Changes with time of the transport rate of sediment mixtures

A novel sediment trap, designed to enable bed-load activity to be related to elapsed time, is described. An account is then given of its use in a short pilot experiment on the armouring of a mixed grain size bed in a small laboratory flume. Despite the limitations of the experiment, significant information was obtained about the variation with time of bed load transport rate and of the composition of the bed load in the early stages of armouring. The changes of composition are presented in terms of log hyperbolic size distributions. It is contended that, for the purpose of validating numerical models of the armouring process, this information about bed load composition has higher value than data on changes in the static bed population, because the latter involve severe sampling problems. Thus the trap provides new opportunities in the development of knowledge about bed winnowing and armouring.