementation of fit-for-purpose monitoring activities aimed at environmental protection and knowledge development. Taking European legislation as an example, it is suggested to adopt standardized monitoring protocols for the enhanced usage and utility of environmental indicators. Towards this objective, the use of the FoR approach is advocated since it provides guidance for the definition and use of coherent set of environmental state indicators. After a description of this framework, various examples of applications are provided considering a virtual MRE project located in European waters. Finally, some conclusions and recommendations are provided for the successful implementation of the FoR approach and for future studies.
- Applicability of the “Frame of Reference" approach for environmental monitoring of offshore renewable energy projectsPublication . Garel, Erwan; Rey, Cibran Camba; Ferreira, Óscar; Van Koningsveld, MarkThis paper assesses the applicability of the Frame of Reference (FoR) approach for the environmental monitoring of large-scale offshore Marine Renewable Energy (MRE) projects. The focus is on projects harvesting energy from winds, waves and currents. Environmental concerns induced by MRE projects are reported based on a classification scheme identifying stressors, receptors, effects and impacts. Although the potential effects of stressors on most receptors are identified, there are large knowledge gaps regarding the corresponding (positive and negative) impacts. In that context, the development of offshore MRE requires the implementation of fit-for-purpose monitoring activities aimed at environmental protection and knowledge development. Taking European legislation as an example, it is suggested to adopt standardized monitoring protocols for the enhanced usage and utility of environmental indicators. Towards this objective, the use of the FoR approach is advocated since it provides guidance for the definition and use of coherent set of environmental state indicators. After a description of this framework, various examples of applications are provided considering a virtual MRE project located in European waters. Finally, some conclusions and recommendations are provided for the successful implementation of the FoR approach and for future studies.
- Backbarrier evolution at a medium-term scalePublication . Carrasco, Ana Rita; Ferreira, Óscar; Matias, Ana; Freire, Paula; Dias, J. A.This study reports the medium-term evolution of a sandy backbarrier and its relation with prevailing wind conditions. Obtained results demonstrated that Ancão backbarrier does not behave as a scale-down version of higher energy fetch-limited or oceanic beaches, being considered as a low-energy beach with extremely low wind-induced wave conditions. Volumetric changes during the three years monitoring were small, and the analysis of low-scale changes was divided into four beach compartment: upper beach, beach face, tidal flat, sand bank. There was no significant correlation between prevailing wind conditions, volume and grain-size variation, neither a marked seasonal pattern. Some wind-induced beach changes were perceptive in the grain size variations, related mostly to aeolian offshore sediment transport. Morphological changes were similar in the upper part of the profile, including upper beach and beach face, with changes mostly related to wind and windwaves energy. The lower part of the profile, which includes the tidal flat and the sand bank, frequently react independently from the upper part of the profile, although, sediment exchange between them was noticed during extreme conditions. The lower part of the profile had larger volumetric variability, without any evident wind dominance. The studied beach revealed high morphologic resilience, yet the overall quantities of sediment transported between 2005 and 2008 has particular relevance in the local coastal management context. Further research is needed to develop a broad-scale model of fetch-limited beaches, including the less energetic settings of the spectra such as backbarrier environments.
