PARTICLE-SIZE DISTRIBUTION OF SEDIMENT TRANSPORTED BY SHALLOW FLOW

A small laboratory flume on a 0.035 m m(-1) slope was used to examine particle selection in shallow flows. initial experiments were carried out on smooth surfaces to measure travel rates of different sediment s izes. Grain size fractions were introduced by hand into 1.37 X 10(-4) to 3.96 X 10(-4) m(2) s(-1) flows, and clear selective mobility occurr ed with slowest transport in the 63-90 mu m range. Following those exp eriments, a sediment mixture was introduced aselectively from a vibrat ing overhead hopper into 1.39 X 10(-4) to 8.16 X 10(-4) m(2) s(-1) flo ws on roughened surfaces. The highest discharge had sufficient energy to transport all particle sizes but 355-595 mu m were preferentially t ransported. None of the lower discharges were competent to transport t he full range of particles injected and 45-125 mu m grains were always preferentially transported. Sediment finer than the median bed grain diameter was entrapped in micro bed pockets. The progressively coarsen ing of transported material in time, observed for all flow conditions, may be related to progressive filling of micro bed pockets by fines. Both transport and entrapment selectivity is strongly related to the a bility of flow to transport the full range of grains present, as well as to the relationship of the bed roughness to the particles transport ed.