Vulnerability to relative sea-level rise in the Po river delta (Italy)

Relative sea-level rise poses one of the greatest threats to transitional coastal systems around the world and it is due to changes in both sea level and terrestrial elevation. Although expected sea-level rise due to climate change is the most widespread concern, the effect of land subsidence can be much greater. Analyses of coastal vulnerability to relative sea-level rise generally neglect or over-simplify land subsidence patterns affecting large areas. This can lead to inaccurate assessments of vulnerability that do not take account of the spatial variability of ground movements. The Po delta (Italy) is a highly distinctive example of a transitional coastal environment, which hosts an extensive range of habitats and biodiversity together with archaeological and historical sites. Relative sea-level rise significantly threatens the delta, mostly due to uneven subsidence rates ranging from a few mm/yr to more than 20 mm/yr. This study is aimed at exploring the vulnerability of the Po river delta to relative sea-level rise, considering the heterogeneity of present land subsidence, various sea-level rise scenarios and the hydro-morphological setting, by means of an index-based model. The analysis shows almost 50% of the Po delta (~500 km2) currently at critical-extreme vulnerability to relative sea-level rise, exceeding 80% (~800 km2) by 2050. The model indicates uneven distribution of vulnerability over agricultural, submerged and intertidal, urbanized and forested areas due to land subsidence variability along with the complex hydro-morphological setting of the Po delta area. Considering an average subsidence value instead of the actual uneven pattern, vulnerability is over- and under-estimated in the inland and lagoon sectors respectively.