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- The influence of coastal processes on inner shelf sediment distribution: The Eastern Algarve Shelf (Southern Portugal)Publication . Rosa, F.; Rufino, M. M.; Ferreira, O.; Matias, Ana; Brito, Ana C.; Gaspar, MiguelThis study examines sediment distribution patterns in the Southeastern Algarve inner shelf (southern Portugal), an area characterized by marked variations in its coastal environment and low continental supply of sediments. The specific goals of this study were to identify the principal sediment sources and the factors influencing sediment transport paths and deposition. A total of 199 samples, collected along the shelf from the Guadiana River mouth to Olhos de Agua, were analyzed. Grain-size distribution and parameters were measured for all the samples. Terrigenous and biogenic components of sand were identified in 38 samples, and results analyzed using multivariate non-linear multidimension scaling (MDS) and cluster analysis. Patterns of sediment distribution in this area of the inner shelf vary according to water depth and exhibit significant longshore variation, related mainly to coastal processes (littoral drift and storm currents) and to a lower degree to sediment sources. Sand is dominant at all depths, reflecting the influence of littoral drift in the supply and redistribution of shelf sediments. Fine and gravel-sized deposits are significant in specific areas and are usually associated with changes in sediment composition. Five sectors have been identified according to sedimentary dynamics. The results, based on geostatistical and multivariate analysis, have allowed detailed sediment distribution maps to be generated, which represent an update of the existing cartography and serve as a tool for the management of coastal and marine resources. They have been furthermore compared with inner shelf sediment dynamics in other regions worldwide, to distinguish between specific regional responses to forcing mechanisms and processes that are more generalized within this type of shelf environments. In this context, the results obtained results in the Algarve study area are of great interest for the understanding of sediment dynamics of sand dominated inner shelves with reduced continental supply.
- 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.
- Towards assessing the resilience of complex coastal systems: examples from Ria Formosa (South Portugal)Publication . Kombiadou, Katerina; Matias, Ana; Carrasco, Rita; Ferreira, Oscar; Costas, S.; Vieira, G.The present paper contributes to assessing the resilience of a complex barrier island environment, namely of the Ria Formosa multi-inlet system in southern Portugal. The long-term morphologic evolution of four study areas during the last 60 years (1947 to 2014) is analysed based on aerial photographs, including the environments of oceanic and backbarrier beaches, dunes and salt marshes. The results show that each study area responded to external drivers (inlet stabilisation works, storms, etc.) differently, evolving in distinct patterns during the study period. All four study areas appear resilient to external pressures and/or forcing conditions, since they are either transforming (Barreta and Culatra islands), or adapting (Cabanas island and Cacela peninsula) or remaining stable at a near-equilibrium state (Tavira island). Based on the analysis of the multi-decadal evolution of the sites, four resilient barrier states are identified, related to the maturity and growth of the barrier. In the next stages, the research will focus on the relation between medium to short-term changes, aiming at understanding the response and feedbacks of the environments to specific drivers of change and relating them to resilience indicators.
