Salt-marsh retreat on different time scales: Issues and prospects from a 5-year monitoring campaign in the Venice Lagoon

Lateral edge erosion is one of the main mechanisms leading to loss volume of salt marshes. Given the relatively large number of field measurements of eroding salt marshes reported in the literature, and the different ways they were performed, some natural questions arise. (i) What is the relationship between wave climate and erosion rate at different time scales? (ii) How can the surveys based on a single meteorological event support large-scale monitoring campaigns? (iii) What are the main meteorological and morphological parameters correlated to the erosion process? (iv) Why do only some marshes show cantilever profiles? (v) What are the most common issues to deal with in monitoring campaigns? Most of the previous studies considered large spatial and temporal scales only, providing an estimation of the overall behaviour of the system without identifying the specific role of each event. In this study, we attempted to answer these questions by investigating the lateral evolution of salt marsh margins located in the Venice Lagoon at different time scales, combining marsh retreat measurements, remote sensing data and numerical modelling. Field data were collected monthly for 5 years (2014–2018), covering 26 storms. A linear relation linking erosion rate to wave power is confirmed at different time scales, with a decreasing slope at higher elevation of the marsh bank that explains the occurrence of cantilever profiles. Mass failures can occur over long-period cycles, related to the geomorphic characteristics of the area. Finally, the major issues affecting the in-situ monitoring campaigns have been highlighted.