- Barrier island resilience assessment: Applying the ecological principles to geomorphological dataPublication . Kombiadou, Katerina; Matias, Ana; Costas, Susana; Carrasco, A. Rita; Plomaritis, Theocharis A.; Ferreira, ÓscarApplying the ecological resilience principles to barrier island geomorphological evolution requires approaches that perceive and interpret resilience far from predefined barrier characteristics and static views. Accepting that barrier islands, like all natural systems, are dynamic and adaptively changing in response to external disturbances is fundamental to the formulated approach. To this aim, geomorphological units and dimensions were used to describe barrier island stability landscape as an actively shifting ‘topography’, reshaping in response to exogenous events and in relation to intrinsic properties. The structure of the subaerial barrier was characterised using the environmental units of Beach, Dune and Marsh (or BDM), where different combinations of BDM structure define distinct barrier stable states, under a simplified framework that is applicable over a wide range of barrier structures. The methodology is based on reconstructing resilience trajectories of barrier islands through identifying the distinct BDM states and related shifts (thresholds crossed) and assessing resilience dimensions (latitude, resistance and precariousness defined as barrier width and height and proximity to mainland, respectively) that, jointly, define the shape of the stability domain. The approach was applied to the Ria Formosa barrier island system (S. Portugal), using multi-decadal geomorphological data and gradually decreasing spatial discretisation, passing from individual barrier transects to sectors and to entire barriers. The joint evolution of two inland-migrating barriers (Cabanas Island and Cacela Peninsula) was used as an exemplar of adaptive capacity in barrier geomorphic change and, therefore, of resilient response to external pressures. Resilience pathways showed that the Ria Formosa barriers have been resilient over the studied timeframe, with a tendency for maintaining or increasing BDM structural complexity. In general, the stability domain tends to shift from low latitude and high resistance forms (narrow-deep basins of attraction) in the west part of the barrier chain, to higher latitudes and lower resistance ones (wide-shallow basins) towards the east. Precariousness peaks near the edges of the system (low lagoon width) and minimises towards the central part (most detached barriers). Scaling issues regarding smoothing of longshore variability and potential consequences on masking thresholds and critical dimensions are highlighted and discussed, along with the key role of the meaning of specified resilience (of what?) in the assessment. The methodology is a novel approach, easily transferable to different systems and spatiotemporal scales of analysis, representing a step forward in interpreting and assessing barrier island resilience.
- 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
- Benthic foraminifera on the Guadiana inner shelf as indicators of rainfall variability and human impacts on the river basinPublication . Mendes, Isabel; Lobo, F. J.; Ferreira, Óscar; Schönfeld, J.; Hanebuth, T.; Lebreiro, S.; Lantzsch, H.; Antón, L.; Reguera, I.Prodeltaic depositional systems can be used as high-resolution archives, since they sensitively record environmental changes occurred during their formation. On the inner shelf off the Guadiana River, northern Gulf of Cádiz, the recent deposition is mainly driven by river discharge, which reflects both climate variability and human-induced processes in the river basin. The purpose of this study is to unravel rainfall variability and human impacts on the river basin, recorded on the Guadiana prodeltaic wedge, over the past decades. A multiproxy study including the analyses of benthic foraminiferal assemblages, grain size and geochemical element distribution (X-ray fluorescence core scanning), constrained in a chronological framework (based on radiocarbon dating and Pb-210), was performed on core GeoB19522. This core was collected in March 2015, during the RV Poseidon cruise POS482 CADISED, on the Guadiana prodeltaic wedge at 13.5 m water depth, which is an area with high sedimentation accumulation rates and consequent detailed temporal resolution. The upper part of the sediment core (from 1.6 m depth to the top) is dominated by the silt-clay fraction (77-99%), occasionally with higher percentages of sand (0.2-23 %). The element ratios and the benthic foraminiferal fauna show significant temporal variations. Some of the identified species can be regarded as successful colonizers (Textularia earlandi, Reophax arctica) and others as opportunistic species linked with river discharges and the limits of productivity (Bolivina ordinaria, Hopkinsina atlantica). These variations point to changes in the environment, promoted by periods of increase or decrease sediment supply to the shelf.
