Although much research has been done on Manning's roughness coefficient, n, for stream
channels, very little has been done concerning the roughness values for densely vegetated
flood plains. The n value is determined from the values of the factors that affect the roughness
of channels and flood plains. In densely vegetated flood plains, the major roughness is caused
by trees, vines, and brush. The n value for this type of flood plain can be determined by
measuring the vegetation density of the flood plain.
Photographs of flood-plain segments where n values have been verified can be used as a
comparison standard to aid in assigning n values to similar flood plains.