On the role of biomass on coastal morphodynamics in natural and urban Mediterranean beaches

TROGU, DANIELE

2022-10-27

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Abstract

The continuous sea level rise and the increase in the intensity of wave storms, amplified by global climate change, implies an adequate response from coastal management and environmental protection. To counter these phenomena, urgent measures should be taken to mitigate anthropogenic impacts and restore natural coastal resilience. This thesis deals with the role that biomasses (mainly Posidonia oceanica and, in some cases, Arundo donax rests) have within three Mediterranean beaches differentiated from each other by more or less marked human impacts. On a beach with almost irrelevant anthropic impacts, the deposition of the banquettes at the end of the most significant wave storms is analysed through a video monitoring system. It has been shown how this phenomenon generates a rapid accretion of the emerged beach, which restores its original extension, reduced during the storm, in two or three days. The spatio-temporal shift of the shoreline was investigated along two others urban beaches in Cagliari Gulf (Southern Sardinia, Italy), through orthorectified photographs, across a time frame of 62 years, in order to quantify and distinguish the factors controlling the shoreline evolution (erosion and/or accretion rates) in relation to a number of anthropic and natural forcings. The evolution of the geomorphological setting of the urban coastal belt of Cagliari was also carried out through a sea-land approach and a multidisciplinary and multi-temporal investigation summarized in a geomorphological map. Furthermore, the results of the water infiltration tests, carried out in different areas of the dry beach, showed that the presence of biomasses, above and inside the sand, considerably increases its permeability. This allows the beach to drain the overwashed water and its return to the sea during the most energetic storm events, limiting runup and flooding extension. Finally, another original contribution of this work is related to the analysis of the banquette deposition and dismantling dynamics, through a four years video monitoring system database in one of the most urbanized portion of the Cagliari beaches. This study looked for a correlation between the wave and wind climate and the dynamics of the banquettes, and the results showed that wave motion plays a more important role in the deposition of the banquettes than in their erosion. For this last process the offshore and inshore winds plays a more significant role than the waves.