Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

Abstract

AaSTRACr: Heavy-mineral transport and deposition by shallow unidirectional flows in a gravel-bed channel were studied in a water-recirculating sediment-feed flume 6 m long and 0.15 m wide. The sediment was poorly sorted gravel with mean size 3 mm and 3% by weight of finer magnetite, lead, and tungsten.The heavy minerals became concentrated into a thin layer (the heavy infralayer) composed of nearly 100% heavy minerals lying beneath a thin surficial layer of low-density sediment (the light supralayer). Heavy minerals were not transported past a given location on the bed until the heavy infralayer was fully developed there. Heavy minerals were transported on the upper surface of the heavy infralayer only when local and temporary erosion of the light supralayer, caused by large clasts, clast jams, or downstream-moving dunelike bedforms, exposed the heavy infralayer.In two runs, one with sediment feed and one without, overall degradation of the bed was forced by slow lowering of a tailgate at the downstream end of the channel. In the sediment-feed run a downstream-prograding heavy infralayer formed as in the nondegrading runs. In the run without feed, the surficial heavy-mineral concentration increased gradually at all points along the channel to the end of the run. In light of these results, we hypothesize that ultimately the surficial heavy-mineral concentration would reach a steady state and the bed slope would change until the flow can transport all of the heavy-mineral sediment supplied by local exhumation.Although no runs were designed for aggradation, a too-small initial slope caused overall bed aggradation in the early parts of two runs. Heavy minerals in the feed sediment were transported only a short distance downstream to form a zone of higher but downstream-decreasing heavy mineral concentration extending vertically through the deposit. We hypothesize that the downstream length of this zone would increase as the heavy-mineral feed rate increases relative to the bed aggradation rate.