average velocity of each section is the same, and the average shear stress can be calculated using where is the river bed shear stress, is the water density, g is the gravity acceleration, R is the hydraulic radius, and S is the slope of energy line. This study investigated the boundary shear stress distributions by energy and momentum method in semi-parabolic channels. In order to determine the variation of the local shear stress, a Preston tube with the inner diameter of 4 mm equipped with

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... amic pressure-sensitive cells were used. The Patel calibration curve was used to convert the pressure reading to boundary shear stresses. Finally, based on the dimensional analysis, dimensionless parameters were identified, and the relationship was presented for the prediction of the shear stress in semi-parabolic channels with both smooth and rough walls. The experiments were carried out at the hydraulic-model laboratory of the Faculty of Water Science and Engineering of Shahid Chamran University of Ahvaz, Iran in a rectangular channel with 8.3m length and 1 m height. The general slope of the channel was considered as 0.002. A knife-edge flume (frictionless flume which is located on knife-shaped columns) was used in the experimental section. Data have been taken using a Preston tube (the energy method), and Preston equations were used for converting the pressure into stress. In order to avoid waves in the experiments, a mesh screen was used at the beginning of the channel, and a gate was located at the end of the channel. The semiparabolic section was made using a 2 mm metal plate. In order to minimize the roughness and simulate the smooth wall, several turns were stained and placed in a rectangular flume. With 20% of laboratory data, the verification of the proposed equations was studied. Results and Discussion The results show consistent relationships with the experimental data presented. The results indicate that the shear stress of the wall channel in smooth and rough conditions is, on average, 3.7% and 4.6% higher than the bed shear stress. The measurement of shear stresses of the wall and the bed by the momentum method in conditions of a smooth and rough wall showed a mean of shear stress values of 2.7% and 5.85%, respectively, in comparison to the energy method.