Paula Freire

Wind-induced sand transport in Tagus estuarine beaches – First results

Sand beaches and spits of the southern embankment of the inner Tagus estuary indicate the influence of locally generated wind waves. The aim of this study is to evaluate the ability of these waves to resuspend bottom sediments, and sustain the longshore transport. For this purpose, local wave climates were obtained by running a wave forecasting numerical model for three selected target areas.Results suggest that local wave climate is largely responsible for sand movement and accumulation in the marginal zones of the inner estuary facing the longest effective fetch. Two main areas of longshore transport were identified in the inner Tagus estuary with similar annual gross transport rates, of about 12 000 m3 yr−1. Results obtained for the annual net drift amount and direction are in agreement with geomorphology and accretion direction of the beaches. Wave-induced longshore transport rates agree with field observations and erosional evolution of the Alfeite sand beach during the last century.

Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios

ABSTRACT Rilo, A., Freire, P., Guerreiro, M., Fortunato, A.B., Taborda, R. 2013. Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios. Sea level rise (SLR) is increasing the risk of flooding in marginal low-lying estuarine areas. High water levels in estuaries are associated with the simultaneous occurrence of various phenomena ranging from high tidal levels to large fresh-water discharges. The intense and diverse human occupation of some of these marginal areas increases the flood vulnerability, endangering human lives and causing heavy economic and environmental losses. This issue becomes even more relevant since the European Floods Directive establishes mandatory guidelines to all member states regarding flood risk assessment, integrating flood maps with territorial characteristics. The present study aims to assess the vulnerability to floods of a specific area in Tagus estuary, by integrating hydrodynamic modelling results with land use cartography. Results suggest that about 16.1% of the estuarine marginal fringe could be flooded for the 2050 scenario, rising up to 23.7% for the 2100 scenario. Moreover, industrial and urban areas will be the most affected sites. Considering future development scenarios planned for that site, the results point to an increasing vulnerability of urban areas and therefore to the relevance of this knowledge in future planning and management guidelines as well as measures to adapt to SLR. The uncertainties and implications of the study are also discussed, and future research topics are indicated.

A local-scale approach to estuarine flood risk management

New challenges in flood risk management are raised by climate change and land-use development. These challenges are particularly complex in estuarine and coastal systems, where different hazard sources interact in a dynamic socio-economic context. This paper presents an innovative approach to support flood risk management in estuaries. The approach, developed at a local-scale basis, is applied in the case study of the Tagus estuary (Portugal). The methodology is supported by the regional framing of the study area and integrates hazard, exposed elements, territorial vulnerability and risk assessments considering different climate scenarios. Through the involvement of the various risk management dimensions, the results allow the definition of a new decision-making supporting framework for emergency and land-use planning. At the emergency level, the results include a WebGIS interface providing an early warning system for the locations with highest risk of flooding and the definition of emergency planning guidelines. A set of flood adaptation actions based on land-use and occupation measures are recommended to increase resilience in face of flooding and future sea level rise. The institutional capacity-building is achieved through the availability of information and tools that can effectively support decision-making. Additionally, the outcomes contribute to better understand flood risk in estuaries and to strengthen its prevention, preparedness and response, priorities defined in the Sendai Framework for Disaster Risk Reduction 2015–2030.

Short-Term Sediment Transport at a Backbarrier Beach

Abstract This 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. Resumo O presente artigo define os padrões de transporte sedimentar, a curta escala de temporal (a nível do ciclo de maré), para uma praia interna localizada no interior de um sistema de ilhas barreira. Os trabalhos de campo decorreram durante um clima de não tempestade, em regime de marés vivas, permitindo a definição das condições morfodinâmicas de base da praia. A experiência incidiu em duas morfologias, na face de praia e num banco arenoso paralelo à praia, localizado na parte terminal do perfil. Em cada uma dessas morfologias foi colocado um correntómetro. Nos mesmos locais procedeu-se ainda à colocação de traçador fluorescente, para aferir a taxa e direcção de transporte sedimentar. Foi ainda efectuado um levantamento topográfico da área total, e colheita de sedimento superficial. Na generalidade, observou-se fraca deslocação do traçador desde o ponto de injecção, bem como a ocorrência de alterações morfológicas pouco significativas. O clima de não tempestade a que se associam estas fracas alterações morfológicas representa as condições hidrodinâmicas típicas da praia. O transporte sedimentar é maioritariamente longilitoral, sendo controlado pela velocidade da corrente de maré. As correntes de maré apresentaram potencial para desenvolvimento de transporte sedimentar durante um curto período do ciclo de maré, com maiores velocidades em vazante. Em resultado, o transporte sedimentar é preferencialmente orientado para a barra de maré. Assim, sugere-se a existência de uma dependência entre a magnitude do transporte na área de estudo e as correntes de maré, directamente relacionadas com a Barra do Ancão.

