| dc.description.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. | |
