Selective bedload transport during the degradation of a well sorted gradedsediment bed

The paper presents an analysis of the composition of bedload transport and changes to bed structure and topography during three graded sediment degrad ation experiments. The analysis suggests that variations in channel hydraul ics and active layer composition alone may not explain the observed reducti ons in sediment transport. Further, the experiments appear to cover a cruci al range of mean bed shear stresses for armouring studies, ranging between a condition of passive winnowing, to one of more active armour development in which the coarse grains play a role in determining bed structure. This i ndicates that the active layer concept, commonly applied in computer models of graded sediment transport, may be Limited in its application.