FIELD MONITORING OF A RETREATING SALT MARSH IN THE LAGOON OF VENICE (ITALY)

A field monitoring campaign is carried out in the Lagoon of Venice (Italy) in order to investigate the role of wave climate in the evolution of a retreating salt marsh. Erosion data were systematically collected by means of erosion pins located horizontally along the marsh scarp for a time period of over one year. Pressure transducers were used to measure the wave climate close to the bank edge during three storm surges. A relationship between measured significant wave height and estimated wave height from wind, fetch and depth data has been calibrated to avoid recurrent field measurements. Wind and water level data are collected hourly from measurement stations located in the Lagoon of Venice. Field observations revealed that marsh retreat is characterized by continuous erosion alternate to mass failures, mostly of cantilever-type. Retreat rates are in the order of 50-90 cm/year. A linear relationship between wave energy flux and erosion rate is identifiable in case mass failures are not accounted for in the erosive process. Furthermore, mass failures can almost double the annual erosion rate, even if at shorter time scale, the slumped block can temporarily protect the bank from wave attacks.