- Benthic foraminiferal and sedimentological response to the evolution of the Adra submarine delta, northern Alboran SeaPublication . Mendes, Isabel; Lobo, F. J.; Ferreira, Óscar; Schönfeld, J.; Rosa, F.; Bárcenas, P.; Fernandez-Salas, L. M.; López-González, N.; Dias, J. A.The Adra submarine delta is located on the northern Alboran Sea shelf in the western Mediterranean Sea. The genesis of this deltaic system is associated with the discharges of the short and mountainous Adra River. The area is under the influence of a Mediterranean climate with sporadic winter torrential flows and increased summer aridity. Major anthropogenic activities in the river system occurred in 1872 AD, with the deviation of the main fluvial course to the east. The channel was silted up in 1910 AD as result of a flood event and relocated further west, at its present position. These artificial changes are reflected in the submarine morpho‐stratigraphy of the delta that is composed of two main lobes. In order to understand the interaction between river discharges and the evolution of the submarine delta at different timescales, two sediment cores were collected from both lobes. A chronological framework was performed and combined with sedimentological and benthic foraminiferal analyses. Radiocarbon dating of plant debris from the base of the cores indicates that the sedimentary record goes back 250 years BP. The correlation of sediment cores with seismic records indicate that both cores penetrated the same seismic unit, deposited between ca. 1070 to ca. 1872 AD, under the direct influence of the ancient river course. The predominant sedimentary facies is sandy silt with intercalated sand layers. The uppermost core sections are pure sand. The number of benthic foraminifera is generally below 100 specimens per gram. The most abundant species in both cores are Ammonia tepida, Bolivina ordinaria, Nonionella stella, Reophax arctica and Textularia earlandi. The increases of sand and the low faunal density, followed by increased abundance of successful colonizers and opportunistic species, is interpreted as result of periods of high precipitation, and sediment supply to the shelf, and the subsequent establishment of an environment with new ecological constrains.
- Coastal barrier stratigraphy for Holocene high-resolution sea-level reconstructionPublication . Costas, Susana; Ferreira, Óscar; Plomaritis, Theocharis; Leorri, EduardoThe uncertainties surrounding present and future sea-level rise have revived the debate around sea-level changes through the deglaciation and mid-to late Holocene, from which arises a need for high-quality reconstructions of regional sea level. Here, we explore the stratigraphy of a sandy barrier to identify the best sea-level indicators and provide a new sea-level reconstruction for the central Portuguese coast over the past 6.5 ka. The selected indicators represent morphological features extracted from coastal barrier stratigraphy, beach berm and dune-beach contact. These features were mapped from high-resolution ground penetrating radar images of the subsurface and transformed into sea-level indicators through comparison with modern analogs and a chronology based on optically stimulated luminescence ages. Our reconstructions document a continuous but slow sea-level rise after 6.5 ka with an accumulated change in elevation of about 2 m. In the context of SW Europe, our results show good agreement with previous studies, including the Tagus isostatic model, with minor discrepancies that demand further improvement of regional models. This work reinforces the potential of barrier indicators to accurately reconstruct high-resolution mid-to late Holocene sea-level changes through simple approaches.
- Coastal lagoons and rising sea level: a reviewPublication . Carrasco, A. Rita; Ferreira, Óscar; Roelvink, D.Sea-level rise (SLR) poses a particularly ominous threat to human habitations and infrastructure in the coastal zone because 10% of the world's population lives in low-lying coastal regions within 10 m elevation of present sea level. There has been much discussion about projected (and the sources of projection) vs. measured SLR rates. Which rates should coastal scientists and managers apply in their studies, and what is the degree of confi- dence of such forecasts, are still open questions. This paper reviews the patterns and effects of relative SLR (RSLR) in coastal lagoons. Three main components are presented in the review: (a) a summary of the main approaches used in predicting medium- to long-term trends in RSLR, (b) a summary of the main evolutionary trends of coastal lagoons and the tools used to examine such trends, and (c) an identification of future research needs. The review reveals that the major source of uncertainty is how and when RSLR will manifest itself at different spatio-temporal scales in coastal lagoon systems, and how its effects can be mitigated. Most of the studies reviewed herein articulate a natural ‘defence’ mechanism of barriers in coastal lagoons by landward barrier retreat through continuous migration, and a gradual change in basin hypsometry during the retreat process. So far, only a relatively small number of detailed studies have integrated and quantified human impacts and coastal lagoon evolution induced by RSLR. We conclude that much more research about adaptation measures is needed, taking into consideration not only the physical and ecological systems but also social, cultural, and economic impacts. Future challenges include a downscaling of SLR approaches from the global level to regional and local levels, with a detailed application of coastal evolution prediction to individual coastal lagoon systems