On Bidisperse Grain Size Distributions in High Density Turbidity Currents

Abstract

Physical experiments on high density turbidity current (HDTC) have been performed in a laboratory flume to investigate the effect of bidisperse grain size distributions on flow properties. In total three parameters are varied: slope (4-11.3 [deg]), sediment volume concentration (9-21v%) and grainsize fraction, with the latter being the ratio between two distinct sized clastic sediments (D50 = 144 & 50 μm). With high resolution U(D)VP (Ultasonic Doppler Velocimity Profiler) measurements, velocity profiles are constructed for each individual experiment, which are then compared. The obtained data shows that the addition of a small portion of fine grained materials extensively enhances the carrying capacity and efficiency of turbidity currents. The influence of sediment fraction on the stability field (experimental conditions in which an equilibrium is reached) is substantial, but not as much on average velocities. The effect of sediment fraction on maximum (Umax) and average velocities (Umean) is substantially smaller than what was found with varying gradient or initial concentrations. The outer region velocity gradient decreases with higher proportions of fine materials. The more vertically uniform velocity structures must be attributed to more homogeneous suspended sediment distributions due to the addition of fines. This is explained by the fact that the fine fraction is more easily affected by the turbulence kinetic energy and therefore to entrainment with the ambient fluid. As an addition shear stresses are approximated with the law of wall. This method can however not be applied to the aggrading runs, since the inner region profile deviates from a logarithmic function and shows more similarity with proposed ‘traction carpet’ model structures.