A Virtual Element Model for the prediction of long-term salt-marsh dynamics

Salt marshes are vulnerable environments hosting complex interactions between physical and biological processes. The prediction of long-term vertical dynamics, i.e., marsh growth and/or reduction, is crucial to estimate the potential impacts of different forcing scenarios on such systems. The most significant processes influencing the elevation of the salt-marsh platform are accretion, auto-compaction, and the variation rates of the relative sea level rise, i.e., land subsidence of the marsh basement and eustatic rise of the sea level. The accretion term considers the vertical sedimentation of organic and inorganic material over the marsh surface, whereas the compaction reflects the progressive consolidation of the porous medium under the increasing load of the overlying younger deposits. The present work describes a novel mathematical approach, based on the Virtual Element Method, for the long-term simulation of the salt marsh vertical dynamics. The Virtual Element approach is a grid-based variational technique for the numerical discretization of Partial Differential Equations allowing for the use of very irregular meshes consisting of a free combination of different polyhedral elements. The modelling approach provides the pore pressure evolution within a compacting/accreting vertical cross-section of the marsh, coupled to a geomechanical module based on Terzaghi’s principle of effective inter-granular stress. The model takes into account the geometric non-linearity caused by the large salt marsh deformations by using a Lagrangian approach with an adaptive grid, where the domain geometry changes in time to follow the deposit consolidation and the new sedimentation. The use of Virtual Elements ensures a great flexibility in the element generation and management, avoiding the numerical issues often arising from strongly distorted meshes. The numerical model is developed, implemented and tested employing two different configurations of the sedimentation r ate. The preliminary numerical results provide evidence of the flexibility of the proposed approach, which appears to be a promising computational tool for the accurate simulation of real-world applications.