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Wind-induced airflow acceleration over irregular foredunes promotes the genesis of sandy depressions named blowouts. During their development, they transfer sediment to the back-dune helping to maintain the available barrier sediment budget while promoting eco-geomorphological feedbacks that regulate the plant community and biodiversity. However, the mechanisms controlling their dynamics and evolution are still not well understood and demand further research, as these are key landforms for the future management of coastal dunes under rising seas and climate change. This work aims to identify the internal (e.g. blowout morphometric characteristics) and/or external factors (e.g. metocean conditions, human interventions) influencing the migration and spatiotemporal evolution of a series of blowouts present in the foredune of a coastal stretch of 1.3 km situated in Ancão Peninsula, South Portugal. To achieve this, their morphometric characteristics (area, orientation, width, length, width-length ratio and centroid position) were mapped and their changes analysed over time, together with the storm frequency, wave power, dune toe location and anthropogenic interventions in the area. The previous was done using a 49-year set of historical aerial photos, orthophotos, and Google Earth images as well as time series of metocean conditions (from 1972 to 2021). The estimated Kendall’s bivariate correlation coefficients showed that, with 0.1 significance level, blowout migration rates were positively dependent on blowout area, width, length, orientation and dune toe retreat. Besides, fastest migration rates occurred in narrower and lower dune crest areas as these offer less resistance to erosion. It was not possible to obtain certainties on the statistical dependencies with the metocean conditions due to the low temporal image resolution at the beginning of the study period. Nevertheless, two main phases of significant dune toe retreat (1996-2001 and 2008-2011) were concomitant with the impact of several extreme storm clusters reported in the literature (1998-2000 and 2008-2009). The role of extreme events in the study area is threefold: (1) shoreline erosion and dune scarping, which further debilitates the foredune, (2) increase in the total blowout area as the widths and lengths of the medium and large blowouts increase, and (3) disappearance of small blowouts as well as blowout genesis. Lastly, the jetty construction updrift (finished by 1972) seemed a very likely trigger of the initial dune instability in the area while foredune fencing promoted the artificial sealing of blowouts, which showed approximate sealing times of 4 years after fencing implementation.

Blowouts are sandy depressions of different shapes formed by wind-induced erosion of foredunes or dune fields. Their initiation has been linked to air flow acceleration due to the occurrence of irregularities in the topography caused by natural or human causes, or by the lack of vegetation, and they contribute to maintain the available sediment budget in barrier islands migrating inland. In this study, we investigated the morphometric characteristics (area, orientation, and shape/structure) and spatiotemporal evolution of a series of blowouts present in the foredune of a coastal stretch of 1.3 km situated in Ancão Peninsula (South Portugal) that has been retreating for the last 60 years. For that, a set of historical aerial photos, orthophotos, and Google Earth images, covering a 45-year period from 1972 to 2017, was analysed. In addition to the blowout mapping, the dune foot, trampling paths and human occupation (e.g. restaurants, walkways, umbrellas) observable in the previous imagery were mapped in order to find possible causes that could help explaining the observed foredune fragmentation and blowout development. Finally, we characterised the present-day plant species distribution along and across the study area in order to understand the impact of these landforms in the plant community and possible eco-morphological feedbacks. The findings showed that during the analysed period: (1) blowout dimensions ranged from 1.2 m2 to of 2200 m2, with 50 to 80% of the blowouts displaying sizes below 100 m2; (2) the orientations of the smallest blowouts (below 100 m2) showed high variability (from SSE-NNW to W-E orientations), whereas the bigger ones (above 400 m2) were mostly SW-NE oriented, coinciding with the dominant winds in the area; (3) most of the blowouts had mixed shapes and branched structures, likely enhanced by human trampling; (4) the number of blowouts and their morphometric parameters were not clearly related with the shoreline retreat/progradation; and (5) the transition between the blowout lobes and the stablished foredune suffered a change in the plant community dominated by species such as Artemisia campestris subsp. maritima, suggesting a shift to burial tolerant species.Besides wind conditions and shoreline changes, human pressure seems a very likely trigger of blowout morphology reinforcement and even blowout initiation in the area, although the origin of some of these features seems related also to already-existing irregularities in the topography, suggesting the fragility of this sector and supporting its tendency to migrate inland.

Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment transport and wave overtopping. Frequent short-term monitoring of the island and further research coupling variations in the different factors driving island change (i.e., sediment availability, reef-wave interactions, and extreme events) are needed to shed light on the future trajectory of OTI and other rubble islands under a climate change scenario.

