Entrainment,displacement and transport of tracer gravels

The mass and size distribution of grain entrainment per unit bed area may be measured by replacing a volume of the bed with tracer gravels and observing the mass difference before and after a transport event. This measure of spatial entrainment is relevant to any process involving size-selective exchange of sediment between transport and bed and may be directly used in calculations of sediment transport rate using an elementary relation for fractional transport components presented here. This relation provides a basis for evaluating tracer data collected by different methods and may be used to provide physical insight regarding the expected behaviour of tracer grains. The variation with grain size of total displacement length L_ti depends on the degree of mobilization of the individual fractions on the bed surface: L_ti is independent of D_i for smaller, fully mobile sizes and decreases rapidly with D_i for larger fractions in a state of partial transport (in which a portion of the surface grains remain immobile through the flow event). The boundary between fully and partially mobile grain sizes increases with flow strength. These inferences are supported by values of L_ti calculated from flume experiments and provide a physical explanation for a summary relation between L_ti and D_i based on field data. © 1997 John Wiley & Sons, Ltd.