Great differences in the critical erosion threshold between surface and subsurface sediments: A field investigation of an intertidal mudflat, Jiangsu, China

Understanding of bottom sediment erodibility is necessary for the sustainable management and protection of coastlines, and is of great importance for numerical models of sediment dynamics and transport. To investigate the dependence of sediment erodibility on degree of consolidation, we measured turbidity, waves, tidal currents, intratidal bed-level changes, and sediment properties on an exposed macrotidal mudflat during a series of tidal cycles. We estimated the water content of surface sediments (in the uppermost 2 cm of sediment) and sub-surface sediments (at 2 cm below the sediment surface). Bed shear stress values due to currents (τ_c), waves (τ_w), and combined current–wave action (τ_cw) were calculated using a hydrodynamic model. In this study, we estimate the critical shear stress for erosion using two approaches and both of them give similar results. We found that the critical shear stress for erosion (τ_ce) was 0.17–0.18 N/m² in the uppermost 0–2 cm of sediment and 0.29 N/m² in sub-surface sediment layers (depth, 2 cm), as determined by time series of τ_cw values and intratidal bed-level changes, and values of τ_ce, obtained using the water content of bottom sediments, were 0.16 N/m² in the uppermost 2 cm and 0.28 N/m² in the sub-surface (depth, 2 cm) sediment. These results indicate that the value of τ_ce for sub-surface sediments (depth, 2 cm) is much greater than that for the uppermost sediments (depth, 0–2 cm), and that the τ_ce value is mainly related to the water content, which is determined by the extent of consolidation. Our results have implications for improving the predictive accuracy of models of sediment transport and morphological evolution, by introducing variable τ_ce values for corresponding sediment layers, and can also provide a mechanistic understanding of bottom sediment erodibility at different sediment depths on intertidal mudflats, as related to differences in the consolidation time.