Estimating current-related bed shear stresses during flooding phase on intertidal flats from measured water depths

Abstract

Current-related bed shear stress is vital for the prediction of the morphological evolution of intertidal flats. The present study develops a simple method to estimate the current-related bed shear stress during the flood phase on intertidal flats, which is based on the easy-to-measure water depth. To verify this simple method, comparisons were made to the current-related shear stress derived from both the LP and the TKE methods based on the measured velocities. The results show that the widely used approach on intertidal flats, which relates the current-related bed shear stress to the square of the depth-averaged velocity with a constant factor, underestimates the bed shear stress for water depths smaller than ca. 0.5 m when high sediment concentrations and significant bed-level changes however usually occur. For water depths smaller than approximately 0.5 m, the uniform horizontal velocity assumption in the vertical direction in the shallow water theory is inappropriate, and velocity shear must be considered. The current-related bed shear stresses could be estimated using the measured water depths during flood phase, even when the water depth is smaller than 0.5 m. The developed method also shows that the scale of current-related shear stress is u‾/h, not u‾2, at least with a water depth smaller than 0.5 m.