Modeling the consolidation of coastal marshes in the Venice Lagoon by a coupled 3D mixed finite element approach

Coastal marshlands are valuable habitats for wildlife and important coastal defences. With global climate change and relative sea level rise, saltmarshes are becoming increasingly important as buffer areas between the land and the sea. In recent years, coastal marshes around the world have declined dramatically. Their existence is severely threatened by erosion and sea level rise. On the other side, marshes aggradation (increase of land elevation) is controlled by the deposition and autocompaction of sediments transported by lagoon water over the marsh platform during high tide conditions. To understand these dynamics, the response of salt-marsh landforms to applied vertical stress must first be quantified. However, the interactions between sedimentation and autocompaction are currently understudied. Recently, a series of in-situ loading experiments was conducted on marshlands of the Venice Lagoon to assess long-term autocompaction relying on measures of subsurface displacements and pore water pressures collected during the tests. The four identical experiments, performed in four different sites of the Venice Lagoon, have shown large differences in behavior and maximum deformations from site to site, proving that coastal marshes consolidation is strongly influenced by soil heterogeneity. In this thesis, the results obtained from experiments have been analyzed to characterize the marshlands from a geomechanical view point and then they have been used to setup and calibrate numerical models able to simulate the compaction of the marshes during the loading tests. The numerical model is based on a coupled 3D mixed finite element approach and solving the Biot consolidation equations. The comparison between model outputs and the measurements emphasizes that, overall, the model is able to reproduce quite well salt marsh autocompaction dynamics both in terms of vertical displacements and pore water pressures, with some limitations due to the complexity of the phenomenon.