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 sizes. Grain size fractions were introduced by hand into 1.37 × 10−4 to 3.96 × 10−4 m2 s−1 flows, and clear selective mobility occurred with slowest transport in the 63–90 μm range. Following those experiments, a sediment mixture was introduced a selectively from a vibrating overhead hopper into 1.39 × 10−4 to 8.16 × 10−4 m2 s−1 flows on roughened surfaces. The highest discharge had sufficient energy to transport all particle sizes but 355–595 μm were preferentially transported. None of the lower discharges were competent to transport the full range of particles injected and 45–125 μm grains were always preferentially transported. Sediment finer than the median bed grain diameter was entrapped in micro bed pockets. The progressively coarsening 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 ability of flow to transport the full range of grains present, as well as to the relationship of the bed roughness to the particles transported.