Analysis of the effects of different storm events on shoreline dynamics of an artificially embayed beach

摘要

A five-year dataset of Argus-derived mean intertidal positions has been analysed to characterize the shoreface variability in a beach protected by a system of groynes and a parallel low crested structure (Lido di Dante Ravenna, Italy). For the period 2004-2009, 84 intertidal beach bathymetries and shorelines at the zero sea level were used as indicators to assess beach changes in between a number of selected surveys and to determine characteristic patterns of the beach response to storm events from different directions. Variations in the shoreline at the zero sea levels have been quantified and analysed in conjunction with nearshore wave conditions and provenance linked to storm events. These fall into two categories: (1) storm events occurring during Bora (north-eastern) wind conditions and (2) storm events occurring during Scirocco (south-eastern) wind conditions. The results show that, apart from main beach advances of the whole protected beach due to nourishments periodically carried out, a marked variability is observed among the four sub-cells into which the shoreface behaviour has been separately analysed. In particular, a dependence of beach rotation in the 'artificially embayed' area on the substantially bi-directional wave climate has been shown: Bora and Scirocco storm events produce shoreline rotation in counterclockwise and clockwise directions, respectively, due to the occurrence of longshore currents in the opposite direction in the nearshore. An attempt was made to correlate the shoreface dynamics for the main rotation events (14 selected ones) to the wave attack intensity (as the total energy flux due to storm events). A relationship seems to occur (for each storm category) between the shoreline displacements estimated for each sub-cell and the total energy flux computed for inter-survey periods, supporting the occurrence of a link between the observed morphological changes and the hydrodynamic forcing associated with storm events in the five-year monitoring period.