  This paper studies the effects of velocity gradients and secondary currents
on the distribution of the shear force between the walls and bed of rectangular
open channels. We show that neglecting the effect of secondary currents and
assuming zero-shear division lines does not yield acceptable results. We,
accordingly, introduce a method to determine the percentage of the total shear
force acting on the walls and bed of rectangular open channels, which takes
both the velocity gradients and secondary currents into account. Using the
channel bisectors, along which there is no secondary flows effect, and
orthogonal trajectories to isovels, along which there is no shear stress, we
divide the channel cross section into three major subsections, namely bed area,
walls area, and shared area. The geometry of each subsection is derived given
the location of the maximum velocity. The share of the bed and wall shear
forces from the shared area, are calculated afterward. The results for bed and
walls shear forces agree well with the experimental data with an average
relative error less than 5% for both regular flows and flows carrying suspended
sediment. This method also provides a physics-driven range for the wall and bed
shear forces which nicely covers the experimental data.
