Abstract

So far, few studies have focused on the concept of critical flow velocity rather than bed
shear stress for incipient sediment motion. Moreover, few studies have focused on sediment mixtures
(graded sediment) and shape rather than uniform sediment for incipient motion condition.
Different experiments were conducted at a hydraulic laboratory at the University of Guilan in 2015
to determine hydraulic parameters on the incipient motion condition. The aim of this study is to
conduct a comparison between uniform and graded sediments, and a comparison between round
and angular sediments. Experiments included rounded uniform bed sediments of 5.17, 10.35, 14
and 20.7 mm, angular uniform sediment of 10.35 mm, and graded sediment. Results demonstrated
that angular sediment has a higher critical shear velocity than rounded sediment for incipient motion.
Results also showed that for a given bed sediment, although critical shield stress and relative
roughness increased with the bed slope, the particle Froude number (based on critical velocity)
decreased. In terms of the sediment mixture, the critical shear stress (Vc*) was higher for the graded
sediment than for the three finer uniform sediment sizes. The finer fractions of the mixture have a
higher particle Froude number than their corresponding uniform sediment value, while the coarser
fractions of the mixture showed a lower stability than their corresponding uniform sediment value.
Results demonstrated that the reduction in the particle Froude number was more evident in lower
relative roughness conditions. The current study provides a clearer insight into the interaction
between initial sediment transport and flow characteristic, especially particle Froude number for
incipient motion in natural rivers where stream beds have different gravel size distribution.