Carlos Astudillo-Gutierrez

The Mediterranean coast is known for its great tourist attractions, concentration of population, and economic activities. Specifically, in the autonomous regions like Catalonia and Valencia, more than half of the population lives in coastal counties, and the population during the summer months increases due to the influx of tourists. Furthermore, in this stretch of coast, there are some areas of natural interest such as the Delta del Ebro or the Albufera, which are two of the most important wetland areas in the Mediterranean. However, according to studies by Day Today, the retreat of the coastline has increased in recent years, and this influences management of coastal territory both directly and indirectly, mostly harming all sectors with low levels, creating spaces with significant problems. It is for this reason that reporting on climate change and the impact on the coasts is assuming an important role in society, because they are essential tools for planning and management costs. In this thesis, the ground that would be affected by a +1 m, +2 m, and +3 m increase in average sea level, as simulated by the existing flood simulator, has been quantified. And a methodology has been developed for determining the vulnerability of the land based on flooding provided by terrain elevations, and each area studied was evaluated with different degrees of vulnerability: very high, high, moderate, or low. Finally, a first estimate has been made of economic loss that could involve a meter rise in the average sea level for Catalan beaches, and major damage to natural parks, urban areas, and major infrastructure has been identified. This study shows that there are nine areas with high vulnerability due to the low heights of their territory, and the majority of the flooded land is concentrated in the Ebro Delta and the Albufera, which jointly dominate the totals across scenarios.

Seagrass banquettes, formed primarily by Posidonia oceanica, play a critical role in coastal protection and sediment dynamics. However, due to their perceived negative impact on seaside tourism, local authorities frequently remove these accumulations using heavy machinery, contributing to beach erosion. This study surveyed a 100 km stretch of the southern Catalan coastline to map the spatial distribution of Posidonia oceanica banquettes and to investigate their relationship with adjacent submerged seagrass meadows. The results indicate that banquette development is most prevalent in meadows with a depth of less than 5 m, under local wave conditions with significant wave heights ranging from 0.4 to 0.8 m. Pixavaques Beach was selected as a case study to evaluate the physical characteristics and ecological functions of banquettes. Granulometric and permeability analyses were conducted on sand and banquette samples, alongside in situ penetration resistance measurements. The results indicate that banquettes located near the shoreline accumulate around 12 times less sediment than those located further inland, primarily due to their continuous exposure to wave action, which produces a natural washing and removal of sediments. The mean permeability values of the banquette were 16.41 cms^{− 1} in the zones closest to the shoreline and 0.32 cms^{− 1} at a distance of 1.5 m inland. The values obtained for these samples were higher than those measured in bare sand (0.05 cms^{− 1}). The maximum in situ penetration resistance values were recorded as 1.84 MPa in bare sand and 1.68 MPa in the banquettes. The highest values measured within the banquettes indicate zones of greater sediment accumulation. Additionally, the artificial nourishment implemented to counteract erosion from banquette removal has led to increased grain size and steeper beach profiles. These findings highlight the protective function of banquettes and support alternative, sustainable management strategies, such as repositioning or partial retention, that balance ecological integrity with the demands of coastal tourism.

The Mediterranean coast is known for its great tourist attractions, concentration of popu-lation, and economic activities. Specifically, in the autonomous regions like Catalonia and Valencia, more than half of the population lives in coastal counties, population during the summer months increases due to the influx of tourists. Furthermore, in this stretch of coast, there are some areas of natural interest such as the Delta del Ebro or the Albufera, which are two of the most important wetland areas in the Mediterranean. However, ac-cording to studies by Day Today, the retreat of the coastline has increased in recent years, and this influences management of coastal territory both directly and indirectly, mostly harming all sectors with low levels, creating spaces with significant problems. It is for this reason that reporting on climate change and the impact on the coasts is assuming an im-portant role in society, because they are essential tools for planning and management costs. In this thesis, the ground that would be affected by a +1 m, +2 m, and +3 m increase in average sea level, as simulated by the existing flood simulator, has been quantified. And, it has developed a methodology for determining the vulnerability of the land based on flooding provided by terrain elevations, and each area studied was evaluated with dif-ferent degrees of vulnerability: very high, high, moderate, or low. Finally, it has made a first estimate of economic loss that could involve a meter rise in the average sea level for Catalan beaches and has identified major damage to natural parks, urban areas, and ma-jor infrastructure. This study shows that there are 9 areas with high vulnerability due to the low heights of their territory, and half the land that would be affected corresponds to the Ebro Delta and Albufera.

