The laboratory studies have been carried out in this research. After determining the sensitivity analysis of the Preston tube diameter in shear stress, four Preston tubes with external diameters of 3.2, 3.9, 4.7, and 6.3 mm were used. The aspect ratios of 2.86 to 13.95 were examined. For measuring the pressure di erence of the Preston tube, a 200 millibar di erential pressure transducer with 0.01 accuracy of the original scale was used. Laboratory results demonstrated that Preston tubes with a diameter of 3.9 mm presented the minimum di erence in the average value of the shear stress resulting from the Bechert and Patel calibration equations. Therefore, by using the Preston tube with an optimal diameter, transverse distribution of shear stress in channel beds and walls was determined. The outcome of this part of the study is two dimensionless relationships for determining the local shear stress both on the bed and wall. These relationships are a function of aspect ratio B=H and the bed relative coordinates b=B in cross-section and Z=H sidewall. The survey showed that the dimensionless bed shear stress distribution was considerably in uenced by the aspect ratio. The transverse distribution of wall shear stress was independent of the aspect ratio for B=H > 3.

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