Virtual velocity of tracers in a gravel-bed river using size-based competence duration

Virtual velocity (Vi) of river gravels is commonly used to determine sediment transport rates and gravel dispersion dynamics. Virtual velocity is calculated from tracer gravel step-length data as the distance travelled divided by the duration of competence. However, no allowance is usually made for differences in competence duration according to grain size. In this investigation, Vi of gravel tracer clasts for the River Rede, Northumberland, UK, is calculated using a method that takes into consideration an approximation of size-based competence duration. Although scaled transport distance data compared favourably to past studies, it was the comparatively smaller clasts that tended to have lower Vi in comparison to coarser clasts, because of their longer competence duration, contrasting with previously published research. The key findings of the study serve as proof of concept and should be adopted in future studies that focus upon Vi estimation.