Coastal areas are experiencing increasing degradation and risks driven by climate change and human activity. These areas, which host the highest population densities globally, are rapidly urbanizing and developing, making them particularly vulnerable to extreme weather events and sea level rise. Historically, coastal management has focused on conserving natural landscapes and habitats. However, recent efforts have shifted towards expanding targeted habitats and promoting natural processes to enhance resilience. Nature-based solutions (NbS), such as restoring seagrass meadows and wetlands, offer promising approaches to protect these vulnerable areas by combining ecological restoration with protection measures. Since the 2010s, growing concern about climate change has highlighted the critical role of the Wadden Sea in flood risk management. The shallow Wadden Sea, with its barrier islands, tidal flats, banks, and salt marshes, acts as a natural buffer, shielding the mainland coast from flooding. This renewed focus has led to innovative approaches aimed at integrating flood protection with nature development, exemplifying how NbS can address complex environmental challenges.The Ebro Delta, located on the Spanish Mediterranean coast, is one of the most vulnerable coastal regions in Europe due to the near-total reduction of river sediment supply. Currently, the protective function of the sandy beach belt has been lost in several areas, leaving inland zones exposed to coastal flooding and disrupting their current usage. This is the case of the Trabucador barrier beach where a restoration effort has been implemented by recreating past embryonic dune systems. This NbS was designed using a morphodynamic modelling to account for different hydrodynamic scenarios.This study highlights the development of an operational early-warning system (EWS) for coastal flooding, which leverages advanced hydrodynamic and wave models capable of forecasting flood events up to three days in advance. Initially applied in Barcelona (NW Mediterranean) and Jade Bay (Southern North Sea), this system demonstrates high adaptability across diverse coastal settings. The modeling framework used in the EWS can also inform the design and implementation of NbS, optimizing strategies for habitat restoration and enhancing coastal protection.The integration of NbS into the EWS framework allows for comprehensive scenario analysis, exploring the effects of different restoration configurations on flood risk reduction. By simulating the impact of restored habitats such as seagrass meadows and tidal marshes, the system provides actionable insights for optimizing NbS layouts. These pilot areas serve as showcases on how these natural features act as buffers, reducing wave energy and mitigating flood risks. The analysis highlights that strategic restoration not only enhances ecological functions but also strengthens coastal defenses, providing co-benefits like carbon sequestration and improved water quality. The risk assessment outcomes support decision-making processes, encouraging broader adoption of risk-based tools to facilitate coastal adaptation efforts and investment planning.

An analysis of the interactions between wave-induced velocities and seagrass meadows has been conducted based on the large-scale CIEM wave flume data. Incident irregular wave trains act on an initial 1:15 sand beach profile with measurement stations from the offshore of a surrogate meadow until the outer breaking zone, after crossing the seagrass meadow. The analysis considers variability and peaks of velocities, together with their skewness and asymmetry, to determine the effects of the seagrass meadow on the near bed sediment transport. Velocity variability was characterized by the standard deviation, and the greatest changes were found in the area right behind the meadow. In this zone, the negative peak velocities decreased by up to 20.3%, and the positive peak velocities increased by up to 11.7%. For more onshore positions, the negative and positive peak velocities similarly decreased and increased in most of the studied stations. A progressive increase in skewness as the waves passed through the meadow, together with a slight decrease in asymmetry, was observed and associated with the meadow effect. Moving shoreward along the profile, the values of skewness and asymmetry increased progressively relative to the position of the main sandbar. The megaripple-like bedforms appeared earlier when the meadow was present due to the higher skewness, showing a belated development in the layout without the meadow, when skewness increased further offshore due to the proximity of the breaker sandbar. To assess the sediment transport capacity of a submerged meadow, the SANTOSS formula was applied, showing that in front of the meadow, there was a higher sediment transport capacity, whereas behind the meadow, that capacity could be reduced by up to 41.3%. In addition, this formula was able to produce a suitable estimate of sediment transport across the profile, although it could not properly estimate the sediment volumes associated with the bedforms generated in the profile.

Posidonia Oceanica is a phanerogamous species endemic to the Mediterranean. It is characterised by its habitat in sandy and rocky areas at depths of less than 40 m in waters with constant salinity and the necessary clarity for the plant to obtain nutrients (Marín Guirao et al., 2011; Ruiz et al., 2009).  In seasonal periods (autumn and winter), part of the Posidonia meadow is dragged by the highest energy waves, mainly associated with storm surges, depositing these remains on the shoreline of the beaches. The constant accumulation of this Posidonia remains can generate vertical growth, transforming it into Posidonia banquettes. The Posidonia banquettes accumulation have been continuously found covering the shoreline of the Mediterranean beaches. Over years these accumulations have been retired from the beach mainly by heavy machinery. The movement of these materials has caused an alteration of the beach-dune system, as the removal of banquettes has also removed part of the beach sediment, which is generally not quantified. This work will carry out the quantification and characterisation of sediments trapped in the Posidonia banquette of a sandy beach located on the south coast of Catalonia (Spain). For this purpose, several transects perpendicular to the beach line will be sampled. One transects considers a sample of clean sand from the beach and another one from the upper and lower part of the Posidonia banquette. For the sediment analysis of the banquette samples, the separation of the sediments from the Posidonia remains by a simplified method will be considered. This method removes the organic matter present by pyrolysis of the samples in an oven at 500ºC for 30 minutes. In this way, the amount of sediment by weight per volume of banquette is precisely quantified. The type of sediment retained by these accumulations is characterised with respect to the type of sand present on the beach. In addition, the fieldwork considers the measurement of the in-situ compactness of the banquettes using a handheld penetrometer. Thus, the compactness of the accumulations can be related to the amount of sediment retained by the banquette. Acknowledgements The fields and laboratory experiments to be described in this work have been funded by the Spanish Government within the Research, Development and Innovation Program through a grant to the ECOPLANTS project (REF PID2020-119058RB-I00). References Marín-Guirao, L., Sandoval-Gil, J. M., Ruíz, J. M., and Sánchez-Lizaso, J. L. (2011). “Photosynthesis, growth and survival of the Mediterranean seagrass Posidonia Oceanica in response to simulated salinity increases in a laboratory mesocosm system”. Estuarine, coastal and shelf science, 92(2), 286-296. Ruíz, J. M., Marín-Guirao, L., and Sandoval-Gil, J. M. (2009). “Responses of the Mediterranean seagrass Posidonia Oceanica to in situ simulated salinity increase”. 52(5), 459-470.