Hydrodynamic conditions of Posidonia oceanica seagrass berm formation and dismantling events.

Seagrass berms are a natural accumulation of seagrass leaves along the shoreline, which play an important role in coastal protection. Seagrass berms have been shown to reduce wave energy, helping to shield the coast from erosion. However, their protective role is debated in coastal management, where there are contrasting views on whether berms should be left in place or removed to improve beach aesthetics or accessibility. Although seagrass berm accumulation and erosion are a globally recognized phenomenon, the physical processes underlying their formation and dismantling remain poorly understood, limiting insights into their role as coastal defense. This study fills a critical knowledge gap by analyzing the first long-term dataset that directly connects Posidonia oceanica seagrass berm dynamics to specific wave conditions. Measurements were collected at Cala Millor beach in Mallorca over almost a decade and the research utilized topo-bathymetric surveys, video monitoring, and wave measurements. Our research reveals that berm formation follows an annual cycle, driven by wave height, period, and direction, alongside seasonal availability of seagrass material. Numerical modeling further shows that high-energy waves facilitate berm formation, while low-energy, spilling waves contribute to their dismantling. These findings highlight the significance of wave conditions in berm dynamics, advancing our understanding of their potential role in coastal protection.