SEDIMENTARY AND MORPHODYNAMIC PROCESSES OF MICROTIDAL WAVE DOMINATED MEDITERRANEAN BEACHES (VILLASIMIUS - SE SARDINIA)

This paper shows the results of the morpho-sedimentogical study of Porto Giunco-Simius-Is Traias beach system; the beaches are situated in the Site of Community Importance (SCI) area ITB 040020 “Isola dei Cavoli, Serpentara, Punta Molentis”, in the municipality of Villasimius (Sardinia SE). The research was carried out as part of the European project Life+ Nature & Biodiversity Providune (LIFE07NAT/IT/000519) using the standards and the survey methodology identified by the European project Interreg IIIA GERER “Gestion intègrèe de l’environnement à haute risque d’èrosion”. The aim was to understand the sedimentological and hydrodynamic processes of the studied area and to map the granulometric distribution of the emerged and submerged beach (drybeach and shoreface). The survey was carried out by taking topographic measurements referring to a geodetic network purposefully created, based on the point IGM95 ETRF2000 n.235902, near the port of Villasimius. The dry beach was examined using the DGPS system on 1Hz frequency; the submerged beach, using Reson Navisound 215/DGPS Omnistar 8300HP with Reson PDS2000 navigation software, installed on the Marino660 ship belonging to the Osservatorio Coste E Ambiente Naturale Sottomarino (OCEANS) of Cagliari University. The survey was carried out on the emerged and submerged beach at the same time, on an acquisition grid with 50m per side. The DTM created was the basis of the land-sea sedimentology study. 117 samples were collected and studied from the submerged beach (using Van Veen 5 lt grab), the drybeach, the shore-line and the dune area. The samples were first studied according to the Folk and Ward (1957) method, and then a compositional analysis was carried out through the semi-quantitative identification of quartz, feldspars, micas, accessory minerals, lithoclasts and bioclasts. Through the data elaborated (DTM and sedimentology) wave and hydrodynamic models were constructed using Delft3D software, WAVE and FLOW modules. The models were based on the meteo-marine data recorded in the area and calibrated with observations and measurements in situ. Mz values on dune zone show sediments that consist of fine sands, compared to those of the backshore that consist of medium sands. The sediments of the shoreline is characterized by medium and fine sands while the submerged beach is characterized by fine and very fine sands. This distribution is typical of coastal systems dominated by the waves, characterized by a decrease in energy towards the sea. From a compositional point of view the results show sediments prevalently formed by siliciclastic materials (quartz, feldspars, biotites, heavy minerals and lithoclasts). The poor bioclastic component is formed by the foramol association related to the presence of Posidonia oceanica meadow. The hydrodynamic models allowed the identification of the main events that control the sedimentary exchange processes and structure the bars and troughs zone (surfzone). The hydrodynamic models demonstrate that Sirocco wind (135°) mainly controls the exchange processes and sets the surfzone. These events produce longshore and rip-currents and several circulation cells which shape a system of bars and troughs, with a surfzone extension over 100 metres. The several thematic maps produced (bathimetry, refraction, bottom current distribution, texture, facies distribution, surfzone dynamic etc.) and the reults obteined show the main mechanisms of sediment flows. The general results underline clearly the main mechanisms of control and behavior of sedimentary fluxes between dune-drybeach-shoreface.