Coupling a hydro-maritime model and remotely sensed techniques to assess the shoreline positioning uncertainty: The Marsala coast study case

The severe erosion phenomena affecting the Mediterranean coasts are strictly related to geophysical characteristics and socio-economic pressures. This suggests the need of monitoring and modelling the phenomenon in order to quantify its strength. In fact, the shoreline position, as well as its temporal evolution, provides important information for designing defence structures and for the development of a coastal management plan. The shoreline has a dynamic nature as it changes both in the short and long period. Those changes are caused by geo-morphological (e.g. bars and barrier island development etc.) and hydrodynamic (wave motion, tides and flows) processes, as well as by sudden and fast events such as sea storms, earthquakes and tsunamis. The research examines the uncertainty in positioning the shoreline coupling remotely sensed images and a hydro-maritime model. Although the assessment accuracy strongly relies on data availability and consistency, the resulting assessment of the shoreline erosion and accretion is crucial for an overall understanding of the hydro-maritime geo-morphological interaction. The study case is the Marsala coastline (western coast of Sicily, Italy), named 12th island physiographic unit. It is characterized by a low coast with sandy sediments from Holocene age. These sediments are in continuity of sedimentation on whitish debris composed by organogenic limestone from Pleistocene age. The diachronic analysis was carried out on both emerged and submerged parts of the beach and involves two distinct phases. In the first phase, geo-morphological in situ data have been compared with maps and georeferenced remote sensing images referred to the period 1994-2006. It allowed the identification of shoreline indicators [2] such as the beach cross-section and the shoreline positioning including its spatial and temporal variations. It should be noted that the comparison between EO (Earth Observation) images and cartographic maps is subjected to several uncertainties, due to graphic error, geo-referencing accuracy and spatial resolution. Moreover tidal and climate waves data refer to an acquisition time different to that of the EO images. In the second phase, a maritime hydraulic modelling accounting for sea fluctuations has been performed. The run-up is related to wave's amplitude and phase, as well as to the composition and particle size of the beach sediments determining the beach slope [3]. Prior to run-up calculation, an investigation aiming to evaluate how the waves propagate from offshore to inshore (a third-generation spectral wave numerical model, SWAN - Simulating WAves Nearshore), has been carried out. Wave data have been acquired by a buoy belonging to the National Network Waves Data, located at the SW of the Mazara del Vallo harbour (Trapani), while tide data were recorded by the national marigraph of Porto Empedocle (Agrigento). The results allowed assessing the uncertainty and the consequent accuracy in the shoreline positioning for given slope, highlighting that it is not always possible to assess the shoreline rise and fall, for values lower than 10-15 m. © 2010 Copyright SPIE - The International Society for Optical Engineering.