Managing Marsh Nourishment Projects: An Integrated Measuring and Modeling Approach

In recent decades, numerous projects aimed at restoring tidal morphologies have been planned and implemented. These initiatives are driven by the objective of reestablishing the vital services provided by these ecosystems. A key parameter for the ecological functionality of restored tidal ecosystems is the elevation of the landforms with respect to mean sea level. Non-optimal elevations can result in permanently submerged areas by the sea only a few years after their construction (i.e. when elevation is too low), and/or vegetation cover remains more patchy and less biodiverse than on natural marshes (i.e. when elevation is too high). In this contribution, we present recent work conducted between 2021 and 2023 to study a nourishment project realized in the central basin of the Venice Lagoon (Italy) to restore a 6.1-ha salt marsh. The marsh area was enclosed by wooden poles connected by geotextile and subsequently filled by dredged sediments using a nourishment pipe. The area was partially nourished in October 2021 and 2022 after the establishment of a specific monitoring network aimed at measuring the consolidation of the pristine lagoon bottom and the new infilled sediments. The network consists of 10 Nourishment Elevation Change (NEC) stations by which vertical movements were monitored with a topographic intersection technique at millimeter accuracy. Additionally, drone photogrammetry was employed to monitor the nourishment landform and elevation evolution through time. Additionally, a bathymetric survey and a 10-m deep core were carried out before the restoration activities to characterize the local morpho-geological setting of the lagoon platform. The data obtained by the campaign surveys provided detailed spatio-temporal evolution of nourishment infilling and consequential post-depositional sediment compaction. To simulate the evolutional process, the data has been integrated into a numerical simulator which couples groundwater flow and sediment consolidation equations in a 3D setting under the hypothesis of large deformations. Using Finite Elements, the numerical model describes sedimentation and compaction in an evolving domain where new elements are incorporated into the original domain when sedimentation takes place, and these elements undergo deformation as consolidation occurs. The model enables us to successfully reproduce the subsurface sediment beneath the salt marsh (based on the 10-m coring) and the marsh nourishment itself in 3D with appropriate sediment lithology characterization. This preliminary marsh reconstruction is fundamental for a comprehensive assessment of the salt marsh's ability to cope with future sea-level rise while also accounting for autocompaction of both the newly infilled sediments and the pre-existing, underlying deposits, which significantly contributes to lowering the marsh platform. Moreover, the model is used to quantify post-depositional compaction and provides a valuable tool to estimate required nourishments to eventually achieve the optimal marsh elevation for ecological functioning.