Tidal channel hydrodynamics and salt marsh deposition under varying sea-level rise scenarios

In this study, two process-based modelling approaches built in the Delft3D domain were applied to predict changes induced by sea-level rise (SLR) in the tidal circulation and bathymetric evolution of the Ria Formosa coastal lagoon. An unstructured grid modelling approach was calibrated and validated to predict changes in the tidal circulation in the year 2100 of the western sector of the lagoon. The model was forced by tides and a static sea level, with four scenarios of sea-level rise considered. To predict bathymetric changes in the lagoon, a structured grid modelling approach was forced by tides and a linear sea-level rise, with one scenario of sea-level rise considered. Model results indicate that sea-level rise (SLR) will change residual sediment circulation patterns over the 89-year study period, with the intensity of the impact directly influenced by the intensity of SLR. Under sea-level rise, flood-dominance of the Ancão Inlet increases, hinting to an attenuation of flood dominance in the Faro-Olhão Inlet and a higher role of the Ancão Inlet in capturing the tidal prism. Bathymetric changes over time are considerable in the Ancão and Faro-Olhão inlets and marginal in the channel and tidal marsh areas. Despite some limitations, the results of this study contribute to the scientific knowledge about the impacts of SLR in the hydrodynamics and morphodynamics of coastal lagoons. This study also provides a first exploration of the morphological changes induced by SLR in the Ria Formosa coastal lagoon and recommendations for future research.Neste estudo duas abordagens de modelação baseada em processos (construídas no domínio Delft3D) foram aplicadas para prever mudanças induzidas pela subida do nível do mar na circulação das marés e evolução batimétrica da lagoa costeira da Ria Formosa. Uma abordagem de modelação de malha não estruturada foi calibrada e validada para prever mudanças na circulação das marés no ano 2100 no setor oeste da lagoa. O modelo foi forçado pelas marés e um nível do mar estático, com quatro cenários de subida do nível do mar considerados. Para prever mudanças batimétricas na lagoa, uma abordagem de modelação de malha estruturada foi forçada pelas marés e uma elevação linear do nível do mar, com um cenário de subida do nível do mar considerado. Os resultados do modelo indicam que a subida do nível do mar (SNM) mudará os padrões da circulação residual de sedimentos ao longo do período de estudo de 89 anos, com a intensidade do impacto diretamente influenciado pela intensidade da SNM. Sob o aumento do nível do mar a dominância de enchente da Barra do Ancão aumenta, sugerindo uma atenuação da dominância de enchente na Barra de Faro-Olhão e um maior protagonismo da Barra do Ancão na captura do prisma das marés. As alterações batimétricas ao longo do tempo são consideráveis nas barras do Ancão e Faro-Olhão e marginais nas zonas do canal e sapal. Apesar de algumas limitações, os resultados deste estudo contribuem para o conhecimento científico sobre os impactos da SNM na hidrodinâmica e morfodinâmica de lagoas costeiras. Este estudo também fornece uma primeira exploração das alterações morfológicas induzidas por SNM na lagoa costeira da Ria Formosa e recomendações para pesquisas futuras

Salt marsh response to changing hydrodynamics: the case of Ancão Inlet migration (Ria Formosa coastal lagoon)

