How Compressible Are Salt Marshes? Unique Field Experiments Combined With Advanced Modeling Provide Novel Insights From the Venice Lagoon

Coastal depositional environments, particularly salt marshes, are among the morphologies most vulnerable to relative sea-level rise (RSLR). Their ability to withstand changing conditions is closely related to their capacity to accrete sediment, gaining elevation at a rate comparable to RSLR. Sedimentation, decomposition of organic remains, and autocompaction are among the main processes that govern the long-term evolution of marsh elevation, with soil compressibility that controls the salt marsh thickening. In fact, autocompaction may significantly offset elevation gain from sediment deposition. Autocompaction is especially apparent in the shallowest decimeter depth where sediments have the highest porosity and are most compressible. However, the geomechanical characterization of these landforms is uncommon worldwide. Laboratory geotechnical testing faces challenges in terms of (a) reliability due to sampling disturbance of these soft soils, and (b) representativeness of in situ conditions because of the dense root network of halophytes in the upper 0.1-0.3 m depth. Here we quantify for the first time the compressibility of the shallowest decimeters of salt marsh soils by novel field-scale loading tests carried out in representative salt marshes of the Venice Lagoon, Italy. The data collected are analyzed using an advanced 3D coupled flow-consolidation simulator with nonlinear constitutive models. Our analysis reveals that salt marshes exhibit considerable variability in soil compressibility within the upper 1-2 m, with differences of up to 2 orders of magnitude. This variability is attributed to several factors, including lithology, depositional environment, and soil age. Moreover, they are characterized by a preconsolidation stress ranging from 2 to 8 kPa, linked to the specific depositional environment, which plays a major role in their mechanical response. The quantification of these parameters will allow a reliable prediction of the salt marshes fate in the Venice Lagoon following global environmental changes and local human interventions.