Comparison of spatio-temporal evolution of experimental particulate gravity flows at two different initial concentrations, based on velocity, grain size and density data

Flume experiments were conducted to investigate the spatio-temporal structure of subaqueous particulate gravity flows with an initial concentration of 14% by volume. Time series of downstream flow velocity and its calculated degree of turbulence, median grain size and sediment concentration at different positions along the path of nominally identical flows are analysed and combined to constrain the spatio-temporal evolution of a single idealised flow. Comparison of the 14% flow with a flow of 5% initial concentration reveals similarities in the basic spatio-temporal structure of velocity, turbulence, grain size and concentration. Both flow types exhibit a velocity maximum at about 1 / 3 of the flow height above the flume floor. At that level, velocity decreases slowly in the flows' body and more rapidly in their tails. Moreover, turbulence intensity is highest in the head and at the base of the flows, whereas the level of maximum velocity and the tail of the flows typically are weakly turbulent. The zones of high turbulence are associated with shear at the front and base of the gravity flows. The flow of 5% and 14% initial concentration also agree in stratification patterns of median grain size and concentration. Grain populations are relatively well mixed in the head, show normal grading in the main part of the body and normal to inverse grading in the rear of the body and tail. The inverse grading is thought to originate from particles transported from the head upward and backward into the body of the flows, where they subsequently settle. The main difference between the flow of 5% and 14% initial concentration is that the higher-density flows appear to develop from a jet into a turbidity current closer to the inception point than the lower-density flow. This difference is interpreted from dimensionless vertical profiles of the flow parameters: horizontal velocity, concentration and grain size distribution. In the turbidity current phase of both flows, the dimensionless variables collapse well. This indicates that the flows behave in a dynamically similar manner and inspires confidence that the dimensionless variables can be used to predict the dynamic behaviour of particulate gravity flows across the measured concentration range in the flume, which due to dilution/sedimentation effects, was from ∼7 to < 1 vol.% concentration.