Unusual shore-normal and barred-like rhythmic features were found in Camposoto Beach (Bay of Cádiz, SW Spain) during a monitoring program using unmanned aerial systems (UAS). They appeared in the backshore and persisted for 6 months (October 2017–March 2018). Their characteristics and possible formation mechanism were investigated analyzing: (1) UAS-derived high-resolution digital elevation models (DEMs), (2) hydrodynamic conditions, and (3) sediment samples. The results revealed that the features did not migrate spatially, that their wavelength was well predicted by the edge wave theory, and that they shared characteristics with both small-scale low-energy finger bars (e.g., geometry/appearance and amplitude) and swash cusps (e.g., wavelength, seaward circulation pattern, and finer and better sorted material in the runnels with respect to the crests). Our findings pinpoint to highly organized swash able to reach the backshore during spring tides under low-energy and accretionary conditions as well as backwash enhanced by conditions of water-saturated sediment. This study demonstrates that rhythmic features can appear under different modalities and beach locations than the ones observed up to date, and that their unusual nature may be attributed to the low spatiotemporal resolution of the traditional coastal surveying methods in comparison with novel technologies such as UAS.

Human occupation of and alteration of the world's coast has transformed large stretches of it into Coupled Human-Natural Systems (CHANS) in which humans both influence and are influenced by coastal evolution. In such systems, human activity is as critical on natural resilience as processes and sediment supply derived from the natural setting. Pre- and post-storm observations of these interactions on the intensively developed Atlantic coast of the Gulf of Cadiz, (Spain and Portugal) are examined to determine natural and engineering resilience. Three case studies are used in three CHANS, showing that human interventions interact in complex ways with the natural system influencing post-storm recovery. In natural coasts, storm impact is assessed in terms of geomorphological response; on developed coasts, it is quantified as damage to infrastructure or loss of amenity. Preparedness, availability of resources, choice of response and the speed at which human agencies respond affect resilience for post-storm beach behaviour. Results show in some sites natural resilience adjusting by post-storm sediment transfers and an equilibrium morphology that may differ from pre-storm morphology; engineering resilience ensured that CHANS regained their pre-storm human infrastructure and amenity. Their management requires a fundamentally different approach to that of natural coastlines. The current immature stage of understanding of CHANS (especially the human preparedness and response components) is illustrated by the case studies presented where short-term political decisions and reactions to storms play a strong role in post-storm response. The nature and extent of many developed coasts as CHANS is slowly becoming more widely acknowledged, but to increase natural resilience and decrease vulnerability in CHANS better planning is required so that future storms are anticipated and when they happen, pre-planned human response actions are activated. Storms are an integral and inevitable element in the behaviour of coastal CHANS, not a disaster or emergency. (C) 2021 Published by Elsevier B.V. Coastal Environments Research Group (RNM 911 of the Plan Andaluz de Investigacion y Desarrollo e Innovacion); project EW-COAST [ALG-LISBOA-01-145-FEDER-028657]; Ministerio de Economia y CompetitividadSpanish Government [CGL2014-53153]; RNM 328 Research Group of the Plan Andaluz de Investigacion

The increasing pressures and hazards derived from human action and climate change make the implementation of coastal monitoring plans a requirement for assessing coastal responses and evolution. Fo...

Talavera, L.; Del Rio, L.; Benavente, J.; Barbero, L., and Lopez-Ramirez, J.A., 2018. UAS & SfM-based approach to Monitor Overwash Dynamics and Beach Evolution in a Sandy Spit. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 221–225. Coconut Creek (Florida), ISSN 0749-0208.The role of overwash processes is of great relevance in the long-term evolution of sandy barriers, and it can also affect coastal infrastructures in the short term. In this work, a 10-month monitoring program based on the use of Unmanned Aerial Systems (UAS) in combination with Structure from Motion (SfM) algorithms was performed in order to monitor morphological changes and ulterior evolution at Camposoto beach, an overwash-prone coastal spit located in SW Spain. This UAS&SfM approach allowed the reconstruction of 6 Digital Elevation Models (DEMs) of the area with high temporal and spatial resolutions (RMS vertical errors spanning from 5 to 8 cm). The morphological changes occurred were detected using Geomorphic Change Detection Software in ArcGIS, and they were correlated with the oceanographic conditions that prevailed during the analysed period. The results obtained provided insight into the response of the system against storm-induced overwash, which caused significant erosion/accretion patterns over a pre-existing washover fan, as well as the landward migration of the system. In addition, this methodology captured the progressive partial recovery of the system, which included onshore transport of sand, fore-beach accretion, and berm reconstruction. Despite the partial recovery observed, spring tides also triggered non-storm overwash in the area during fair-weather conditions, allowing to rethink the actual vulnerability state of the spit against rollover processes.

