Entrainment, displacement and transport of tracer gravels

  • Wilcock P
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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.

Author-supplied keywords

  • bed-load transport
  • displacement length
  • field methods
  • grain
  • gravel-bed rivers
  • sediment entrainment
  • sediment transport
  • tracer techniques

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  • Peter R. Wilcock

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