Estimating Manning's roughness coefficient for shallow overland flow in non-submerged vegetative filter strips

Results of an experimental study showed that overland flow in non-submerged vegetative filter strips can be described by the Petryk and Bosmajian's modified Manning's formula with a variable roughness coefficient. The roughness coefficient is made up of the shear stress caused by the boundary roughness and the drag force caused by the vegetative elements. The resistance due To vegetation was related to vegetation density and flow depth. For small flow rates, the shear stress at the boundary played the major role. As the pow rare increased, the drag force due to the vegetative elements became dominant. The model can be used to estimate flow velocity in vegetative filter strips (VFS).