Clay influence on the threshold of movement and physical parameters of sand-mud deposits
The erosion of mixed sediment deposits is described, under the action of unidirectional steady currents and (simulated) waves, separately, and in combination. The experiments were undertaken using a rectangular recirculating flume, incorporating an oscillating tray. The mixtures consisted of angular fine-grained quartz sands (D50=152.5 m and 215 m) combined with a very cohesive estuarine mud. Time-averaged erosion threshold current speeds, during the unidirectional and combined flow experiments, were measured. In addition, pore (water) pressure measurements, during the oscillatory and combined flow experiments, have been monitored. The results obtained under the action of currents show that there is an incremental increase, with clay content, in critical erosion shear stress. This increase is small for clay percentages lower than 11% (dry weight); it is larger for clay contents in excess of 11-14%. The quantity and cohesive nature of the clay fraction are suggested as the mechanisms to explain the bi-modal pattern of sediment erodibility. When the mixtures were subjected to different pre-threshold current speeds, together with various time-periods of flow, the critical erosion shear stress was higher than the original. In this process, current velocity is more important than flow duration. Data obtained under the influence of simulated wave action show that, for clay contents 11%, sediment erodibility is unaffected by the increasing clay concentrations. However, with clay contents in excess of 11-14%, a positive linear function may describe the variation in erosion threshold with clay content. Results obtained under the co-linear combined action of waves and currents demonstrate a significant and positive linear relation between erosion threshold and the cohesive additive. Furthermore, waves protect the sediment/water interface from the eroding competence of the steady currents. The resistance to erosion increases with a decrease in wave period (from 10 s to 6 s).