The erodibility of sediment mixtures is a key factor in sediment dynamic processes and morphological evolution in coastal environments. However, it remains insufficiently understood. In the current study, the critical shear stress of sediments is analyzed with different mud contents and consolidation degrees from experimental results and previous studies. The results indicate that the critical shear stress increases with clay content, peaking at 30% clay content, and then gradually decreasing. Compared to the solid volume fraction of mud (clay and silt), the solid volume fraction of clay shows a higher relation with the critical shear stress of sand-mud mixtures. The role of the consolidation degree in the erodibility of sediment mixtures was quantified through consolidation experiments, revealing an exponential relation between critical shear stress and consolidation coefficient. An empirical equation for the critical shear stress is proposed to consider the mud content, the solid volume fraction of clay, and the consolidation degree. This equation is applicable to mixed sediment over the full range of mud content and varying consolidation degrees. It has a simple form, is easier to apply, and outperforms other empirical equations (RMSE = 0.62; R² = 0.73).

Tidal flats are critical coastal ecosystems, with their geomorphic characteristics traditionally understood to be primarily influenced by tidal, wave, and storm forces. This study investigates the impact of rainfall on the morphodynamics of upper tidal flats by combining hydrodynamic-sediment data, meteorological rainfall records, and video monitoring at the Chongming Dongtan tidal flat in the Yangtze River Estuary, China. We show that rainfall significantly increases suspended sediment transport and accelerates tidal channel elongation. Notably, rainfall events—though occurring during only 25 % of observed tidal inundation periods—accounted for 62 % of cumulative net sediment transport. This disproportionate efficiency compared to tidal forcing stems from the rainfall-induced hydraulic connectivity between expansive supratidal areas and tidal channels, where concentrated runoff convergence intensifies scour dynamics. These findings challenge the traditional view of tidal flat dynamics, suggesting that rainfall is a more influential driver of morphodynamic change than previously recognized.