Evolution trend of La Cinta beach (San Teodoro) after the flooding on 24th September 2009

A few days after the flooding on 24th September 2009, which was declared a natural distaster, the staff at the Osservatorio Coste E Ambiente Naturale Sottomarino (O.C.E.A.N.S.) carried out investigations and instrumental surveys on La Cinta beach. The beach is located in the municipality of San Teodoro and is within the protected area Area Marina Protetta Tavolara Punta Coda Cavallo; it extends N-S, is approximately 4km long and between 50 and 70m wide (FIG. 1). Following the intense rain, which continued for 9 days (168mm/24hr was recorded at the weather station in Siniscola on 24th September) and the flood wave, the ridge of the foredune, which defined the upper beach, collapsed forming 25 lagoon openings. New topographic-bathymetric and sedimentological-stratigraphic data were acquired, following the creation of a specific geodetic network. The study began within the agreement between O.C.E.A.N.S. and the province Olbia-Tempio and continues thanks to two Fellowship grants to young researchers and the Project RIAS (a study of Sardinia’s coastal systems and global climate change), funded by the Sardinian Regional government (co-fund PO Sardegna FSE 2007-2013 L.R.7/2007). The survey was carried out by taking topographic measurements referring to a specific geodetic network, based on the point IGM95 ETRF2000 n.182704, located in Murta Maria. The dry beach was tested using DGPS Thales ProMark3; points with the coordinates xyz were acquired at the frequency of 1 Hz. The Reson Navisound 215/DGPS Omnistar 8300HP interface echosound system using Reson PDS2000 navigation software, was used for the shoreface (De Muro S. & Pusceddu N., 2009). The survey was carried out on emerged and submerged beach at the same time. The limits of the morpho-sedimentological structures were identified and the profile of the lagoon and the new openings, caused by the flood, were defined. The survey covered the area of the beach from 3 m up to a depth of -12 m. The examination carried out a few days after the event allowed the variations in the dimensions and slope of the dry beach and shoreface to be calculated. It was also possible to determine the sedimentary volumes altered due to the event and to measure numerous morphological parameters (e.g. depth, width, axis of new channels, planimetrical layout of alluvial fan lobe, the flood event direction, etc). Seven stratigraphic logs were also carried out and sampling of the areas opened up by the flooding, in order to define the ancient coastal ridge. Digital Full HD images were acquired, taken from a helicopter and on field. From the data collected, models of wave motion and hydrodynamics were created using Delft3D software, modules WAVE and FLOW. The models were based on marine forecast data recorded in the area as well, and were gauged on observations and the new measurements in situ. The overflowing of the lagoon caused the breach of 25 openings on the drybeach. When the initial data were taken six were still open, with dimensions varying between 25-30m and depth between 1 to 4m (FIG 2). In the submerged beach the greatest variations were found immediately in front of the lagoon openings. Measurements have been taken repeatedly in the field over the year following the event and in-depth study has been carried out in order to examine the resilience of the system. A second digital model of drybeach and shoreface has been built and the hydrodynamic chart has been updated which is based on the Delf 3D model and on the new observations and measurements. The data taken in the second survey of measurements show that all the openings caused by the flooding had closed. The slope of the entire beach shows strong modifications in the dimension of the sand bars and troughs with relative hydrodynamic modification.