The influence of clay on the threshold of movement of fine sandy beds

The erosion threshold (i.e. surface shear strength) of mixed sediment deposits has been tested under the action of unidirectional currents (mean critical speeds, 4 mm above the bed, ranging from 12.4 cm s −1 to 18.2 cm s −1) or simulated waves (mean critical periods ranging from 2.0 s to 6.7 s, for near-bed wave amplitudes of 0.28–0.57 m) using an oscillating tray in a rectangular recirculating flume. The sediment mixtures investigated consisted of angular fine-grained quartz sands ( D 50 = 152.5 and 215 μm) and cohesive estuarine mud. The investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits. The rate of increase is smaller for mud contents < 30% (clay mineral content < 11–14%, by dry weight) and is attributed to an increase of the internal angle of friction. When the clay mineral content exceeds 11–14% by dry weight, then the sand particles are not in contact with each other. Consequently, the clay component controls the erodibility of the sediment mixture. Finally, a prolonged and intense stress history can increase considerably (by a factor of 1.3–1.7) the original erosion threshold value of these sediments.