3D Modelling of Enhanced Groundwater Dynamics in Artificial Saltmarshes of the Venice Lagoon

Salt marshes are coastal wetlands that provide essential ecosystem services, such as wave attenuation, sediment trapping, nutrient buffering, and carbon sequestration. Despite their recognized value, they have undergone extensive decline in recent decades, making their conservation and restoration a global priority. Salt marsh restoration is a common practice to recreate a part of these landforms lost over the last decades because they may have eroded or drowned. However, these areas often have no vegetation, as the low elevation of the deposited sediments exposes the marsh to frequent and longer inundation periods and the absence of a creek network prevents proper soil desaturation. Indeed, creeks play a main role in natural salt marshes to facilitate groundwater drainage when tidal levels lower and marsh platform emerges above the mean sea level. The presence of internal channels is recognized as a crucial factor for the establishment of vegetation. Recent experimental efforts have focused on facilitating the formation of tidal creeks in restored or artificial salt marshes. This study aims to apply a three-dimensional variable-saturated groundwater flow model to simulate and visualize drainage dynamics within a real artificial salt marsh in the Venice Lagoon. Simulations were conducted using the GroundWater Simulator. Initially, modelling has been carried out on an idealised marsh and subsequently on a real artificial marsh, assessing the impact of tidal creek networks on saturation and pressure head evolution. Both sinusoidal and real tides were investigated, and the influence of different boundary conditions was also evaluated. Results showed that channels significantly enhanced drainage, reducing full saturation duration and allowing the marsh to reach lower saturation values. Simulations with real tidal regimes revealed more complex saturation patterns, where only the highest tidal peaks induced saturation of the whole marsh body and the creek allows subsequent decrease of the water content in the area surrounding the drainage element. A model sensitivity analyses revealed that mesh refinement, which is a critical issue in modelling the evolution of the water content in large 3D landform domain, affects result accuracy. Moreover, as expected, higher hydraulic conductivity of soil used to restore a salt marsh accelerates drainage and expands the desaturated area. Overall, this study provides insights to guide restoration practices, improving salt marsh resilience and mitigating sea level rise impacts.