Natural variability of shoreline position: Observations at three pocket beaches

International audienceWe investigated the variability of shoreline position of three adjacent pocket beaches at Barcelona city (NW Mediterranean) over a period of two years. Daily measurements of shoreline position were extracted from high-resolution video images and used to determine shoreline variability. Using empirical orthogonal function analysis we determine two dominant modes of shoreline variability. For all beaches analysed the mode that explains most of the variability (around 70%) describes the beach plan-form rotation which primarily occurs at the seasonal scale. The cross-shore translation of the beach profile, is described by the 2nd mode which explains around 30%. Both types of shoreline movement have been related to the previous nearshore conditions. This relationship was explored standardizing the translation/rotation of the shoreline and comparing it with time-averages of the cross-shore energy flux (EFcsh) and the energy flux direction (EFD). Averages over 7 to 12 days of EFcsh were significantly correlated to the translation movement. Plan-form rotation was well described by 28 to 40 day-averaging of EFD. Comparing results from the three beaches we determined that the time required to average the previous wave conditions depends on the beach characteristics (beach length and sediment grain size) in such a way that the greater the length of the beach and its sediment grain size, the slower its response