Percorrer por autor "Freire, Paula"
A mostrar 1 - 6 de 6
Resultados por página
Opções de ordenação
- 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.
- Morphodynamics of fetch-limited beaches in contrasting environmentsPublication . Freire, Paula; Ferreira, Óscar; Taborda, Rui; Oliveira, Filipa; Carrasco, Ana Rita; Silva, Ana; Vargas, Catarina; Capitão, Rui; Fortes, Conceição Juana; Braga Coli, Alexandre; Santos, JoãoSandy beaches can be found in fetch-limited environments that are protected from ocean generated waves, as estuaries, lagoons, and backbarriers, and where fetch characteristics allow local wind-generated waves to develop and maintain a beach. The morphodynamics of these low-energy beaches present a peculiar behaviour and general open-ocean models are inappropriate for their study.
- Morphological changes in a low-energy backbarrierPublication . Carrasco, Ana Rita; Ferreira, Óscar; Freire, Paula; Dias, J. A.Profile characteristics of low energy sandy beaches include narrow foreshores that are often steep, with reflective swash zones. Seaward of the foreshore a low gradient terrace is generally present, acting as a wave energy filter. Low magnitude changes are usually associated to this type of environments, either cross-or longshore. To examine the short- (between months) and medium-term (from months to years) morphological changes at Ancão backbarrier (Ria Formosa), data from monthly cross-shore surveys was used. The degree of morphological mobility was given by the volumetric variability within specific morphologic units (foreshore, tidal flat and sand spit) and through the analysis of three cross-shore sections. The results show low medium-term variability, not seasonally distributed, and without a direct connection to changes on average wind intensity. The three morphologies are not interdependent and do not show a clear relation on sediment exchanges between them. Low intensity short-term changes are however present, which seem to be related with net sediment adjustments.
- Natural and human-induced coastal dynamics at a back-barrier beachPublication . Carrasco, A. Rita; Ferreira, Óscar; Matias, Ana; Freire, PaulaThis study contributes to the understanding of very low-energy fetch-limited environments by reporting the evolution of a back-barrier beach (Ancão Peninsula, southern Portugal). It considers two timescales: a large-scale evolution for the past 60 years based on aerial photograph analysis, and a small-scale beach evolution based on monthly topographic surveys performed during three years of monitoring. Each timescale revealed a different rate of evolution, the first reporting a modified beach response-type (from human activities), and the second reporting a natural beach response-type. Human activities caused significant changes in the back-barrier shore, whereas changes under natural forcing were much smaller, were less influential on the area's evolution, and were not sufficient to counteract or mask the consequences of human activities. The findings of the study should contribute to a better understanding about the large- and small- scale changes in other back-barriers characterised by similar very low-energy conditions.
- Short-term sediment transport at a backbarrier beachPublication . Carrasco, Ana Rita; Ferreira, Óscar; Matias, Ana; Pacheco, André; Freire, PaulaThis article defines short-term (tidal cycle) sediment-transport patterns at a backbarrier beach based on detailed field studies. Fieldwork was planned to record nonstorm, spring tide conditions that enable the definition of background sediment dynamics at the study area. The experiment was set at two beach morphologies: beach face and sandbank. Current meters were deployed at both sites. Fluorescent tracer techniques were applied to determine rates and direction of transport. Topographic surveys and sediment sampling were undertaken. During the experiment, limited tracer displacement and small morphological changes occurred. The low magnitude changes are representative of predominant, low-energy hydrodynamic conditions. Sediment transport was primarily longshore orientated and dependent on the velocity of tidal currents. Tidal currents had the potential for sediment transport only during a short period of the tidal cycle and had higher velocities during ebb, which results in a net sediment transport orientated towards the nearby inlet. It is suggested that there is dependence on sediment transport rate at the study site and on the variability of tidal currents, which are greatly influenced by the distance to, and conditions of, the nearby Ancão Inlet.
- Temporal assessment of the water quality along an urban estuary (Tagus estuary, Portugal)Publication . Rodrigues, Marta; Cravo, Alexandra; Freire, Paula; Rosa, Alexandra; Santos, DanielaUnderstanding the natural dynamics of estuaries and their response to changes in the human-related or climatic drivers is fundamental to guarantee their environmental quality. This study aims to contribute to a better understanding of the water quality dynamics in an urban estuary, the Tagus estuary (Portugal), across different temporal scales. This estuary supports diverse uses and activities and its ecological value is well recognized. Three field campaigns were performed covering distinct seasons (Spring, Summer and Autumn - 2018). Physical, chemical and biological data were measured along the estuary. The acquired data was then integrated into a set of indicators and compared with historical data, to assess the long-term evolution of the water quality in the estuary. Results showed marked spatial gradients, typically with larger concentrations of chlorophyll a, nitrate and silicate upstream. Silicate and nitrate presented a conservative behavior. Ammonium and phosphate were not conservative, which suggests an anthropogenic pressure along the estuary. Seasonally, the highest nutrients and suspended solids concentrations were found in Autumn, after a period of rainfall, pointing out to the relevance of land runoff for material supply into the estuary. These conditions were favorable for phytoplankton development upstream, once chlorophyll a was maximum during this campaign (24 mu g/L in the upper estuary). Regarding the nutrients classification, results suggest that the upper estuary is at risk with a "Medium" status. The middle and downstream areas presented a "High" status, with the exception of the middle-right margin that is also at risk ("Medium" status). The assessment of the historical data suggests that high loads of nutrients have been reaching the Tagus estuary over time, with a decreasing trend in recent years. The trophic index (TRIX) suggests a "Moderate" trophic status in the middle and upstream areas of the estuary. As for nutrients, a "Poor" classification was obtained in the middle-right margin of the estuary, mainly due to ammonium concentrations, confirming an intensification of anthropogenic pressure at this site. Similarly to the nutrients status, TRIX suggests some improvement of the water quality in the Tagus estuary in recent years compared to the 1980s. These results provide further insight regarding the water quality dynamics in the Tagus estuary and are useful to support management.