Optimizing CVI-based heuristic approach for regional coastal vulnerability assessment in Sicily and 2000-2019 shoreline change calibration.

Coastal vulnerability can be defined as its proneness to be damaged by changing climatic  and/or geomorphic conditions. In this study, coastal vulnerability focuses entirely on the  physical impact of erosion, distinguishing the coastal behaviour between negative  (susceptibility) and positive (resilience) responses to climate change-induced conditions,  without taking into account any socio-economic variables. 

The Coastal Vulnerability Index (CVI) approach has become one of the most used and simple  assessing method for coastal regions around the world. CVI approach requires the selection  of a set of critical variables which are expected to significantly drive system processes, which  are then reclassified assigning to each obtained class a score expressing its expected  incidence, following to an expert based analysis. In this study we propose a modified version  of the CVI approach, for application to the entire coastline of Sicily as a first-pass regional scale coastal-vulnerability assessment. In particular, we modified the set of considered  variables, maintaining coast classification and slope, but excluding the shoreline change,  which was rather used for validating the obtained model. At the same time, long term or  negligible variables (sea level change and tidal range, respectively) were not considered and substituted by three covariates much more directly related to the short-term sea wave energy  (number of storm surges; mean wave power; dominant wave direction). Finally, a new  variable expressing the sediment yield from the fluvial system was also included, exploiting  the estimation offered by the Watem-Sedem model. To apply the CVI at a regional scale,  centroids along the coastline were generated and the vulnerability class for each factors  captured, so to compute the composite vulnerability CVI, which was reclassified into its four  quartiles. 

By crossing the obtained CVI classes and the observed 2000-2019 shoreline variation, a largely  satisfactory validation of the heuristic model was obtained, with the advancing and retreating  coastal sectors being strongly concentrated in the I and IV CVI classes, respectively. A general  negative trend between advancement and CVI score prediction arose from class I to class IV,  with a less marked discrimination between class II and class III. At the same time, it was  observed how the sediment yield factor allowed to correctly classify some advancing coastal  sector, which otherwise would have been predicted as subjected to retreatment. 

The obtained results produced a reliable picture of the regional coastline status, posing also  the basis for potential steps toward a stochastic modelling of coastal erosion processes in  Sicily.