Accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts. This study used high-frequency in situ measurements of water depths and near-bed velocities to estimate bed shear stress on an open intertidal flat in the Yangtze Delta, China. To determine the current-induced bed shear stress (tc) the in situ near-bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches: a moving average (MA) and energy spectrum analysis (ESA). tc was then calculated and evaluated using the log-profile (LP), turbulent kinetic energy (TKE), modified TKE (TKEw), Reynolds stress (RS), and inertial dissipation (ID) methods. Wave-induced bed shear stress (tw) was estimated using classic linear wave theory. The total bed shear stress (tcw) was determined based on the GranteMadsen waveecurrent interaction model (WCI). The results demonstrate that when the ratio of significant wave height to water depth (Hs/h) is greater than 0.25, tcw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds. In addition, wind enhances the total bed shear stress as a result of the increases in both tw and tc generated by the greater wave height and reinforcing of vertical turbulence, respectively. From a comparison of these various methods, the TKEw method associated with ESA decomposition was found to be the best approach because: (1) this method generates the highest mean index of agreement; (2) it uses vertical velocities that are less affected by Doppler noise; and (3) it is less sensitive to the near-bed stratification structure and uncertainty in bed location and roughness. @en

Short-term bed-level variability in tidal wetlands has important implication both for ecology and engineering. In this study, we combined in situ measurements with model simulations to quantify short-term bed-level changes on a meso-macrotidal wetland in the Yangtze River Delta. On the middle flat, we observed erosion during neap-to-mean tides under onshore moderate-to-strong winds, and bed recovery during subsequent spring tides, when winds were both offshore and weaker, suggesting that winds can overturn the neap–spring cyclicity of bed-level changes even on meso–macrotidal mudflats. The magnitude of bed-level changes was smaller on both sides of the middle flat, while the smallest changes occurred on the salt marsh. Observed bed-level changes were reconstructed using a single-point bed-level change model, which incorporates in situ measured parameters of hydrodynamics (waves and currents), suspended sediment concentrations, and bed sediment properties. We conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally (due to changes in balance between wave and tidal energies) and spatially (because of changes in elevation and vegetation in the cross-shore profile). Our study also reflects the advantage of combination of in situ measurement with simulation in detecting short-term variability of tidal flats.@en