In order to understand local alluvial flow sorting processes of grains of different densities, studies were made of 4 bed configurations in a large recirculating flume. These bed configurations included flat beds,

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transition flow beds, and 2 kinds of dune beds. Bed material used in the study was a moderately well sorted sand (median = 0.286 mm) from the Rio Grande near Bernardo, New Mexico. Opaque heavy minerals consisting mainly of ilmenite and magnetite (median = 0.144 mm) made up 0.38% of the bed material by volume.

Concentrations of opaque heavy minerals were formed as 3 basic types: (1) small thin concentrations associated with dunes that lacked topset beds and on the stoss sides of large dunes with topset beds; (2) concentrations associated with the topset deposits of large dunes and with dunes formed in the transition flow; and (3) widespread concentrations associated with the flat-bed condition.

The most important factors influencing the type and degree of sorting of the opaque heavy minerals from light minerals were bed configuration and grain density. The thickest deposits of heavy minerals were associated with the topset deposits of large dunes, but the most widespread deposits were associated with the flatbed type of bed form. Density is important to segregation because the differences in shear stress necessary to move the light minerals and to move opaque heavy minerals are large. Equivalent fall velocities of grains of two different densities were determined to have little importance in local segregating mechanisms.

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