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.

A novel flume experiment was conducted to compare the sheltering effect of surrogate seagrass meadows of two different lengths against a bare beach (benchmark). The analyses focused on assessing the impact of meadow cross-shore extent on wave height attenuation, behaviour of wave orbital velocity components, sediment transport, and shoreline erosion. Throughout the tests conducted in the large-scale CIEM wave flume at LIM/UPC Barcelona, meadow density and submergence ratio remained constant, while irregular waves were run over an initial 1:15 sand beach profile. In both meadow layouts, a persistent decrease in wave height from the offshore area in front of the meadow to the breaking zone was found. This reduction was directly correlated with the length of the seagrass meadow. As a result of the reduction in wave energy, less erosion occurred at the shoreline in accordance with the decrease in wave height. The mean velocities exhibited changes in the velocity profile from the meadow area to the immediate zone behind the meadow, a phenomenon not observed in more onshoreward positions. Orbital velocities displayed a reduction exclusively for the long meadow case. This decrease was persistent up to the breaking zone. As a consequence of these changes, the long meadow layout led to a decrease in the volume of sediment transport and a breaker bar closer to the shoreline. The short meadow layout resulted in a higher volume of sediment transport compared to the long meadow layout, although still less than the benchmark layout. Furthermore, in the short meadow layout, the final bar was situated in a location similar to that observed in the benchmark layout.

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.

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.