Operational forecast framework applied to extreme sea levels at regional and local scales

ABSTRACT The design, implementation and demonstration of a novel and generic computational forecast framework for multi-scale prediction of extreme sea levels and associated flooding is presented. Denoted Water Information Forecast Framework (WIFF), it integrates process-based models for waves, tides and surges from regional to local scales, predicting the flooding of coastal areas, and supporting the routine and emergency management of coastal resources. WIFF manages the simulations and the real-time monitoring data, archives the data and makes the information available through a WebGIS that targets users with distinct access privileges. Additionally, the web component of WIFF adapts automatically and transparently to any device. WIFF also provides ways to assess the model accuracy and generates tailored products based on model results and observations. WIFF is demonstrated in the prediction of extreme water levels in the Portuguese coast, simulating processes at different scales: at basin scales, waves are simulated in the North Atlantic and in the Portuguese shelf, and sea levels due to tides and atmospheric forcings are simulated in the Northeast Atlantic; at estuarine scales, high-resolution, fully coupled wave/circulation predictions are performed in the Tagus estuary to account for wave–current interactions. User-oriented georeferenced products are generated, including automatic model/data comparisons, targeting the needs of civil protection agents and combining for the first time an agile, service-oriented platform with high-resolution, process-rich predictions of the Tagus dynamics.

Geomorphological response of the salt-marshes in the Tagus estuary to sea level rise

ABSTRACT Silva, T.A., Freitas, M.C., Andrade, C., Taborda, R., Freire, P., Schmidt, S., Antunes, C., 2013. Geomorphological response of the salt-marshes in the Tagus estuary to sea level rise. Salt-marshes are highly valuable ecosystems due to their role in supporting the aquatic and bird life. Furthermore, many anthropogenic activities such as agriculture, salt production or aquaculture targets these areas. They also act as protective barriers to shores, given their ability to dissipate most of the wave and current energy in high tide. Sea level rise can place these intertidal zones at risk, reinforcing the need to understand their morphosedimentary and dynamic response to the variations on the forcing factors, thus allowing for a better management of these environments. Large expansions of salt-marshes and tidal flats are among the morphodynamic contents of the Tagus estuary. This study focuses on the recent past evolution of Tagus marsh areas in order to understand their geomorphological response to higher sea level scenarios. Cores were taken in four contrasting high salt-marsh expansions in estuarine margins (Trancão – TR, Mouchão da Póvoa – MP, Pancas – PA and Corroios - CO). Marsh surfaces were surveyed using DGPS-RTK and tidal regime characterized at each location. The cores reached at least 1.20m in depth and were sub-sampled every cm for 210Pb and 137Cs radioisotope determination, allowing the derivation of sedimentation rates. In all locations, accretion rates clearly exceed the post-1920 mean rate of sea level rise (+0.21cm/year, Cascais gauge). Their linear extrapolation into the future, until reaching the upper threshold of marsh surface aggradation (MSHT), suggests that Tagus marginal marshes will not drown under the projected sea level elevation scenarios for the end of the 21th century.

Defining beaches and their evolutionary states in estuaries

ABSTRACT Freire, P., Jackson, N.L. and Nordstrom, K.F., 2013. Defining beaches and their evolutionary states in estuaries Projected rates of global sea level rise and human pressures have increased attention to the potential for landform change in estuaries. This paper assesses the status of the fetch-limited beaches in the Tagus estuary, one of the largest estuaries in Europe, with a focus on distinguishing active beaches from inactive vegetated banks and identifying conditions under which they change state. A total of 26 beaches were identified in the inner estuary and 49 in the tributary basins on 2007 aerial photographs and compared with conditions on older photographs (1944–1958). Lengths, widths and maximum fetch distances for beaches were measured and site visits were made to determine their origins and present conditions. Beaches occur at eroding uplands or marshes or on spits extending from eroding uplands. Human-created beaches occur on spoil areas, within niches formed by structures and where vegetation is eliminated or prevented from colonizing (e.g. boat launches and recreational surfaces). Basin infilling, with increase in the elevation of low tide terraces and the formation of bars, is reducing wave energies, and some beaches are reverting to vegetated banks. Beaches that become vegetated banks because of human actions occur where use for boating or recreation is abandoned and where spits that form off spoil deposits reduce fetch distances upwind.