- Detailed investigation of overwash on a gravel barrierPublication . Matias, Ana; Blenkinsopp, Chris E.; Masselink, GerdThis paper uses results obtained from a prototype-scale experiment (Barrier Dynamics Experiment; BARDEX) undertaken in the Delta flume, the Netherlands, to investigate overwash hydraulics and morphodynamics of a prototype gravel barrier. Gravel barrier behaviour depends upon a number of factors, including sediment properties (porosity, permeability, grain-size) and wave climate. Since overwash processes are known to control short-term gravel barrier dynamics and long-term barrier migration, a detailed quantification of overwash flow properties and induced bed-changes is crucial. Overwash hydrodynamics of the prototype gravel barrier focused on the flow velocity, depth and discharge over the barrier crest, and the overwash flow progression across and the infiltration through the barrier. During the BARDEX experiment, overwash peak depth (0.77 m), velocity (5 m s−1 ) and discharge (max. 6 m3 m−1 ) were high, especially considering the relatively modest wave energy (significant wave height = 0.8 m). Conversely to schemes found in the literature, average flow depth did not linearly decrease across the barrier; rather, it was characterised by a sudden decrease at the crest, a milder decrease at the barrier top and then propagation as a shallow water lens over the backbarrier. The barrier morphological evolution was analysed over a series of 15-min experimental runs and at the timescale of individual overwash events. Overall, the morphological variation did not result from an accumulation of many small consistently erosive or accretionary events, but rather the mean bed elevation change per event was quite large (10 mm) and the overall morphology change occurred due to a small imbalance in the number of erosive and accretionary events at each location. Two relationships between overwash hydrodynamic variables were deduced from results: (1) between overwash flow depth and velocity a power-type relation was obtained; and (2) a linear relation was observed between overwash flow depth and maximum overwash intrusion distance across the barrier top (i.e. overwash intrusion). Findings from this study are useful to enhance the knowledge of overwash processes and also have practical applications. On the one hand, results shown here can be use for the validation of overwash predictive models, and additionally, the simple empirical relations deduced from the dataset can be used by coastal managers to estimate overwash intrusion distance, which in turn can assist in the location of areas under risk of overwash and breaching.
- Managing flood risk in fetch-limited environmentsPublication . Carrasco, Ana Rita; Ferreira, Óscar; Matias, AnaThere is little unanimity concerning the environmental modifications potentially forced by the acceleration of global climate change in the coming century, or the future actions required to work towards a sustainable development of natural habitats in the coastal zone. A simple framework to assess the potential implications of flooding in fetch-limited coastal environments (with no relevant wave setup), experiencing both coastal and river flooding, was recently developed. It identifies tide, storm surge, sea level rise and river discharge as the main sources of flooding hazards. Hazard maps differentiate distinctive zones of flooding: the constantly flooded zone, the frequently flooded zone, and the infrequently flooded zone. The framework prioritizes site-based local management, and identifies alternative tools for sustainable coastal resources, to face non-desired flood effects. It was applied to a morphologically complex (tidal channels network, salt marshes, dunes) occupied backbarrier coastal stretch located in Culatra Island, South Portugal (max. fetch ~ 4 km). The obtained inundation levels predicted for 1, 10 and 100 year-return periods were 2.02 m, 2.39 m, and 2.84 m above MSL, respectively. The high-, moderate-, and low-risk zones include about 34 m2 , 1,073 m2 and 31,821 m2 of occupied area, respectively. Besides houses, flood impacts also affect other infrastructures (e.g., walkways and recreational structures). Several best practices principles and guidelines were proposed for the three risk zones, namely the adaptation to uses with ‘added value’, which enhances the local ecological values and strengths economic activities. This study contributes to the method validation at a complex morphological area and enriches the understanding about the potential of flood in fetch-limited conditions, praising the perspective of ‘living with climate change’.
- 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.
- Historic monuments threatened by coastal hazards at Boca do Rio, Algarve, PortugalPublication . Carrasco, A. Rita; Ferreira, Óscar; Matias, Ana; Dias, J. A.The coastal stretch of Boca do Rio, western Algarve, Portugal, is experiencing shoreline retreat. Roman ruins are located along the beach section of the coast, and a 17th-century fort is positioned over the cliff section. Both historic monuments are being destroyed by erosion but need to be protected due to their heritage status. Aerial photograph analysis was used to define the evolution of the coast for the period 1945–2001. Shoreline retreat of 0.3 m/year was measured for the sandy coast and 14 mass movements in the cliff face were identified. Considering the rate and style of shoreline evolution, several types of management scenario were evaluated, with the favored measures being the removal of the Roman ruins to a museum and the relocation of the fort inland.
