Laboratory Determination of Average Shear Stress in Smooth Rectangular and Trapezoidal Open Channels by Direct Method

Aim of this study is to measure shear stress in smooth rectangular and trapezoidal open channels. A new approach is presented to measure direct shear on wet perimeter of open channels. A water flume with a section on a frictionless support (called knife edge flume) along with a system of load cell was used to measure total force on the wet perimeter. The local boundary shear stress in rectangular and trapezoidal cross sections was measured using a 4mm diameter Preston tube connected to a differential pressure transducer and a data acquisition system. The calibration curves proposed by Patel were used to convert the pressure readings to shear stress. Local and total shear measurements were used to differentiate the channel wall and bed shear forces. This method allows minimizing of the error as result of the Preston tube diameter. Comparison of the results confirmed advantage of the momentum method over the energy method in term of accuracy. Results are presented for smooth rectangular and trapezoidal channel cross sections with side slopes of 1, 1.5, and 2.