Critical Bed-Shear Stress of Mud–Sand Mixtures

The critical bed-shear stress (cbs) for erosion of mud–sand bed mixtures was studied in laboratory and field conditions with currents, waves, and combined currents and waves. Three types of erosion were distinguished: particle/floc erosion, surface erosion, and mass erosion. Important influencing parameters were found to be sediment composition (percentage clay, silt, and sand), presence of organic and carbonate materials, type of bed (homogeneous, remolded, layered by depositional processes), and the dry bulk density (consolidation stage). The laboratory and field results reveal that the critical bed-shear stress is not much influenced by cohesive effects if the percentage of fines ($<63\mu \mathrm{m}$) is smaller than approximately 15%. For $p_{\text{fines}}>15\%$ up to approximately 50%, the critical bed-shear stress increases for increasing values of $p_{\text{fines}}$ but decreases again for $p_{\text{fines}}>50\%$. The fine fraction includes the clay fraction ($p_{\text{clay}}\cong 0.2$ to 0.4 $p_{\text{fines}}$), which contributes primarily to the cohesive effects. In addition, the bulk density depending on the soil composition (clay, silt, sand content, and consolidation stage) has a strong effect on the cbs for surface erosion. Small bed irregularities (local disturbances) also play an important role by creating local accelerations and decelerations with enhanced turbulence and initiating particle movements. A new empirical relation is suggested to predict the critical bed-shear stress for erosion based on the dry bulk density of the sediment samples. This relation is successfully applied in a morphodynamic model of a schematized tidal basin to represent spatial variations in the erodibility of fines between sandy channels and (relatively) muddy shoals.