Sediment dynamic responses of coastal salt marsh to wind waves and swells in a semi-open tidal flat

Coastal salt marshes provide effective protection to the coastal environments they front against coastal erosion by reducing the incoming wave energy. Understanding sediment dynamic processes in coastal salt marshes environments is of crucial importance for coastal defense. The objective of this study is to assess the impact of Spartina alterniflora (S. alterniflora) marshes on wave attenuation, sediment transport, and morphodynamics through extensive field records on the Cixi tidal flat in Hangzhou Bay. Results demonstrate that wave attenuation by S. alterniflora marshes increases proportionally with the intensification of wind waves at a consistent water depth or significant wave height. Moreover, wave attenuation in the context of wind waves surpasses that of swells. On average, the wave attenuation provided by S. alterniflora marshes during both wind waves and swells is more than six times greater than that offered by the adjacent mudflat. Additionally, net sediment fluxes within S. alterniflora marshes decrease by 37 % in the presence of swells and 84 % with wind waves, in comparison to the mudflat. The influence of S. alterniflora marshes on tidal flat accretion is more pronounced with wind waves than swells. Notably, observed from summer to winter, the surface accretion of tidal flats is highest (∼26 cm) at the edge of S. alterniflora marshes. This study contributes valuable insights into the complex interactions between salt marshes and hydrodynamic forces, essential for informing coastal management strategies.