Intratidal erosion and deposition rates inferred from field observations of hydrodynamic and sedimentary processes: A case study of a mudflat-saltmarsh transition at the Yangtze delta front

An understanding of erosional and depositional processes in intertidal wetland environments is of great importance to coastal geomorphologists, ecologists, and engineers. To evaluate the morphodynamic response of intertidal mudflats and saltmarshes to tidal inundation, we measured water depth, wave activity, current velocity profiles, suspended sediment concentration (SSC), and sediment composition at a dynamic mudflat-saltmarsh transition on eastern Chongming Island, at the Yangtze delta front, China. Based on these data, we calculated bed shear stresses generated by the combined current-wave action (τ_cw), and the critical shear stress required to erode the surface sediments (τ_ce), and so were able to calculate the erosional (E) and depositional (D) fluxes. The erodibility parameter (M_E) and settling velocity (w_s) used in the calculations of E and D were calibrated using daily measurements of bed-level change. Our results showed that the mudflat experienced alternating phases of net erosion and net deposition during tidal inundation. The burst-based changes in bed level ranged from -0.92 (net erosion) to +0.43mm/10min, and the cumulative bed-level changes over an entire tidal cycle ranged from approximately 0 to -5.4mm (net erosion), with an average change of -3.4mm/tide (net erosion) over five consecutive spring tides. In contrast, only net deposition was recorded on the saltmarsh during our observations. The burst-based changes in bed level ranged from around 0 to +0.56mm/10min, and the cumulative changes over a tidal cycle ranged from around 0 to +5mm, with an average of +2.6mm/tide for the five consecutive spring tides. We conclude that net erosion and net deposition during tidal cycles alternate on the mudflat, but that only net deposition occurs on the saltmarsh.