Localizzazione della linea di riva, in spiagge sabbiose stabili, considerando le fluttuazioni da onde e maree.

Abstract

The shoreline is defined as the contact line between the land and sea surface. Due to the dynamic nature of water levels at the coastal boundary, such as waves and tides, the shoreline position incessantly changes in time. The shoreline variability and coastal trends have been the main object of several researches and it is the main target of this thesis. The continuous changing of shoreline positi on depends on several causes: that due to the hydrodynamic (e.g. waves, tsunami, tides , sea level rise, storm surge) and geomorphological change (e.g. barrier island formation, spit development). Consequently, the exact understanding of shoreline dynamic is important for a wide range of coastal studies such as: a) management and planning of coastal zones, b)hazard mapping, c) defining the exact boundary between state and private owned areas and d) for conceptual or predictive modelling of coastal morphodynamics (erosion- accretion analysis). Moreover, the location of the shoreline can provide information about shoreline changes due to man-made structures(groins, breakwaters, harbors, ecc.)and about beaches dynamic (shape and volume). The aforementioned elements are useful to quantify rates of change in time. For all these reasons the shoreline position is the most common morphologic indicator of coastal ar eas.Usually, the shoreline is positioned by means of aerial images interpretation and then the evolution in time is obtained by a time se ries images analysis. However, the shoreline position extracted from aerial images only represents the wet/dry line that describes the instantaneous land-water boundary at the instant of the acquisition without providing any information concerning to the “normal” or “average” conditions. In this thesis a novel multidisciplinary method which allows the estimation of the shoreline position by means of remotely sensed images, considering the effects of waves and tides, has been proposed. The application of this method involves several techniques used in other disciplines which were int egrated in order to trace the shoreline. The key steps of the proposed method involve the topographic-geomorphic and hydraulic studies. Even if the geological and the geomorphological survey were mainly descriptive, they are functional to the hydraulic study. The understanding of the geomorphological characteristics and the surface water level fluctuations (waves and tides), provides a greater level of accuracy for shoreline positioning. In fact the dynamic nature of the shoreline suggests that this water-land boundary cannot be defined as a single line but it has to be positioned within a strip. The method has been applied to a Mediterranean beach located in the western Sicily. This beach is geomorphologically in equilibrium as demonstrated by the geomorphological study and by the physical description of the submerged and emerged beach. Physical characteristics description has be en completed with the size and composition of sediments analysis. Moreover, the beach profiles and their average slopes were also obtained. The hydraulic study was divided into four steps: 1)wave and tide data collection, 2)identification of “ordinary” sea storm and tides fluctuation, 3) propagation of waves from offshore to nearshore and 4) run-up computation. In order to take into account the wave effects on the shoreline position during one y ear, the concept of “ordinary” sea storm was used. As defined by the Italian law the shoreline (“lido” in Italian), is the beach zone in contact whit the sea and covered by water during an “ordinary” sea storm. In this study 1 year return period was considered as representative of “ordinary” conditions (as reported in the Italian juridical definition - Corte di Cassazione, Sez., Un., 02/05/1962, n.849). In order to evaluate the parameters of a “ordinary” sea storm, a statistic analysis of extreme events has been performed.