Boundary Shear in Smooth and Rough Channels

A series of flume experiments are reported in which the walls and bed were differentially roughened, normal depth flow set, and measurements made of the boundary shear stress distribution. An empirically derived equation is presented giving the percentage of the shear force carried by the walls as a function of the breadth/depth ratio and the ratio between the Nikuradse equivalent roughness sizes for the bed and the walls. The results are compared with other available data for the smooth channel case and some disagreements noted. The systematic reduction in the shear force carried by the walls with increasing breadth/depth ratio and bed roughness is illustrated. Further equations are presented giving the mean wall and bed shear stress variation with apsect ratio and roughness parameters. Although the experimental data is somewhat limited, the equations are novel and thought to indicate the general behavior of such open channel flows with some success.