## 4 Results and Discussion
The southern part of the coastal plain influences large subsidence, reaching a rate up to 8 \(\pm\) 2.46 mm/year in the Lentini area. The mobile coastal system showed phase of decrease of sediment supply, particularly over the last decades. The reduction of suspended load from Simeto, Gurnazza and San Leonardo determined a negative sedimentary balance, which is exacerbated by the intense beach-dune erosion. Figure 7: Retreat of the Simeto river mouth on the Catania coastal plain. The shoreline at 2003 epoch is marked in light orange (**left**) while the shoreline for 2020 epoch is marked in light blue (**right**). _The sandy coast of Siracusa._ This coastal zone shows a rate of shoreline changes between 4.01 and 3.9 m/year (linear regression rate of 4 \(\pm\) 1.9 m/year). The main evidence of shoreline changes occurred south of the Ciane river mouth. This area, which is characterized by salt-marashes, has been affected by a coastline retreat of about 70 m, with local breaching of the beach-dune system (Figure 6). The salt-marsh is subsiding at about 2 \(\pm\) 2.46 mm/year, while the northern sector of this coast, displays higher subsidence rates mainly located in the Siracusa harbor, with velocities of about 5 \(\pm\) 2 mm/year. The narrow beaches that separate the salt-marsh from the sea are not nourished by inland sediments due to dams that highly reduce the supply of sediments on the coast. _The coast of Vendicari._ This area shows a rate of shoreline retreat between 0.95 and 0.4 m/year, with a mean rate of 0.68 \(\pm\) 0.4 m/year. The InSAR analysis showed significant rates of local land subsidence, at about \(-\)4 \(\pm\) 2.46 mm/year. Foredune system showed a given stability over the last few decades, while the dynamic of the coasts seems to be correlated to the longshore drift toward south. Figure 9 and Table 1 show reference RSRR projections for RCP 2.6 and RCP 8.5 for the investigated coastal zones most prone to SLR in 2050 and 2100. Figure 8: (**a–c**) Horizontal displacements obtained following a geometric steady-state approach: sea-level rise determines a landward shoreline movement without accretion, where the shoreline rate change is dependent only on the coastal slope extracted from the DEM of LiDAR data. Figure 10: The Catania coastal plain. In color are reported the expected extension of flooding in 2050 and 2100 for (**a**) RCP2_6 and (**b**) RCP8.5 emission scenarios, respectively. The expected maximum loss of land in 2100 is 4.094 km\({}^{2}\) (high-resolution maps are inserted in Supplementary Materials Maps S2 and S3). The coastal strip is characterized by about null VLM (\(-0.7\pm 0.35\) mm/year) with respect to the inner zones that show subsidence rates exceeding 10 \(\pm\) 2.5 mm/year, likely due to local fluid withdrawal. Figure 9: Sea-level projections relative to 2016 AD for the coast of South-eastern Sicily obtained by combining the IPCC-SROCC 2019 projections at regional scales with the contribution of the most common rates of land subsidence as derived from InSAR analysis (Figures 10–15): (**a**) 0.7 \(\pm\) 0.35 mm/year; (**b**) 5.2 \(\pm\) 2.5 mm/year; (**c**) 7.6 \(\pm\) 2.5 mm/year and (**d**) 10 \(\pm\) 2.5 mm/year. Full lines correspond to the 2.6 (blue) and 8.5 (red) SROCC scenarios and the coloured bands highlight the 90% confidence interval. Dashed lines correspond to the raw SROCC projections when no measured vertical land motion is included. The small-scale variations observed in the data are related to the ocean component contribution accounting for the effects of dynamic SSH, the global thermosteric SSH anomaly and inverse barometer effects. Figure 11: The bay of Augusta. In colors are reported the expected extension of land flooding in 2050 and 2100 for (**a**) RCP 2.6 and (**b**) RCP 8.5 climatic scenarios for a mean land subsidence at \(3.4\pm 2.5\) mm/year. The expected maximum loss of land is 1.659 km\({}^{2}\) (high-resolution maps are inserted in Supplementary Materials Maps S4 and S5). Figure 12: The bay of Siracusa.