QUANTIFYING THE EVOLUTION OF A BEACH PROTECTED BY LOW CRESTED STRUCTURES USING VIDEO MONITORING

The purpose of this study is to analyse the shoreline response of a beach protected by a system of low-crested-structures (LCS) and to apply the results to the management of coastal stability problems. This contribution aims to provide heretofore scarce detailed data and observations of the shoreline variability of coasts protected by LCS. These aims are achieved by selecting and monitoring the appropriate Coastal-State-Indicator (CSI) to describe the state of the beach. The CSI used is the Mean-Intertidal-Position (MIP), which has been detected by the ARGUS system installed in Lido di Dante, Italy. The error in the shoreline estimation from video was assessed by comparing shorelines detected with GPS and image analysis over a large number of cases. The horizontal error in the shoreline detection is estimated to be on the order of 0.90±0.74m, which is acceptable because it is on the same order of magnitude as the swash width. The MIP, which has been monitored over a forty months period with elevated spatial and temporal resolution, was monitored in several transects of the beach protected by a system of LCS and boundary groynes, creating two cells. The behaviour of the section in the middle of the cell was comparable to the behaviour of the total beach. Results were analyzed with respect to the fixed beach state of the initial beach position (January, 2004). The MIP regression rate is around 2.3m/year, which is modest considering the local subsidence magnitude (1.8cm/year). Therefore, the actual data do not show any evidence of immediate risk either for the present structures on the beach or for the infrastructure immediately behind; however, the results do show a risk for the sustainability of the characteristics that permit tourist development. The use of video-derived CSIs is confirmed to offer a valuable tool for coastal management.