Risposta morfologica della spiaggia compresa tra Lido di Dante e Lido di Classe ad eventi di mareggiata

The impact of sea storms on low-lying sandy coasts may induce hazardous erosive beach and dune processes and, in the most severe situations, flooding of coastal areas which can cause, on a very limited time scale, significant changes in the littoral. The risk level increases for highly developed areas, where the negative effects on the local economy of the territory are added to the direct consequences of the physical impact of storms. In front of the rapid and continuous population growth in coastal areas, the increased human activities and the threat posed by rising sea level, understanding and forecasting the possible storm induced damage to the coastal environment may represent a useful tool in support decisions in coastal management. In this context dunes play a role of fundamental importance in granting natural protection to the backbeach during storm events. For the areas behind the shore placed below the mean sea level (which is the case of quite all Emilia-Romagna coastal environment), dunes are at the same time the first line of defense as well as the last barrier against phenomena of marine ingression. In the present study the response to the storm events befallen in the period between September 2008 and March 2010 is discussed in detail for one of the few remaining dune cordons along the coast of the Emilia-Romagna Region, located between Lido di Dante and Lido di Classe and characterized by an extension of about 6 km. Storm surge phenomena and wave conditions were analyzed to identify marine storms that may induce significant morphological changes in the beach-dune system. The characteristics of extreme sea levels that occur along the northwestern Adriatic coast were identified through the examination of the longest mareographic series available, highlighting their highly seasonal pattern. The quantification of the impact of storm surges was carried out comparing the pre and post-storm profiles, spaced a hundred meters apart, and simulating the morphological evolution of beach profiles through SBEACH numerical model. The results show that the combination of storm surge and wave conditions induces the most serious consequences for the considered physical system. The analysis clearly shows that the combination of high sea levels and average wave intensity has a decisive role in erosion and inundation processes that affect the regional coast. The study results indicate that the magnitude of the impacts caused by storm surges is generally controlled by the topographical and morphological beach features (such as slope and amplitude) as well as by dune state (ridge elevation, amount of frontal dune reservoir, distance to sea): these features may prevent extensive erosion in some areas and promote duneface retreat or washover in others. In spite of the smaller wave heights associated to Scirocco winds, storm surges, which generally are higher during these conditions, may result in particularly dangerous effects for the resilience of the dune system due to the longer wave period depending on the morphology of the Adriatic basin. An evaluation of the volume lost from the impact of sea storms further indicates that a system like the one located in the northern part of the coastal stretch under consideration, already severely degraded by high human impact and effects of headlong land subsidence, which in this area reaches the highest value of the entire regional coast, becomes increasingly susceptible to storm events and easier involved during less intense storm conditions.