Holocene-dependent sediment requirement for supporting the resilience of the Venice Lagoon marshlands against expected (relative) sea-level rise

Marshlands in the Venice Lagoon (Italy) are among the most valuable morphological environments, yet they face the risk of disappearing by the end of the century. This risk is associated to relative sea-level rise due to climate change, land subsidence caused by various processes, and decrease in sedimentation rate above the marshland platform because of an increasing frequency of MoSE activation. In the micro-tidal conditions characterizing the Venice Lagoon, marshlands need to maintain an elevation between 20 and 40 cm above mean sea level to keep pace with relative sea level rise,. Recent research worldwide has clearly revealed how aggradation (i.e., net elevation gain) of transitional landforms can be significantly smaller than the accumulation rate of newly deposited sediments on their surface. The difference between aggradation and sedimentation rate is primarily related to the self-compaction of Holocene deposits induced by the progressive  load applied by subsequently deposited (younger) sediments. To investigate the amount of sediments needed for saltmarshes to keep pace with the expected (relative) rise in lagoon water level, we applied the NATSUB3D simulator, that is based on finite element discretization and accounts for sediment deposition and consolidation over time in the context of large  vertical deformations. NATSUB3D uses an adaptive mesh, with the hydro-geomechanical properties (porosity, hydraulic conductivity, compressibility) of the heterogeneous growing sedimentary body that vary in space and over time depending on the actual vertical effective stress. NATSUB3D is applied to three representative sections of the Holocene sequence in the Venice Lagoon, recostructed through borehole lithostratigraphy, facies analysis, C14 datings, in-situ and lab geomechanical tests. The model allows quantifying the sedimentation needs based on different climatic scenarios and characteristics of the future sedimentation. It also highlights the significant difference between marshlands located in the different sections depending on the Holocene thickness and composition