Concerning the economic and ecological importance of salt marshes, it is of extreme importance to improve our knowledge on the physical processes involved with marsh evolution, as well as the limits and degree of interaction with the surrounding sedimentary sources. The studied salt marsh area is located fronting Ancão Inlet, in the western sector of the Ria Formosa barrier system. A 67-year time series of aerial photographs and ortho-photographs, between 1947 and 2014, was used to infer salt marsh sedimentary evolution (progression or recession) in relation to natural inlet migration and to its relocation in 1997. The following morphologies were mapped: salt marsh, tidal flat, sand banks, and inlet flood delta. The results focus on the analysis of horizontal displacement of these morphologies over time. It was noted that in general, the Salt Marsh presents a growth of its area along time. The different stages of the Ancão inlet migration and the consequent distance to the inlet throat is a limiting factor for salt marsh growth. The sand banks and flood deltas are the main sediment feeders of the Tidal Flat growth. The contribution of the Ancão inlet to the Tidal Flat growth was more significant when the inlet starts reaching its closure phase. The growth of Tidal Flat is an opportunity for salt marsh developmentTendo em conta a economia e ecologia dos sapais de maré, é de extrema importância melhorar os conhecimentos sobre os processos físicos envolvidos na sua evolução, bem como o conhecimento acerca dos seus limites físicos e interações existentes com as fontes sedimentares presentes no sistema. A área de estudo deste trabalho localiza se em frente a uma barra de maré, Barra do Ancão, localizada no sector oeste do sistema de ilhas barreira Ria Formosa, Portugal. Foi utilizada uma serie temporal de 67 anos de fotografias aéreas e ortofotomapas, entre 1947 e 2014, para inferir a evolução sedimentar do sapal (progressão ou regressão) em relação à migração natural da barra do Ancão e à sua relocação em 1997. Os seguintes ambientes foram mapeados: sapal de maré, planície de maré, bancos arenosos nos canais e delta de enchente da barra. A análise de resultados incluiu a análise da variabilidade horizontal destes ambientes ao longo do tempo. No geral observou-se um crescimento na área do sapal ao longo do tempo, influenciado pela migração da barra. Os diferentes estágios de migração da barra, e consequente proximidade à barra, são fatores limitantes para o crescimento do sapal em estudo. Verificou se que os bancos arenosos no canal e o delta de enchente da barra são as principais sumidores sedimentares para o crescimento da planície de maré. A contribuição da barra para o crescimento da planície de maré é mais significativa quando a barra começa a aproximar-se da sua fase de encerramento. Este aumento na planície de mare, por sua vez, leva ao aumento da área de sapal.Would also like to acknowledge the providing of the aerial photographs obtained from the framework of the EVREST project (PTDC/MAR-EST/1031/2014), funded by FCT (Fundação para a Ciência e a Tecnologia)

Variability of nutrients and chlorophyll a in Ancao Inlet during a spring tidal cycle in April 2009

Sao Luis or Ancao Inlet is the most western of the six inlets of the Ria Formosa lagoon, in southern Portugal. This inlet contributes relatively little to the water volume exchange over a tidal cycle (similar to 10%). Temperature, salinity, pH, dissolved oxygen, and chlorophyll a and nutrient concentrations were measured hourly at the surface, at an intermediate level, and near the bottom over a complete spring tidal cycle, at a sampling point in the middle of the inlet. Current velocity was also measured hourly over the entire cross-section of the inlet using an acoustic Doppler current meter. From these parameters, the flow rate and the transport of nutrients and chlorophyll a were calculated and integrated over the complete tidal cycle providing the net transport. The results show that this is a well-mixed dynamic inlet where, during spring tide, the tidal signal is evident particularly for the nutrient and chlorophyll a concentrations. The nutrient concentrations were low (almost depleted), while the chlorophyll a concentrations were high (4-6 mg m(-3)). The estimated transports through Ancao Inlet revealed that there was a net import of chlorophyll a, nitrate, and phosphate to the Ria Formosa lagoon, while silicate was exported to the adjacent coastal area.Center for Marine and Environmental Research (CIMA, Portugal)info:eu-repo/semantics/publishedVersio

Hydrodynamic and sediment fluxes through the inlets of the Ria Formosa

Tese de dout., Ciências do Mar, da Terra e do Ambiente (Ciências do Mar-Oceanografia Física), Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2011Research 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.Fundação para a Ciência e Tecnologia (FCT)União Europeia. FEDE

Aeolian sediment transport potentials and dune evolution in Ancão peninsula (Ria Formosa), Portugal

