Field monitoring of a retreating salt marsh in the Lagoon of Venice.

Michele Bendoni, Riccardo Mel, Simona Francalanci, Hocine Oumeraci, Stefano Lanzoni, Luca Solari

Thursday 2 july 2015

17:36 - 17:39h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Morphodynamics of estuaries and coastal areas, Poster pitches

Parallel session: Poster pitches: 13A. Sediment - Coast

Salt marshes are part of ecotone environments such as lagoon and estuaries. They provide ecosystem services to local population, but in the last decades they are disappearing due to sea level rise, subsidence, increased anthropic pressure and boundary erosion forced by surface waves. The latter is likely the main process leading to marsh edge lateral retreat in several areas of the world. For example, in the last century the whole salt marsh surface of the Venice Lagoon has more than halved and trends predict its completely disappearance over the next 50 years. We are carring out a field monitoring activity on a retreating salt marsh located in the north part of the Lagoon of Venice (Italy), subject to North-East (Bora) strong wind. Comparison of georeferenced aerial photographs showed a retreat rate of the order of 1 m/year during the last decade for the monitored marsh. Ongoing field measurements started by the end of November 2013. Erosion is surveyed by means of erosion pins located horizontally on the marsh scarp. Significant wave height has been measured during three storm surges close to the bank edge by means of pressure transducers (PTs). The measured wave climate was then related to the offshore wave climate estimated using wind (intensity and direction), fetch and depth data. Wind intensity and direction and water level are measured hourly by several measurement stations located in the Lagoon of Venice. In this way, it is possible to extrapolate wave climate hourly at the monitored marsh and calculate the wave power striking the marsh boundary in a given time interval. Field survey revealed that the main retreating mechanisms are particle by particle erosion alternated to cantilever failures. Preliminary results show a linear relationship between erosion rate and wave energy flux and the existence of a critical threshold for the onset of erosion.