Evaluation of the Various Methods of Shear Stress Determination in a Sharp Channel Bend with Developed Topography

Shear stress is one of the important parameters in river flow studies and has a key role on river pattern
and also estimating the amount of erosion and sedimentation of fluvial material within the river.
Therefore, estimating this parameter in different positions of the field flow, especially next to the river
bed and banks, is vital. Despite doing intensive studies by many researchers on the amount of shear
stress and its distribution in straight and curved channels, due to the complex flow regime within the
bends, more studies have to be carried out. In this research, the results obtained from various methods
of shear stress estimation are compared with those obtained by using of mean velocities and their
fluctuations taken by an ADVP instrument. The compared methods include Reynolds shear stress,
Turbulent Kinematic Energy, and Logarithmic Profile, have been previously used and compared in
straight channels, but never have been used and compared in a sharp bend. The results obtained from
the study on a 1930 bend with developed topography showed that unlike the straight channels, the
strong 3D flow changes the Reynolds stress patterns in sharp bend, so that there is no specific order to
get a reliable estimation of bed shear stress in this type of channels. Moreover, because of the effects
of strong secondary current, the longitudinal velocity profiles in sharp bends are flatter than those in
straight channels, and hence there are some uncertainty regarding using logarithmic profile
distribution. The results of this study showed that in general, the Logarithmic method to estimate the
bed shear stress gives better results among the available mentioned methods.