Modern approach to design of grassed channels

The resistance to flow in a grassed channel is linked directly to the relative roughness of the flow through long‐accepted boundary‐layer principles. The difference between flow over grass linings and linings composed of rigid materials such as riprap or concrete lies in the tendency of the grass to deform under an imposed fluid shear. This deformation results in a smoothing of the channel roughness with increasing flow. Mathematically, this can be accommodated with coefficients that define the Darcy‐Weisbach friction factor and are dependent on relative values of boundary shear and vegetative stiffness. The emphasis of the paper is on showing that the relative roughness resistance equation can successfully reproduce the n‐VR retardance curves. Aspects of channel stability are discussed and the paper describes some problems associated with stability design.