Selecting Surface Roughness
How to select a surface roughness that correlates with Manning's n. Flows through rivers, pipes and irrigation channels encounter resistance that depends upon the roughness of the bounding walls. The roughness of a surface can vary dramatically. For instance, a steel surface may be considered relatively smooth, while large boulders in a river bottom would be the other extreme. A roughness variable in FLOW-3D can be selected to account for any type of surface.
To calculate the mean flow velocity, V, in a channel, it is customary to use a table of values for various surface types (available in many hydraulics hand books) that assign a roughness in terms of Manning's n and apply it to Manning's formula shown in Equation 1 in which S is the slope of the channel, Rh is the hydraulic radius (fluid cross sectional area divided by the wetted perimeter). Values can rage from 0.009 for a smooth metal to 0.5 for lots of vegetation.
In FLOW-3D, the roughness variable is specified as the average height variation in the surface roughness. Therefore, if the actual average roughness of the surface is known, it can be applied directly. If it is not known, it can be computed from a Manning's value as shown in Equation 2 in which k is the roughness variable used in FLOW-3D. Both k and Rh must be expressed in feet in this expression.