Erosion status of a sea cliff promontory bounding an ecologically important beach

Abstract

The study attempts to assess the retreat of a section of a bounding sea cliff promontory, exposed to wave forcing from both sides, of an important Mediterranean sea-turtle nesting beach, within a Mediterranean marine protected area (Gerakas, Zakynthos Island, Greece). Morphological changes were monitored within a 2-year period using a terrestrial laser scanner and the findings were compared with those from historical ortho-photographs to estimate sea cliff erosion the last 20 years. An advanced wave model supplemented by a model for cliff notch evolution, as well as an analytical model were also used to estimate cliff erosion due to wave attack. Comparisons of sea cliff profiles and calculations of volumetric changes using the collected and historical topographic data showed an average annual recession of about 0.16 m for the period 1997–2016. By comparison, coastal modeling estimated a retreat of about 0.23 m/yr, whereas a 0.14–0.3 m/y recession was assessed by the analytical approach; this reasonable discrepancy with the empirical results, is probably caused, amongst others, by the fact that the models does not account for the protection of cliff toes by a constant supply of erosion debris from above. The wave model was also used to estimate sea cliff retreat rates under a 0.5 m sea level rise and showed a fourfold increase compared with the current recession rate. On the basis of these results, as well as considerations related to the seismicity of the study area, it appears that the factors responsible for the sea cliff retreat are wave action, precipitation-induced surface flows, and earthquake-related instabilities that may cause rapid, large-scale erosion. The outcomes of this study are highlighting the critical interplay of several natural factors and climatic change effect on driving coastal erosion, which consequently results in the loss of ecologically critical habitats, thereby challenging conservation goals and management effectiveness.