Boundary shear stress distribution in meandering channels

Abstract

Precise estimation of boundary shear force distribution is essential to deal with various hydraulic problems such as channel design, channel migration and interaction losses. Bed shear forces are useful for the study of bed load transfer where as wall shear forces presents a general view of channel migration pattern. With the purpose of obtaining shear stress distribution at the walls and on the bed of compound meandering channel, experimental data collected from laboratory under different discharge and relative depths maintaining the geometry, slope and sinuosity of the channel constant, are analyzed and confronted. Preston-tube technique is used to collect velocity heads at various intervals along the wetted perimeter and within the flow that helps to calculate shear stress values using calibration curves proposed by Patel (1965). The distributions of boundary shear stress along the channel wetted perimeter are plotted for both in bank and overbank flow conditions. Based on experimental results, the effect of aspect ratio and sinuosity on wall (inner and outer) and bed shear forces are evaluated in meandering wide channels (B/H> 5) and having a sinuosity of 2.04. Equations are developed to determine the percentage of wall and bed shear forces in smooth trapezoidal channel for in bank flows only. The proposed equations are compared with previous studies and the model is extended to wide channels. A quasi1D model Conveyance Estimation System (CES) were then applied in turn to the same compound meandering channel to validate with the experimental shear velocity which ultimately relates to the boundary shear stress. It has been found that the CES results underestimate the shear velocity. A three dimensional modeling software ANSYS-CFX 13.0 is employed to derive the contours of longitudinal, lateral and resultant bed shear stress, for a 60 degree meandering channel using Large Eddy Scale (LES) model.