Characterisation of the dynamics of Figueira da foz beach, Portugal

ABSTRACT The characterisation of the dynamics of Figueira da Foz beach, in the West coast of Portugal, is presented based on the analysis of previous studies and results obtained from the application of a numerical model. Advances on the understanding of the beach dynamics have been achieved by the quantification of the annual and seasonal longshore transport and several beach profile parameters. An evaluation of the contribution of the wave climate components on the longshore transport was also obtained. The results allow sustaining a prediction of the effect of a future change of the wave climate on the longshore transport. Figueira da Foz shoreline change was simulated based on different conceptual scenarios and the results, although not quantitatively conclusive, allow the understanding of the influence of the neighbouring beach hard rock outcrops on the recent evolution.

Effect of inlet morphology and wave action on pollutant pathways and sediment dynamics in a coastal stream

Hydrodynamics and water renewal of intermittent coa stal streams are highly variable, at various time scales, due to the very active morphod ynamic behavior of their inlets. Due to this strong dynamics, the pathways of water-born e materials – and the consequences of contaminated discharges – can depend strongly on the morphology and environmental conditions. Predicting the fate of contaminants in these systems requires coupled numerical models accounting for the major physical and water quality processes. We aim at improving the understanding of the impact of inlet morphology and wave action on the pollutant and sediment pathways of th ese small coastal systems, based on a suite of calibrated and validated coupled models. T wo analyses, based on particle simulations, are presented to assess sediment dynam ics and pollutant pathways for several conditions. Results show that waves have a major effect on the fate of waterborne materials in the estuary. Wave-induced curren ts sweep away materials coming out of the estuary, while wave-induced setup has a prof ound effect on tidal propagation, water levels and velocities in the estuary, promoti ng he upstream transport of pollutants. 1 National Laboratory for Civil Engineering, Av. do B rasil, 101, 1700-066 Lisbon, Portugal; {aoliveira, afortunato, mguerreiro, xbertin, nbrune au, mfrodrigues, pfreire, lsimoes, gdodet, amendes}@lnec.pt. 2 Faculdade de Ciências da Universidade de Lisboa, Ca mpo Grande, Ed. C6, 1749-016 Lisboa, Portugal; {rtaborda, candrade, amasilva, cm antunes}@fc.ul.pt. 3 CIMA, Universidade do Algarve, Campus de Gambelas, Ed. 7, 8005-139 Faro, Portugal; cloureiro@ualg.pt.

Estuarine flooding in urban areas: enhancing vulnerability assessment

Abstract The assessment of vulnerability provides valuable knowledge in the risk assessment steps of a risk governance process. Given the multiscale, multilevel, and multisectorial aspects of flood risk, the diversified entities that directly and indirectly intervene in risk management require specific outputs from the assessment studies. Urban areas in estuarine margins are particularly exposed and vulnerable to flooding. Such interface conditions are found in the Old City Centre of the Seixal, located in the Tagus estuary, Portugal. Here, two distinct methodologies were applied for the assessment of territorial vulnerability. A regional, lower-scale, methodology explores the application of the statistical procedure based on the SoVI® at the statistical block level. A second, local and higher-scale, methodology is based in data collected through field matrices at the building and statistical sub-block level. Comparison of results revealed that the lower-scale assessment provides information on the vulnerability drivers at the regional and municipal level. Nevertheless, only at a higher-scale, it is possible to characterize and differentiate the smaller geographical units of analysis that compose the Old City Centre of Seixal. The lower-scale vulnerability assessment allows a strategic response, based on adaptation measures such as spatial planning, institutional capacity building and public awareness. The local level assessment provides more accurate knowledge to support local emergency planning and the allocation of operational and material resources at the urban level. Nevertheless, rather than antagonistic, both models can be considered as complementary, having in mind the requirements of an holistic flood risk governance model.