- Overtopping hazard on a rubble mound breakwaterPublication . Carrasco, A. Rita; Reis, Maria T.; Neves, Maria G.; Ferreira, Oscar; Matias, Ana; Almeida, SílviaA major concern of coastal engineering is not only to access the damage to coastal structures by severe wave overtopping, but also the hazard imposed to users. Local hazard is often associated to the volume of overtopping water per unit of time (called overtopping discharge). Despite two decades of intensive research, it is yet not fully clear to practitioners what is the best method to compute the discharge parameter and its application on the assessment of local hazard. This work provides insight into the overtopping characterization in rubble mound breakwaters, by distinguishing different methods to assess hazardous overtopping. Fieldwork was conducted over a tidal cycle in a breakwater located at Albufeira Harbour (South coast of Portugal) under storm conditions (Hso~ 3 m; Tp ~ 9 s). Mean overtopping discharges were calculated from field measurements of flow depths and velocities at the breakwater slope armour and at the impermeable crest. Two different velocities were calculated: overtopping leading-edge velocity and overtopping peak velocity. The two methods provided similar results, with higher velocities occurring during high-tide (between 2 and 10 m/s). Mean overtopping discharges at the beginning of the impermeable crest ranged between 0.2 and 0.8 l/s/m. Under the measured hydrodynamic conditions, the breakwater offers risk to all types of pedestrians. Additionally it is shown that field measurements compare relatively well with empirical prediction methods (for the overall analysed overtopping events), namely the corrected NN_OVERTOPPING2 neural network tool. Besides contributing to the overall database on wave overtopping in coastal structures, the presented results can also be used for calibration and validation of overtopping evaluation methods (empirical formulae, artificial neural networks and numerical and physical models).
- Estudo morfosedimentar da península de CacelaPublication . Matias, Ana; Dias, Alveirinho; Ferreira, ÓscarA Península de Cacela localiza-se no extremo oriental do sistema de ilhas barreira da Ria Formosa, caracterizado por possuir um carácter extremamente dinâmico, quer ao nível das movimentações sedimentares transversais, quer longilitorais. Devido à elevada vulnerabilidade em que o seu cordão dunar se encontrava foram realizados vários tipos de medidas de recuperação que incluem a alimentação artificial, a colocação de estacaria, a re-vegetação dunar e a colocação de passagens pedonais sobreelevadas. A presente dissertação visou por um lado a caracterização morfodinâmica da Península de Cacela e, por outro, a compreensão da evolução desta zona na sequência das intervenções, tentando encontrar formas de avaliar quantitativamente a sua eficácia. Para tal foi estabelecido um programa de monitorização que consistiu na realização mensal de perfis de praia emersa e na recolha de amostras de sedimentos de praia e de duna, embora com menor regularidade, e na realização ocasional de levantamentos topográficos em alguns locais da península. Foram ainda analisados dois anos de dados de agitação marítima, uma série de fotografias aéreas verticais (entre ~1940 e 1999) e a vulnerabilidade do cordão dunar através de uma checklist. A evolução recente da Península de Cacela (~1940 a 1999) caracterizou-se por uma substancial diminuição do cordão dunar, dado o significativo recuo da linha de costa, a migração para nascente da Barra do Lacém e o aumento das áreas de galgamento oceânico. Este processo conduziu a uma fragilização do cordão dunar desta península, tendo ocorrido ruptura total durante o inverno de 1995/96. No inverno seguinte (1996/97) as características desta península foram drasticamente alteradas pela implementação de um conjunto de medidas de recuperação dunar. A aplicação da checklist de vulnerabilidade dunar demonstrou uma melhoria das condições de praia e de duna devido às medidas de intervenção a que este troço costeiro foi sujeito. No entanto, a aplicação deste método a todos os corpos dunares da Ria Formosa revelou que, genericamente, os resultados são concordantes com as condições reais, mas existe necessidade de reformulação de certos parâmetros e índices. O material usado para a alimentação artificial das dunas da Península de Cacela possuía características granulométricas muito distintas das apresentadas pelas dunas naturais, tendo ocorrido aproximação granulométrica nos locais em que houve recuperação dunar por colocação de armadilhas de areia. Quanto à praia, verificou-se uma grande variabilidade longilitoral e temporal, já notória antes da realimentação. Esta heterogeneidade sedimentar pode constituir vi uma característica intrínseca deste troço costeiro, relacionada com uma fraca selectividade do agente de transporte, com a existência