In the present study is attempted to examine aeolian sediment transport potential and to determine the constituents that promote/restrict the dune system development of Ancão Peninsula, where, apparent alongshore geomorphological differences, i.e. western and eastern sector, exist. In this study were investigated: wind potential for sediment transfer over the last 20 years (1997-2016) using recent granulometric data, dune morphology and elevation changes obtained from RTK-GPS beach profiles (2016), volumetric changes in three backshore polygons extracted from Lidar and UAV datasets (2009-2011 and 2011-2016) in both sectors. Presence of species communities of vegetation and shoreline evolution rates (1958-2014) as an indirect indicator for dune morphology were also used. Additional key parameters, namely littoral drift, updrift nourishment interventions and effect of Ancão Inlet were also considered. A windy event was defined at velocity above 8.9 m/s for a mean beachface grain size of 466 μm. Accounting for onshore winds and steering of the peninsula, only 1.7% of the winds was responsible for potential sediment transport. Moreover, the wind records, revealed that the windier years for potential transfer were 2002 (1.7 m3/m), 2009 (2.1 m3/m) and 2010 (1.8 m3/m). Shoreline evolution showed negative values (retreat) for the western and central sector of the peninsula (-0.2 m/yr and -0.25 m/yr respectively) and accretional (+0.79 m/yr) for the eastern. The volume change rates for 2009-2011 show accumulation with very low positive values for the western sector that progressively increase towards the eastern (from eastern polygon: 0.85 m3/(m.yr) to western: 7.32 m3/(m.yr)). The vegetational communities in the two sectors are compatible to the respective morphologies. In the western sector the absence of typical embryo dune vegetation (e.g. Elymus farctus, Eryngium maritimum) conforms with the absence of new foredune development. In the western part of the eastern sector, sparse embryo dune vegetation with low density is present, whereas in the most eastward part of the peninsula -in the inlet vicinity- two seaward developing foredunes are formed. Assembling all elements, it is concluded that the mild wind regime in conjunction with the existing coarse grain size permits the development of relatively small dunes. In the western sector the vegetation suggests dune maturity, however the updrift nourishments have contributed to shoreline stabilization and subsequently to restriction of dune erosion. On the eastern sector nevertheless, the presence of Ancão inlet fostered creation of accommodation space through shoreline progradation, and hence incipient dune vegetation and new embryo dunes formation.No presente estudo avaliou-se o potencial eólico de transporte sedimentar e determinaram-se os fatores responsáveis pela promoção/restrição do desenvolvimento do sistema dunar da Península do Ancão, onde se verificam diferenças geomorfológicas, por exemplo entre os sectores este e oeste. Este estudo analisou o potencial eólico na transferência de sedimentos nos últimos 20 anos (1997-2016), utilizando para tal dados granulométricos recentes, a morfologia dunar e variações de elevação obtidas a partir de perfis de praia efetuados com RTK-GPS (2016) e a determinação de mudanças volumétricas em três polígonos costeiros extraídos de conjuntos de dados de Lidar e UAV (2009-2011 e 2011-2016). Utilizou, ainda, a presença de comunidades de espécies de vegetação dunar e taxas de evolução da linha de costa (1958-2014) como indicadores indiretos para a evolução da morfologia das dunas. Parâmetros-chave adicionais, nomeadamente a deriva litoral, as intervenções de realimentação de praia e o efeito da Barra do Ancão foram também considerados na análise. Foram definidos como eventos com capacidade de transporte eólica os registos em que a velocidade do vento foi superior a 8,9 m/s, para um tamanho médio de grão de praia de 466 μm. Tendo em consideração os ventos dirigidos para terra e a direção da península, verifica-se que apenas 1,7 % dos ventos foram responsáveis por transporte potencial de sedimentos. Os registos de vento revelaram que os anos com maior potencial de transporte eólico foram 2002 (1,7 m3/m), 2009 (2,1 m3/m) e 2010 (1,8 m3/m). A evolução da linha de costa mostrou valores negativos (recuo) para os setores ocidental e central da península (-0,2 m/ano e -0,25 m/ano, respetivamente) e avanço da linha de costa (+0,79 m/ano) no sector este. As taxas de variação volumétrica, entre 2009 e 2011, mostram acumulação, com valores positivos muito baixos para o setor ocidental que aumentam progressivamente em direção a leste (R5: 0,85 m3/m ano, R3: 1,2 m3/m ano, R2: 7,32 m3/m ano). As comunidades vegetais dos dois setores são compatíveis com as respetivas morfologias. No sector ocidental, a ausência de vegetação típica das dunas embrionárias (por exemplo Elymus farctus, Eryngium maritimum) está em conformidade com a ausência de desenvolvimento de um novo cordão dunar frontal. Para oeste do setor oriental, verifica-se a existência de vegetação de dunas embrionárias, com baixa densidade, enquanto que na parte mais a leste da península - na vizinhança da barra de maré – se desenvolveram duas crista de dunas frontais, em direção ao mar. Conclui-se que o regime de ventos moderado, em conjunção com o tamanho de grão grosseiro existente, limita a promoção do crescimento dunar. No setor ocidental, a vegetação sugere que as dunas apresentam um elevado estado de maturidade. As realimentações de praia efetuadas a barlamar contribuíram para a estabilização da evolução da linha de costa e, subsequentemente, para a redução da erosão das dunas. No sector oriental, a presença da Barra do Ancão fomentou a criação de espaço de acomodação, através da progradação da linha de costa e permitiu, consequentemente, a formação de vegetação dunar incipiente e a formação de novas dunas embrionárias

