Process-based modelling of wave-induced salt marsh edge erosion.

Michele Bendoni, Hocine Oumeraci, Dano Roelvink

Friday 3 july 2015

8:45 - 9:00h at Antarctica (level 0)

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

Parallel session: 14A. Sediment - Coast

Tidal environments are valuable and rich ecosystems whose dynamics are controlled by the interaction of physical and biological processes. Salt marshes are geomorphic features part of these ecosystems, characterized by steep sub-vertical cohesive banks covered by halophytic vegetation. Such a structure makes their boundaries vulnerable to wave forcing when the marsh surface is not completely flooded, even if the bank surface is generally reinforced by vegetation. As a consequence, horizontal retreat of marsh boundaries has resulted worldwide in a significant loss of salt marsh area, and this erosion is expected to get increasingly crucial due to climate change. In order to obtain an improved understanding of the complex dynamics of this system, a process-based numerical model is developed to describe the behaviour of a vegetated bank composed by cohesive sediment and subject to wave attack modulated by tide. For this purpose, several modifications are implemented in the widely used numerical model XBeach by maintaining the global framework of the overall model. Sand and mud fractions mutually interact influencing soil critical shear stress and sediment transport dynamics. The presence and typology of vegetation may affect both hydrodynamics and soil erodibility. Wave impact is accounted for and a simple formulation to describe the effect of partially reflected waves on sediment dynamics is proposed. The effect of the different processes and quantities on the evolution of the system is analysed. Preliminary results show a strong dependence of this evolution on the relative sand-mud composition. An initial profile, characterized by a step-like shape, tends to evolve towards a more gentle profile with decreasing mud fraction, though sediment behaviour remains cohesive. The effect of vegetation in mitigating erosive processes nonlinearly decreases when the relative mud fraction is reduced. The model provides important insights into the knowledge of salt marsh edge erosion.