For rivers in flood, the shear stress at the top of the stationary gravel bed can set
up a sheared layer of contact-load (bed-load). As is well known, movement of
solid particles does not initiate until the shear stress exceeds some critical value
( Tc ) The main objective of this research is the laboratory prediction of the
conditions at which the incipient motion occurs in the gravel bed channels
through fifteen grading of gravel bed. The laboratory critical shear stress for
every bed grading is computed and compared with that resulted from formulae
used in calculating the critical shear stress on channel bed. The experimental
analysis is used to obtain the relations describing the boundary of the initiation
of motion of the gravel bed particles. Also the flow velocity adjacent to bed
and the mean flow velocity are measured at the critical conditions (case of
incipient motion). The influence of the value of the downward angle of the bed
on the critical shear stress is also examined.
From the laboratory experiments, it was concluded that the values of the critical
shear stress are similar to the values from the existingformulae, especially
Shields diagram, and Shulits and Hill formula. From the measurements of the
bed velocity at the incipient motion conditions, a relationship between the
dimensionless critical bed velocity (VbdV,,) and the ratio (Re/Fnd) is
predicted. It is also noticed that there is aninverse proportionality between the
downward bed angle and the critical shear stress required to initiate motion of
the gravel bed