Analysis of Shear Stress in Rectangular Open Channels Using Force Balance Method

One of the most important problems in open channel flow is determination of shear stress. It is difficult to determine boundary shear distributions in real life applications. Laboratory flume studies have still been serving to deal with this problem. This study is focused on experimental measurement of the bed and wall shear stresses as well as estimation of hydraulic resistance coefficient for flow in rectangular open channels. The energy and momentum approaches were used to determine contribution of wall and bed shear forces on total boundary shear. The hydraulic grade line was measured using an indirect method while a frictionless system (knife edge flume) was used for the momentum measurements. The local boundary shear stress was measured using a 4-mm diameter Preston tube connected to a differential pressure transducer and a data acquisition system. The calibration curves proposed by Patel were used to convert pressure reading to shear stress. The results were adjusted by the total shear force calculated from the energy and momentum methods. A nonlinear regression-based technique was carried out to develop equations and to determine the percentage of wall and bed shear stresses on the wetted perimeter of a smooth rectangular open channel. Comparison of results for shear stress measurement by direct and indirect methods shows maximum errors of 4 and 8 percent respectively.