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Implications of sea-level rise for overwash enhancement at South Portugal

Publication . Ferreira, Oscar; Kupfer, Sunna; Costas, Susana

Overwash is one of the most prominent hazards affecting coastal zones, and the associated consequences are expected to increase because of both sea-level rise and intensification of coastal occupation. This study used a 23-year data set of wave heights and tide-surge levels to define return periods of overwash potential for current and future sea-level conditions, namely 2055 and 2100, at two sites from South Portugal. A relevant intensification of both frequency and magnitude of the overwash is expected to occur by mid-century if adaptation measures are not taken and further aggravated by 2100. Current overwash levels with a return period of 100-years can reach a return period lower than 20-years by 2055 and 10-years by 2100. However, these values are rather variable from site to site, highlighting the urgency to develop detailed local studies to identify climate change impacts along coastal sectors, based on validated equations and long-term time series. These could be easily carried by replicating and adapting the here proposed methodology to sandy coasts worldwide. Understating the impact that climate change (namely sea-level rise) may have at the local level is key to contribute to effective management plans that include adaptation measures to minimize risks associated with coastal floods.

2021Journal article

Restricted access

Exploring controls on coastal dune growth through a simplified model

Publication . Kombiadou, Katerina; Costas, Susana; Roelvink, Dano

Process-based morphodynamic models can be useful in understanding coastal dune responses to disturbances, as well as possible evolutionary patterns. To this aim, we employ Duna, a simplified 1D morphodynamic model, to assess the influence of dune morphology (height and slope) on sand transfer and deposition across the dune profile for different beach widths and wind incidence angles through idealized experiments. Simulations of real conditions show good model performance, both in wind flow reproduction and in topographic change along the dune profiles tested. The idealized experiments show that wind speed increases and sand accumulation decreases logarithmically with dune height and linearly with stoss slope along the dune profile. Fetch and cosine transport limiting parameters are reflected in the sand accumulated windwards from the toe, while sand transfer to the dune appears controlled by multiple factors; the higher the dune and/or the narrower the beach, the likelier that maximum accumulation occurs under oblique winds. Results point to two different types of evolution for high dunes. Either the vegetation is dense enough to maintain the stoss position, in which case vertical growth near-ceases and seaward progradation is promoted, or the stoss is eroded and landward retreat dominates, in which case sand transfer to the crest and lee continues as a mixture of low input from the beach and recycled sand from the stoss. Coastal dunes are sensitive ecosystems whose survival depends on their adaption to changing conditions. Thus, it is important to understand how dune characteristics (i.e., shape, vegetation type, and cover) and prevailing conditions (i.e., wind speed and direction, beach width) determine where and when sand is deposited onto the dune, promoting growth. This is the result of a complex balance between winds that bring sand to the dune from the adjacent beach (main sand provider), the dune topography (decelerating winds near the dune toe and accelerating them along the slope, up to the dune crest) and dune plants (slowing winds down in their vicinity and trapping wind-blown sand). The main controls on these complex interactions have been incorporated into the Duna model for aeolian sand transport. After tuning parameters and verifying that simulated results are accurate, Duna is used to assess the impacts of dune shape (height and slope), beach width, vegetation coverage and wind angles to wind flow and topographic changes on the dune. Results show that both wind speed and sand accumulation vary logarithmically with dune height and linearly with slope. The simulated sand distribution along the dune is used as a basis to draw generalized dune growth patterns. Duna morphodynamic model is calibrated and validated against Computational Fluid Dynamics model results and field dataThe influence of dune shape (height and slope), wind incidence, vegetation density and beach width on dune growth is investigatedDune height and plant cover are the main factors controlling accumulation patterns, sand recycling and vertical growth