Measuring and modelling over wash hydrodynamics on a barrier island

Overwash hydrodynamics datasets are mixed in quality and scope, being hard to obtain due to fieldwork experimental difficulties. 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 performed on Barreta Island (Portugal), in December 2013, during neap to spring-tides, when significant wave height reached 2.64 m. During approximately 4 hours, more than 120 shallow overwash events were measured with a video-camera (at 10 Hz), a pressure transducer (at 4 Hz) and a current-meter (at 4 Hz). This high-frequency fieldwork dataset includes runup, overwash number, depth and velocity. Fieldwork data along with information from literature were used to setup XBeach model in non-hydrostatic mode. The baseline model had variable skills over the duration of the overwash episode, performing better during the rising tide than during the falling tide. Model average number of events RMSE (root-mean-square-error) was 7 events each 30 minutes. The baseline model was forced to simulate overwash with different nearshore morphology, grain-size and lagoon water level. An average decrease of about 30% overwash was obtained due to changes in the nearshore morphology, mostly a small vertical growth of the submerged bar. The coarser and finer grain-sizes tests produced an 11% change in overwash, with less overwash on the coarser barrier. Changing lagoon water levels had a reduced effect on overwash hydraulics

Evolution and stability of tidal inlets: a comparison between two natural cases along human-affected coastlines

