Erodibility of Mud–Sand Bed Mixtures

The erodibility of mud–sand bed mixtures has been studied based on experiments in a flume and in a small, vertical tube placed on the bed (Eromes-tube of 100 mm diameter with rotating propeller). Two natural mud–sand mixtures (percentage of $\text{sand}\cong 35\%$ and 20%) were used and artificial mixtures were made by adding fine sand to the natural mud samples; the percentage of sand was varied in the range of 30%–95%. Two types of fine sand have been used: very fine sand of about 100 μm and fine sand of 180 μm. The dry density was varied in the range of $300–\mathrm{1,500}\mathrm{kg}/{\mathrm{m}}^3$ (soft to firm beds). Three types of erosion were identified (visually) during the flume experiments: particle/floc erosion, surface erosion by generation of local craters and grooves, and mass erosion (two types) by local bed collapse or by local breakout of lumps of bed material with clouds of escaping mud. Plots of the critical bed-shear stress for particle, surface, and mass erosion are given based on the data of the flume and Eromes tests. The critical bed shear for particle, surface, and mass erosion of natural mud–sand beds is found to be primarily influenced by the percentage of fines ($<63\mathrm{\mu m}$), percentage of clay-silt ($<8\mathrm{\mu m}$), and the dry bulk density of the top layer of the bed.