Medium and short-term evolution of a microtidal wave dominated Mediterranean beach seaward bordered by Posidonia oceanica meadow. The example of the Poetto beach (South Sardinia, Gulf of Cagliari - Italy)

This work describes the morphology, dynamics and evolution trend of Poetto beach seaward bordered by posidonia meadow located in Gulf of Cagliari. Medium-term evolution was studied over a period of 57 years using ortho-images (years 1954, 1968, 1977, 1997, 2000, 2006, 2008, Atlas-2011) for the reconstruction of shoreline change. Short-term variations were periodically monitored using topo-bathymetric digital models, derived from data acquired in 4 different field surveys (DGPS data and Echo-sounder data) conducted between 2012 and 2013. Shoreline evolution was reconstructed by means of measurements made at 21 homogeneously spaced transects. The surveys were conducted using field measurements referring to a geodetic net based on the IGM95 ETRF2000 n. 234802 point located in Cagliari Harbour. The topography of the emerged beach was surveyed with a DGPS StarFire system, obtaining the points with xyz coordinates on 1Hz frequency. For the submerged beach, a system of interfaced Reson Navisound 215 echosounder/Omnistar 8300HP DGPS through Reson PDS2000 navigation software, was used. The surveys were obtained both on submerged and dry beach. In “post-processing mode” all data were processed and filtered to realize a studied area DTM, and thus a detailed a detailed topography and bathymetry of the study area, reported on the map. A sedimentological survey was then carried out along 10 pre-established land-sea transects. During this survey the following samples were collected: 62 samples from the shoreface (using 5 liter van veen grab), 20 samples from dry beach. The sedimentological map was drawn according to the 9 sedimentary facies. The classification of the facies was realized using a multivariate statistical technique, which distinguishes sample groups on the basis of sediment characteristics (texture, mineralogy, taxonomical groups). The various sedimentary facies show: shoreface sediments, coastal detritic sands, reworked detritic sands, maërl beds, Coralligenous assemblages and two different types of muddy sandy sediments. Modern inner shelves sedimentation are controlled by eustatic, hydrodynamic and morphological features of the basin and by in situ production of marine ecosystems such as Posidonia oceanica (L.) Delile meadows, Coralligenous and Maërl beds. The Posidonia meadow plays a decisive role in the production, selection and transport of bioclastic sediments and contains the terrigenous sedimentation between the coastline and its upper boundary. Hydrodynamic characteristics were also studied. Models of wave and hydrodynamics were created using Delft3D software. The results obtained from these models (longshore and rip current distribution and surf-zone morphology and dynamics) allowed the visualization of the response of Poetto beach to wind/waves stress forced from NW – SW – SE wind.