Il focus della tesi l'evoluzione di due bocche lagunari non stabilizzate, le bocche di Anc\ue3o e S. Andrea, situate in due diversi sistemi multi-inlet influenzati dall'uomo. Vorremmo analizzare le modalit\ue0 secondo la quali le caratteristiche naturali delle bocche possono essere indirettamente condizionate dalla presenza dell'uomo e dagli eventi naturali. L'obiettivo \ue8 quello di riconoscere i conflitti generati dal comportamento libero (senza restrizioni) dei tidal inlet studiati in relazione alle esigenze e agli usi dell'uomo. Attraverso un'analisi di dati morfologici e idrodinamici, vengono proposte alcune linee guida in relazione ai cambiamenti morfologici delle bocche e del sistema di appartenenza. La bocca di Anc\ue3o \ue8 un'apertura artificiale, rilocata lungo il suo conosciuto passato percorso migratorio. Una volta aperta, gli \ue8 permesso di riprendere naturalmente il suo percorso di migrazione. Si tratta di una piccola bocca, migrante verso est, nel sistema multi-inlet della laguna mesotidale di Ria Formosa (sud del Portogallo). La laguna \ue8 stata interessata da alcuni cospicui interventi antropici che hanno modificato l'ex condizione idrodinamica. La bocca di Anc\ue3o. Man mano che la migrazione di Anc\ue3o procede verso est, la bocca riduce gradualmente la connessione con le altre bocche, diventando pi\uf9 indipendente. L\u2019area di Anc\ue3o \ue8 costantemente monitorata dal 1997. I dati idro-morfodinamici sono stati raccolti durante diversi rilevamenti topo-batimetrici. I parametri morfologici e i tassi di migrazione sono stati successivamente misurati e correlati con i dati di altezza d'onda e di marea, considerando anche eventuali storm surge, al fine di valutare l'importanza dei diversi meccanismi nelle fasi evolutive della bocca. Il trend di migrazione e le variazioni dei principali dati morfodinamici sembrano essere legati principalmente alle condizioni dominanti del mare da sud-ovest, dimostrando la predominanza del clima d'onda sulle correnti di marea. La natura non vincolata della bocca di Anc\ue3o, consente di rispondere alle forze naturali attraverso la migrazione del suo canale principale. Ci\uf2 crea problemi nella circolazione dell'acqua nell'area di laguna retrostante oltre a ostacolare la navigazione. Per queste ragioni, la pozione della bocca di Anc\ue3o \ue8 attualmente controllata e gestita con periodiche rilocazioni updrift, per mantenere la navigabilit\ue0, la buona ossigenazione e lo scambio di nutrienti nell\u2019area lagunare. Il secondo caso studio \ue8 la bocca naturale di S. Andrea, situata nella Laguna di Marano e Grado (Mare Adriatico, Italia), un altro sistema multiplo. Questo differisce dalla laguna portoghese per il suo regime microtidale e un poco energico clima d\u2019onda. Nel corso del ventesimo secolo, le bonifiche di terreni, il dragaggio dei canali e, soprattutto, le stabilizzazioni di bocche, hanno causato significativi cambiamenti all'interno del sistema. Questi sono poi stati amplificati da estremi eventi naturali. La bocca di S. Andrea si trova tra le due bocche pi\uf9 efficienti della laguna. Gli interventi, artificiali e naturali, hanno portato ad un aumento dell'efficienza di queste bocche a scapito di quella di S. Andrea. Negli ultimi due decenni i dati di monitoraggio mostrano un progressivo insabbiamento del canale principale e una diminuzione della sua sezione trasversale e del prisma di marea. Al contrario, il volume di delta di riflusso rimane sorprendentemente invariato. La bassa energia d\u2019onda che colpisce questa area \ue8 insufficiente a rielaborare il delta. Anche se non cambia la sua posizione, la bocca di S. Andrea potrebbe mostrare gli stessi problemi/conflitti della bocca di Anc\ue3o (scarsa circolazione, cattiva ossigenazione, difficile navigazione). Per il momento i problemi causati dall'atrofia dell'entrata non hanno ancora raggiunto un livello di criticit\ue0, ma se la riduzione del prisma continuasse, in futuro occorrerebbe adottare alcune misure di gestione.The evolution of two unconstrained tidal inlets (not fixed), Anc\ue3o and S. Andrea Inlets, located in different human-affected multi-tidal inlet systems is considered in this thesis. The focus is on how natural features of tidal inlets can be indirectly conditioned by the presence of man and by the natural events in the system. The aim is to understand the conflicts generated by the unconstrained behaviour of the inlets in relation to human needs and uses. Through an analysis of morphological and hydrodynamic data, some guidelines are proposed in relation to the changes in the inlets and in the system. Anc\ue3o Inlet is an artificial inlet, opened (relocated) on the known natural migration path. Once opened, it is allowed to migrate naturally. It is a small eastward-migrating inlet in the mesotidal multi tidal inlets system of Ria Formosa (south of Portugal). The lagoon system has been affected by some strong human interventions that modified the former hydrodynamic condition. The most important changes were due to the opening and stabilization of Faro-Olh\ue3o Inlet that caused the narrowing of the nearby Armona inlet. Anc\ue3o Inlet, migrating toward the east, the inlet gradually diminishes the connection with the other inlets, becoming more and more independent. The inlet region has been constantly monitored since 1997. Hydro-morphodynamic data were collected during several topo-bathymetric surveys. Morphological parameters and migration rates were subsequently measured and correlated with wave and water level data to evaluate the importance of different mechanisms in the evolutionary phases of the inlet. The migration trend and the variations of the main morphodynamic data seem to be mainly related to the dominant southwest sea conditions, demonstrating the predominance of the wave climate over tidal currents. The unconstrained nature of the inlet allows it to respond to natural forcings by channel migration. This creates some problems in the water circulation in the backbarrier area, as well as hampering navigation. For these reasons, the position of Anc\ue3o Inlet is currently controlled by periodic updrift relocation, to maintain navigability and good oxygen and nutrient exchange. Additional management provisions are proposed and discussed. The second study case is the small natural S. Andrea Inlet, located in Italy, in the Marano and Grado Lagoon (MGL, North Adriatic Sea), another multi tidal inlet system This differs from the Ria Formosa system because of the microtidal regime and low wave-dominance, making this lagoon a relatively tide-dominated environment. During the 20th century, land reclamations, waterway dredging and, especially, inlet stabilizations, caused significant changes within the system. These may have been enhanced by natural extreme events. S. Andrea Inlet is located between the two most efficient inlets in the lagoon i.e. Lignano and Buso Inlets, whose artificial stabilization began in the 1960s in order to improve navigability. These factors led to an increase of efficiency of those inlets at the expense of the S. Andrea Inlet. During the last two decades, morphological monitoring data shows a progressive shallowing of the main ebb channel, as well as a halving of the cross section area and, consequently, tidal prism. In contrast, the ebb tidal delta volume, although compressed in its own area, remained surprisingly unchanged. The low wave energy that affects this region is insufficient to rework the delta. Although it does not changed its position, S. Andrea Inlet could potentially suffer the same conflicts as Anc\ue3o Inlet: poor water circulation and thus a bad oxygenation of the backbarrier area, and a difficult navigation of the channel. Considering the limited extent of the S. Andrea basin, the issues caused by the atrophy of the inlet are not yet critical, but if the prism reduction continues, some possible management measures must be adopted

