Experimental observations on the bank retreat of salt marshes

One of the main mechanisms responsible for the worldwide loss of salt marshes in coastal areas is undoubtedly due to the edge erosion. In order to address this problem, experimental observations in a laboratory flume were conducted to understand the main processes controlling marsh edge retreat, with a focus on the erosion mechanisms associated to effects of wind and tidal waves. A physical model reproducing a salt marsh bank was build inside a long wave current flume where random temporal sequences of surface waves have been generated according to a given frequency distribution spectrum. The physical model was constructed with the original soil of salt marshes from the Venice Lagoon, while the wave climate was reproduced according to field measurements of wind and wave heights. The following measurements have been collected during the experiments: water levels to characterize the wave climate interacting with the bank, vertical pressure distribution due to the impact of the waves on the bank, pore water pressure and water content inside the bank to investigate the seepage processes, bank geometry profile and bottom topography at different times to characterize the evolution of bank retreat, flow velocity measurements in the equilibrium configuration. In addition to this, the bank erosion processes were documented using frontal and lateral video recording.The experiments were conducted in the case of both non-vegetated and vegetated bank. The non-vegetated experiments started from a vertical bank profile. In an initial set of experiments, the bank was subject to the only effect of alternating high and low tides; small failure blocks and tension cracks at the top were observed. In the final set of experiments, wind waves were superimposed to the tide; the bank profile was seen to evolve towards a stable configuration in the shape of gentle slope profile mainly through successive mass failures. These experiments were replicated in the case of a vegetated bank. The combination of tidal and wind waves let the bank evolve from the initial configuration to a more stable one; mass failures were observed during these experiments together with a strengthening effect of the root plants.