de variações longilitorais significativas da morfologia de praia emersa, ou com a existência de várias fontes sedimentares distintas. De um ponto de vista morfológico, a praia emersa apresentou variação entre dois estados distintos, o perfil de tempestade e o perfil de calmaria, desenvolvendo-se o primeiro numa escala de tempo relativamente curta, tendo-se observado que a recuperação para um perfil de calmaria foi relativamente lenta. Para uma análise quantitativa da evolução morfológica do perfil de praia foram estabelecidos cinco parâmetros, a que se designou por descritores (pendor da face, pendor do terraço, largura da berma, profundidade do runnel e volume de praia). O comportamento da praia, em termos de pendor de face, é relativamente coerente com a transição entre os dois estados da praia referidos, variação esta que se torna mais homogénea longitudinalmente no segundo ano analisado. Existe relativa concordância entre as variações volumétricas sazonais das zonas realimentada e natural, excepto no primeiro verão (1997), devido ao reajuste inicial da zona realimentada. Tendo sido efectuada a refracção dos dados de agitação para o litoral em estudo, foi efectuada a comparação entre a evolução dos vários descritores e as variações mensais e semanais da altura significativa da onda na rebentação (Hsb). Verificou-se que o pendor da face mostra dependência de Hsb média mensal na rebentação. A variação de volume da zona realimentada parece relacionar-se melhor com Hsb média semanal enquanto que a zona natural se relaciona com Hsb média mensal, o que pode ser um indicador de uma maior robustez da praia na zona onde se colocaram os dragados, ou resultar de uma fragilização da praia como consequência da remoção dos sedimentos da praia da zona dita natural, para colmatação dos galgamentos. Quanto ao depósito de dragados propriamente dito, ocorreu acentuada erosão (33% do total depositado) durante o período de monitorização. Estas perdas sedimentares foram mais acentuadas na zona adjacente à Barra do Lacém (45% da erosão registada) e nas zonas relativamente salientes do depósito, num processo que tende, provavelmente, para a linearização da frente da realimentação. Foram propostos métodos de avaliação da eficácia de alimentações artificiais e de estacaria de retenção de areia. A sua aplicação ao caso presente mostrou que o tempo de vida para o qual o índice de eficácia da realimentação é óptimo é de aproximadamente 14 anos. O método proposto de avaliação da estacaria mostrou que, globalmente, esta possui uma eficácia de cerca de 40%.
- Overwash experiment on a sandy barrierPublication . Matias, Ana; Masselink, G.; Kroon, A.; Blenkinsopp, C. E.; Turner, I. L.This paper uses results obtained from the large-scale BARDEX II experiment undertaken in the Delta flume to investigate the morphological response of a prototype sandy barrier to wave and tidal forcing during overwash conditions. Since overwash processes are known to control short-term barrier dynamics and long-term barrier migration, the development of a robust quantitative method to define the critical conditions leading to barrier overwash is important both for scientific and practical management purposes. The Overwash Potential (OP), defined as the difference between the wave runup and the barrier elevation is used to define the overwash threshold condition, and to predict the morphological outcome of a particular overwash event. When OP is negative, wave runup is lower than the barrier crest and insignificant morphological changes are noticed at the barrier crest. When OP is positive, overwash occurs because predicted runup elevation is higher than the barrier crest. When OP is close to zero, overtop is expected with limited intrusion of water across the top of the barrier crest. To make effective use of OP it is necessary to identify a reliable runup predictor. Twelve runup equations were tested for this purpose, and the results were compared with the ones obtained using data from BARDEX experiment on a gravel barrier. A most reliable approach for the determination of OP for sandy barrier was similar to gravel barrier overwash experiments, with runup predictions provided by the equation of Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A.,Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53, 573-588]. This is striking, since different runup predictors would have been expected because beach slope, hydraulic conductivity, grain-size, amongst other factors, differ for both types of barriers. Nevertheless, the two main morphologic characteristics for the computation of OP are beach slope and the barrier crest elevation, both accounted for in the proposed equation. The use of OP values provides a practical means by which to identify potential coastal hazards associated with barrier overwash processes and is considered to have a range of practical coastal management applications.