Hydrodynamic and sediment fluxes through the inlets of the Ria Formosa

Tese de dout., Ciências do Mar, da Terra e do Ambiente (Ciências do Mar-Oceanografia Física), Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2011Research 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.Fundação para a Ciência e Tecnologia (FCT)União Europeia. FEDE

Morphodynamic evolution of fetch-limited beaches

Tese de doutoramento, Geociências (Dinâmica Litoral), Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2012This thesis examines the morphodynamic evolution of a fetch-limited backbarrier beach located in the Ria Formosa barrier island system. Three different scales of analysis were ascribed: a short-term scale, a medium-term scale, and a long-term scale. Main issues addressed were: (a) the factors governing sediment transport under very limited-fetch conditions; (b) the morphological evolution at medium to long-term, and (c) the role of human interventions in beach morphology. At short-term (over the tidal cycle) sediment transport patterns were locally determined by employing fluorescent tracer techniques, and quantifying morphologic variations and transport rates. The medium- to long-term evolution (between months and from years to decades) was set based in topographic surveys and aerial photograph analysis. Finally, a general framework was developed for flood hazard assessment at fetch-limited coastal environments. This framework embraces the hazard zones identification, risk analysis and the associated most reasonable management options.A presente tese analisa a evolução morfodinâmica de uma praia lagunar de fetchlimitado, localizada no sistema de ilhas barreira da Ria Formosa. Para tal, consideraramse três escalas temporais de análise: curto-termo, médio-termo e longo-termo. Pretendeu-se determinar: (a) quais os mecanismos forçadores que regem o transporte sedimentar na praia; (b) qual a evolução morfológica da praia de médio a longo-termo, e (c) qual a importância das intervenções humana na dinâmica morfológica da praia. A curto-termo termo (ao longo de um ciclo de mare), o transporte sedimentar foi localmente determinado com recurso à aplicação de traçadores fluorescentes, através da quantificação das variações morfológicas e taxas de transporte. A evolução da praia a médio e longo-termo (entre meses e de anos a décadas) foi obtida através da análise de levantamentos topográficos e análise de fotografia aérea vertical. No final, propõe-se uma breve metodologia destinada a avaliar a suscetibilidade a inundação de ambientes costeiros de fetch-limitado. A metodologia proposta identifica zonas com perigosidade a inundações, quantifica o risco associado e equaciona as medidas de gestão local mais apropriadas.Fundação para a Ciência e Tecnologia (FCT

The role of Ria Formosa as a waste water receiver

Ria Formosa is a system of barrier islands that communicates with the sea through six inlets, situated in the Algarve, the most southern region of Portugal which encompasses the municipalities of Loulé, Faro, Olhão, Tavira and Vila Real de Santo António, covering an area of about 18,400 hectares along a 60 km stretch from the Ancão beach to Manta Rota beach (Figure 4.1). It is an area protected by the status of Natural Park, granted by Law n. º 373/87 of December 9th and its classified as a sensible area according to the law n.º 152/97 of June 19th concerning urban waste water discharges, in order to assure water quality standards for shellfish aquaculture. This ecosystem is very important from the socio-economic point of view, being responsible for 41% of the Portuguese national production of bivalve shellfish, with a small finfish production (IMAR, 2012).info:eu-repo/semantics/publishedVersio