Selective entrainment and transport of mixed size and density sands; flume experiments simulating the formation of black-sand placers

Abstract

A process-based model for the selective entrainment of grains from sands of mixed sizes and densities has been tested by measuring threshold stresses for a range of relative grain sizes in a laboratory flume. The model was found to yield good predictions for the selective entrainment of different grain-size fractions. It predicts that a minimum threshold stress exists for intermediate grain sizes of a mixed-size sediment, while higher threshold stresses are required for the movement of both smaller and coarser size fractions. This prediction is in part confirmed by our flume experiments, and it also agrees with results of experiments by other investigators. We also conducted flume tests with settling-equivalent sediment mixtures to examine grain sorting leading to the concentration of heavy minerals. The experiments yielded systematic increases in grain sizes and decreasing heavy-mineral concentrations in the transport direction, sorting patterns which imitate those found in our parallel studies of placers on ocean beaches.