Video-monitoraggio della variabilità morfologica delle barre sommerse: il caso del litorale di Terracina

The response to a beach face nourishment of the Terracina barred beach (Tyrrhenian sea, Italy) is analyzed based on a bi-weekly data set of timex video-images collected during a 34-month period (January 2008 - October 2010). Proxies for shoreline and sandbar crest positions were extracted from video-images and compared with topo-bathymetric surveys. During the first 17 months part of the nourished sediments were moved seaward and a shore-parallel linear single-bar system was restored. Bar dynamic was mainly influenced by on/offshore movements in relation to storms occurrence. A cluster of southerly extreme storms with maximum wave height of 4m sharply reshaped the nearshore morphology and a new double-barred system developed. A new inner bar generated close to the shoreline and the pre-existing one migrated 50m offshore. After this morphological reset the outer bar remained inactive and preserved a linear morphology for the whole analyzed period. On the contrary, the new inner bar dynamic was characterized by limited cross-shore displacements and high alongshore morphologic variability. During the last 5 months of observations a single barred-system was restored since the outer bar crest lost its morphologic evidence. As a consequence the inner bar moved 20m offshore and straightened. Video-shoreline provided useful information on the alongshore averaged beach face displacements even if the alongshore variability of the inner surf zone was slightly represented by this kind of proxy.