Talavera, L.; del Rio, L.; and Benavente, J., 2020. UAS-based high-resolution record of the response of a seminatural sandy spit to a severe storm. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 679-683. Coconut Creek (Florida), ISSN 0749-0208. Human infrastructures in barrier islands restrict the natural storm-induced overwash needed by these systems to evolve, further increasing their vulnerability in the medium and long term. For this reason, accurate overwash measurements in coastal environments subject to varying degrees of occupation are needed. In this work, two Unmanned Aerial System (UAS) flights were performed over two distinct sectors of a sandy spit located in Cadiz (SW Spain) prior and after the landfall of storm Emma. This storm was a high-energy event that lasted 153 hours,with an average significant wave height of 4 m and a storm peak of 6.81 m., and water levels (tide and surge) of up to 4 m. The images derived from the UAS flights were processed with Structure From Motion (SfM) algorithms using Pix4D software, resulting in high-resolution mosaics and Digital Elevation Models (DEMs) of the study area. These allowed (1) the digitization of the washover deposits in both sectors before and after the storm, and (2) the computation of DEMs of Difference (DoDs), which in turn allowed the analysis of volumetric estimates of erosion/accretion triggered by the significant overwash. The results revealed a nonuniform coastal response between both sectors. The area subject to higher degree of development showed a clear restriction in the onshore delivery of sediment during overwash due to the presence of a channel and a road, the coalescence of the nine pre-existing washovers into a unique deposit of 77546 m2, and much higher damage extents and associated recovery costs. In the natural sector, the overwash processes reactivated the three pre-existing washovers (which enlarged their areas up to 200%) with no significant damages reported. These findings serve as examples of coastal responses to future similar hydrodynamic conditions, and suggest that this spit will likely evolve following different trajectories in the long-term.

The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast. The coast of Cadiz province (SW Spain) constitutes a complex system with a wide variety of hydrodynamic and geological-geomorphological conditions. Morphodynamic behaviour of beaches in this area shows more dissipative states in the Atlantic coast and more reflective states in the Mediterranean coast. Recent shoreline trends indicate a relatively stable behaviour along most part of Cadiz coast over the last decades, although there are numerous sectors with a clearly erosive trend, mainly in the Northern half of the province. The great spatial and temporal variability of shoreline changes observed is related to the heterogeneity of the coast, as well as to the diversity of factors contributing to erosion processes on each sector. Several coastal hazard hotspots appear along the study zone, mostly related to extreme meteorological events and short-term trends, namely coastal erosion and flooding. The frequency and severity of the risks derived from these hazards have increased over the last decades, mainly due to the increased anthropogenic pressure and urban development on the coast.

This chapter examines the origin, evolution and present state of the coastal dunes along the South Atlantic Spanish coast around the Strait of Gibraltar. Wind regime in the zone is strongly affected by the North Atlantic Oscillation (NAO). Low or negative NAO index values give rise to the prevalence of westerly humid winds, while during periods with a positive NAO index easterly, dry winds prevail. Several Holocene and historical aeolian phases have been identified in the zone, represented by different aeolian deposits, many of them containing archaeological remains. The oldest one dates to shortly after the mid-Holocene eustatic maximum. During historical times dune-building episodes were associated with a higher frequency and persistence of easterly winds. During the 20th century, and especially in the last few decades, the prevalence of positive values of the NAO index has favoured the growth and advance of mobile dunes in the vicinity of the Strait of Gibraltar. In contrast, dune ridges associated with westerly winds are much less mobile or stable and form shore-parallel ridges covered by vegetation. In the Atlantic side of the Strait, environmental health of coastal dunes is satisfactory, especially those included in the Strait of Gibraltar Natural Park, where more effective protection measures are applied. The Mediterranean side is characterized by a dense population and intense human interventions, leading to erosion problems. Climate change predictions for the next decades suggest an increase in the frequency and intensity of easterly-Levante winds in the Strait. This situation would favour the reinforcement and growth of the Levante-derived dunes around the Strait, and perhaps recovery of the small dunes existing in the Mediterranean side.