Hydraulic Sorting of Coastal Heavy Mineral Sands

Hydraulic sorting mechanisms responsible for the formation of coastal placer deposits enriched with heavy mineral sands are considered in this dissertation. The likely mechanisms concentrating heavy minerals, as currently formulated in the literature, are evaluated on their own merit as independent sorting mechanisms and in terms of their applicability in the coastal environment. While hydraulic equivalence is a valid conceptual framework to ascertain the likelihood of a particular sorting mechanism, the traditional formulation of equivalence relationships do not often capture the true nature of sorting conditions on the beach. New insight is provided through laboratory experimentation and results from the sampling of beach placer deposits. The sedimentation rate of sand grains in the hindered settling regime has been investigated to assess particle shape effects. The behaviour of various particulate systems involving irregularly shaped sand has been compared with the widely adopted Richardson-Zaki expression using existing published data and new data btained from experiments performed in a fluidisation column. The general form of the expression remains as a convenient means to describe the hindered settling of irregular particles (in the absence of shear). The sedimentation exponent in the Richardson-Zaki expression is however found to be significantly larger for these particles corresponding to an increased hindered settling effect. A simple method to evaluate the sedimentation exponent, and hence the hindered settling effect for sands of various shape is presented with the discussion furthered to consider the sedimentation of multi-component mixtures. The influence of selective entrainment on heavy mineral content in suspension has been studied through the measurement of sediment concentrations above a rippled bed under regular non-breaking waves in the laboratory. The normalized distributions of time-averaged suspension concentrations were found to be similar for the light and heavy minerals, despite their different clear water settling velocities. This is in contrast to the nature of concentration profiles expected under the traditional gradient diffusion process. The observed concentration profiles imply a convective rather than diffusive sediment distribution mechanism. Between the immobile bed and the lowest measurement level, the light and heavy mineral concentration differs significantly. This discrimination at the ripple face is due largely to selective entrainment. The concept of dispersive pressure to sort light and heavy mineral grains and the development of inverse grading for a granular layer under shear is applied to chute flow experiments. In contrast to the traditional view that shear equivalent combinations of grain sizes and densities provides no opportunity for sorting, strong particle segregation is observed. For a different scenario, where interlayer particle percolation is restricted, denser grains do not migrate towards the free surface, again in contrast to the response expected under the current shear sorting formulation. Both sorting scenarios are openly suggestive of an incorrect interpretation of the constitutive relations founding the shear sorting hypothesis. Kinetic sieving exists as a more appropriate mechanism