Shoreline: is still IGM42 Italian datum an updated reference to account for the Sea Level Rise?

The shoreline, the line where the sea and the shore meet, is an important feature of the coastal environment. While the shoreline concept is simple, it's extremely difficult to track in real-world contexts. What are we already measuring considering the ongoing Sea-Level Rise? How far will the shoreline be from the datum's zero in the future? These are our primary research questions. To reach our goal, we gathered topographic data from two dynamic North Adriatic barrier-island systems, using LiDAR and drone surveys referred to the Italian datum IGM42. We analyzed the distance variability between different contour lines (0m, 0.25m, and 0.50m), also considering the various morphology of the beach profiles. Choosing the lowest line that better represents the coastal trend, we found that it's usually higher than the official datum's zero, not only as a result of the recent increased mean water level but also due to the marine forcings (tides, waves, storms, sediment supply) and the presence of landforms (e.g. shoals, tidal flats). In highly dynamic and variable coastal contexts, the IGM42's zero is far from being a good morphological and topographic indicator, and we suggest adopting the 0.25m contour line as a more accurate and updated baseline.