Salt marsh retreat induced by wind waves: experiments, field and modeling

Edge erosion of salt marshes due to surface waves and tide forcing is likely the chief mechanism that models marsh boundaries and by which salt marshes in worldwide areas are being lost. To address this problem, an experimental investigation in a laboratory flume and field measurements collected in the lagoon of Venice were conducted to understand the main processes controlling marsh edge retreat with a focus on the erosion mechanisms caused by the impact of wind waves in the case of various tidal levels. A physical model reproducing a salt marsh bank was built inside a long wave current flume where random surface waves have been generated according to a given wave 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. In order to reveal the effect of vegetation on bank stability, two identical banks were built but for the inclusion of halophytic plants. A first set of experiments was conducted reproducing only tidal waves, a second set with wind waves superimposed to the tide. A third set o f experiments were aimed to investigate the dynamic impact and transmission of the waves on and within the bank. The following quantities were collected during the experiments: water content and pore water pressure inside the bank, water levels and velocities at various distances from the bank, dynamic pressures on the bank edge surface and internal pressure fluctuations due to wave impact. Bank geometry profile and bottom topography at different times have also been collected to characterize the erosion rate with time and the evolution of bank retreat. Two types of mass failures were observed during the experiments: slides and toppling failures. The latter were most frequently observed failures, consisting in the toppling of blocks and were often the consequence of the presence of deep tension cracks. In most cases the impact of wind waves caused the overturning of the block. In both the unvegetated and vegetated experiments, mass failures occurred in the first part of the experiment whereas the remaining part was characterized by particle by particle erosion. Effect of vegetation lead to a delay in block failures due the presence of roots, although the total eroded volume differed slightly between the two scenarios. The field measurements were aimed at quantifying the erosion characteristics of marsh soil and the wave climate close to the bank edge during a moderate wind event. Several pressure transducers installed 0.15 m above the bed and adequately spaced were used to collect wave height and wave direction with respect to the edge of the marsh. Then, on the base of experimental and field evidence, a new toppling model is proposed and test against laboratory data: a block of cohesive material at incipient failing condition is attached to the underlying layer and identified by the presence of tension crack; it behaves as a dynamical system subjected to several forces, until the tensile strength of the material is exceed. Test of the model showed its capability at reproducing the failure process and it highlighted which are the most crucial conditions in promoting the failure of a bank edge subjected to wave attack and tide forcing.