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Impacts of human interventions on the evolution of the Ria Formosa barrier island system (S. Portugal)

Publication . Kombiadou, Katerina; Matias, Ana; Ferreira, Oscar; Carrasco, A. Rita; Costas, Susana; Plomaritis, Theocharis

Human interventions on sandy barriers disturb natural barrier dynamics, to the extent of having become key forces in modifying geomorphological evolution. This work identifies natural and human-induced drivers and analyses their importance to the multi-decadal evolution of the Ria Formosa barrier island system, in South Portugal. Aerial photographs from the last six decades and historical maps are used to assess changes in cross-shore rates, morphological characteristics (barrier and dune widths, inlet morphology and migration) and barrier areas, through systematic methods that can be easily transferred to other barrier systems. Interventions, and especially hard engineering ones (jetties, inlet stabilisations), affected barrier evolution trends. Shore-perpendicular works increased shoreline progradation updrift and initiated coastal retreat downdrift, with strongest erosive impacts along the edges of the system. Inlet stabilisations changed tidal inlet hydrodynamics and initiated ebb-shoal attachment to the barriers on either side of a non-migrating inlet that experienced loss of tidal prism. This shoal attachment was the main factor for the increase in total barrier area of Ria Formosa during the 60 years of analysis. Barrier growth after 2005 was slower, which could indicate that the system is reaching morphodynamic stability.

2019-10Journal article

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The birth of backbarrier marshes in Culatra Island (Ria Formosa, South Portugal)

Publication . Kompiadou, Aikaterini; Carrasco, Rita; Costas, Susana; Ramires, Margarida; Matias, Ana

The rapid elongation of Culatra Island, a sandy barrier in the Ria Formosa chain (S. Portugal), since the mid1940s led to the formation of three new embayments in its backbarrier that were gradually colonised by halophytic vegetation. This provided a rare opportunity to collect information and data on the very early stages of backbarrier marsh plant establishment and evolution. Sediment (surface and subsurface) sampling in two of the recently formed bays, combined with information extracted from vertical aerial photographs, allowed us to assess modern sedimentation characteristics and vertical accretion rates since the shift from a bare sandflat to a vegetated marsh platform. Present-day topography appears largely inherited by overwash or/and inlet-related tidal deposits that provided the necessary sediment pulse for the formation of an intertidal sandy substrate, suitable for colonisation. The variability in accretion rates, noted even within the same embayment, as well as the differences in accretion balance with similarly young backbarrier marshes, highlight the importance of local conditions (sediment import, distance to creeks and marsh edge, storm frequency and intensity) to marsh buildup, even during the very early stages. Variable accretion rates were also identified over intertidal seagrass patches, indicating similar influences. Organic deposition rates were very low in all vegetated intertidal habitats, indicating the dominance of mineral deposition to the vertical growth. A lag, ranging from roughly 10-30 years, was observed between the formation of the intertidal sandy platform and plant establishment in all embayments. The different timescales in the observed lag are likely linked to differences in hydrodynamic conditions, promoted by the embayment morphology (opening width). The lowest lag was observed in protected embayments, which could reflect a 'typical' delay for plant establishment in the system, while the highest lag was associated with higher energy backbarrier environments.

2024-01Journal article

Open Access

Coastal ridge constructive processes at a multi‐decadal scale in Barreta Island (southern Portugal)

Publication . Herrero, Xabier; Costas, Susana; Kombiadou, Katerina

Multiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions. Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro-Olhao inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges. The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics.

2019Journal article

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