2023Journal article

Open access

Biotic and abiotic factors governing dune response to storm events

Publication . Garzon, Juan L.; Costas, Susana; Ferreira, Oscar

The alongshore response of dunes to storm events can be extremely variable and,consequently, their capacity to maintain their services, including the protection ofhinterland communities. In this study, the role of biotic and abiotic factors determin-ing the magnitude of dune retreat driven by a severe storm along a 60 km barrierisland system was investigated. Data from high-resolution satellite imagery, digitalterrain models, and wave propagation models were used in this assessment. Theassessed abiotic factors included the backshore volume, dune height, downdrift inletdistance, and incident wave power. The evaluated biotic factor was the vegetationcover, characterized by a vegetation index retrieved from the multispectral imagery.The results revealed large alongshore variability on dune retreat, ranging from negli-gible impact to ca. 40 m of retreat. All combined factors allowed us to explain up to70% of the dune retreat variability through a multi-regression analysis. Among allinvestigated factors, the major contributor controlling the magnitude of dune retreatwas the backshore volume (more robust berms reduced the retreat) followed by thewave power (normal and longitudinal components). Moreover, the removal of localsalient features in the dune line caused the straightening of the coastline, highly con-tributing to the development of dune retreat hotspots. The other evaluated factorshad a smaller influence on reducing coastal retreat, including the vegetation, whosecontribution to dune protection was around one order of magnitude lower than thatprovided by the backshore volume. The results highlight the importance of regionalassessments to understand the causes behind the large alongshore variability ofstorm impacts at dunes. They also state the relatively low influence of the vegetationfrom this climatic region to enhance dune resistance to storms.

2021Journal article

Restricted access

Effect of survey parameters on unmanned aerial vehicles-derived topography for coastal dune monitoring

Publication . Bon de Sousa, Luísa; Costas, Susana; Ferreira, Óscar

Coastal dunes are fragile ecosystems emerging at the interface between marine and continental environments. They provide multiple services, among which are the protection against the impact of storms and the hosting of diverse and unique species of fauna and flora. However, changes in the topography or biological component of these systems may endanger the perpetuation of service provision. Topographic changes within dunes can significantly differ in magnitude depending on the type of process (i.e., marine or aeolian) and the temporal scale of analysis (event to annual scale), making their monitoring a challenging task. In recent years, unmanned aerial vehicles (UAVs) have been increasingly used to monitor coastal dunes, proving to be a cost-efficient methodology for the collection of topographic data. Yet, the application of UAVs in combination with the structure from motion approach to obtain digital surface models (DSMs) presents some limitations related to the level of accuracy provided for the evaluation of topographical changes in dunes with low sedimentation rates. This work explores different survey configurations using UAVs flying at low altitudes with the aim of obtaining high-quality DSMs with vertical accuracies preferably around or lower than 0.04 m. Several tests were performed to evaluate the influence of different parameters on the accuracy of the DSM, including flight altitude and orientation, density and spatial distribution of ground control points (GCPs), terrain slope, vegetation cover, and sun-related parameters. The results indicate that the intended accuracies can be obtained by combining overlapped perpendicular flights, GCPs distributed regularly following a diamond grid, with densities of at least 6 GCPs per hectare, sun altitudes between 30 and 40 deg, and a total solar radiation per hour between 1750 and 2250 KJ / m(2). In addition, better results were obtained across gentle slope areas, suggesting the eventual need to adapt to the particularities of each site to ensure the accuracy.

2022Journal article

Open access

Ecogeomorphic response of a coastal dune in southern Portugal regulated by extrinsic factors

Publication . Costas, Susana; Gallego-Fernández, Juan B.; Bon de Sousa, Luísa; Kombiadou, Katerina

Coastal dunes are complex landforms whose morphology results from various interactions between biotic and abiotic factors. Here, we explore the longshore variability of the morphological features, plant community dis-tribution and accumulation patterns of a dune segment (1.4 km-long) located at the downdrift end of a sandy peninsula in the Ria Formosa, Portugal. To understand the main drivers of the observed variability and the implications for dune morphological response, this information was combined with recent multidecadal shore-line evolution data. The integrated results document significant differences in dune morphology, sedimentation patterns and plant zonation, with two distinct dune configurations or states identified in close proximity. One (western sector) shows a narrower dune system, vegetation cover characterised by pioneer species with low densities, and squeezed plant zonation. Conversely, the other (eastern sector) presents a wider dune system with a new foredune, a more developed plant zonation and relatively high vegetation density. Both states could be partially explained by the recent shoreline trends and inlet shifts, with stable to retreating trends in the western sector and shoreline progradation in the eastern one. Plant zonation and accumulation patterns suggest that the dune along the retreating sector is in a cycle of inland migration, encouraged by the reduced accommodation space and the low retention capacity of the vegetation across the dune stoss. Alternatively, observations along the prograding sector suggest that the greater accommodation space and the stabilising feedback between vegetation and topography promoted the seaward progradation of the system and the development of an incipient foredune. Outcomes support the importance of biogeomorphic feedbacks for the dune configuration, but they also evidence that the role of vegetation within the feedback is primarily regulated by physical factors that ultimately promote or inhibit vegetation effects on dune topograph

2023Journal article

Open access