Boundary Shear in Symmetrical Compound Channels

Abstract

Experimental results are presented concerning the boundary shear stress and boundary shear force distributions in a compound section comprising of one rectangular main channel and two symmetrically disposed flood plains. The flood plains were roughened progressively in six steps so that the influence of differential roughness between the flood plains and the main channel on the lateral momentum transfer process could be studied. For the maximum applied roughness, the widths of the flood plains were then reduced. Equations are presented giving the shear force on the flood plains as a percentage of the total shear force in terms of four dimensionless parameters. Supplementary equations are also presented giving the apparent shear forces on vertical, inclined and horizontal interfaces within the cross section. The influence of momentum transfer between sub areas on the vertical and lateral distribution of longitudinal velocity is also assessed. The discharge and apparent shear force results reveal certain weaknesses in four commonly adopted design methods used to predict the discharge capacity of compound channels.