Vulnerability assessment of a coastal dune system at São Francisco do Sul Island (Santa Catarina, Brazil)

Beach dunes are geomorphological forms that are frequent in the coastal environment. They represent a vital factor in the evolution of these systems because they affect coastal areas from several standpoints: economy, culture, tourism and nature are aspects that are tightly connected to the conservation of coastal dunes. Excessive exploitation and mismanagement of dune systems lead to their degradation, erosion and eventual disappearance: extreme use of natural resources and uncontrolled population growth are among the human-related factors that often put the existence of coastal dunes in jeopardy. Based on the necessity to preserve these systems, the aim of this study is to apply a Coastal Dune Vulnerability Index (CDVI) on a beach at São Francisco do Sul Island (southern Brazil) in order to identify the areas that result most sensitive to environmental changes. The CDVI has been applied along six transects traced out on two sectors of Grande beach. The sectors have been selected based on dune characteristics: Zone A (northern sector) is characterized by well developed parabolic dunes up to 15 m high, often interrupted by wide blowouts; Zone B (central sector) is characterized by transverse dunes. The analysis involved 51 quantitative and qualitative variables, divided into five groups: geomorphological dune system condition (GCD), marine influence (MI), aeolian effect (AE), vegetation condition (VC) and human effect (HE). Data were collected through fieldwork (sediment sampling, topographic survey, vegetation coverage, human impact), laboratory analyses (grain-size), digital processing (photo-interpretation); marine and wind database were provided by local authorities and literature data. The index was calculated associating each variable to a five point scale, ranging from 0 (no vulnerability) to 4 (very high vulnerability). For each group, the sum of the variables was divided by the sum of the maximum achievable rating within each group, thus generating a partial index expressed as a percentage. The total CDVI was computed as the unweighted average of the partial vulnerability indices with the algorithm: CDVI = (GCD+MI+AE+VC+HE)/5. ANOVA was used to test if there were any significant differences between transects. Partial CDVI values do not show significant differences between transects, ranging from 0.37 to 0.48. The parameter showing higher partial CDVI for both areas was GCD (maximum value 0.78 – T2), then MI (maximum value 0.55 – T1). AE had lower values in Zone A (0.33 to 0.16) rather than in Zone B (0.50 to 0.53). VC values were similar in both areas. The HE was higher in Zone A (0.35) relative to Zone B (0.14). The CDVI evaluation needs to be repeated periodically, but presently the total vulnerability can be classified as medium: summarizing, the geomorphological factor must be monitored at Grande beach, in particular the blowouts in Zone A and the frontal dune retreat in Zone B. The results of the study further suggest that assessing the vulnerability of coastal dune systems with an easy to use instrument as the CDVI might be a valuable support to improve the management of coastal areas, in particular those subjected to erosion issues.