Abstract: Trapezoidal section is widely used in the design of channels. The regularity of boundary shear stress distribution is an important factor in the research of flow structure and resistance.In this article, the expressions of the relative values of average bed and sidewall shear stress with width-depth ratio of flow section are derived through integration. Comparison between the present calculation method and Knight’s experimental data reveals relatively small error. The variation regularity of average bed and sidewall shear stresses of optimal hydraulic section in trapezoidal open channels with different side angels is analyzed. The result shows that the optimal hydraulic section in trapezoid open channel is narrow and deep, the average bed shear stress changes little while the average sidewall shear stress reduces with the decrease of side angel.

Key words: trapezoidal open channel, average bed shear stress, average sidewall shear stress, width-depth ratio of flow section, optimal hydraulic section