Determination of flow resistance caused by non-submerged woody vegetation

This paper investigates the determination of flow resistance caused by stiff and flexible woody vegetation. A new procedure has been developed which allows the determination of friction factor f or Manning’s n using measurable characteristics of vegetation and flow. The procedure is capable of predicting flow resistance due to: (1) leafless bushes or trees and (2) leafy bushes or trees. The application of the procedure is limited to non-submerged flow (h ≤ H ) and relatively low velocity (U 1 m/s), which are typical conditions in low-gradient stream valleys, floodplains and wetlands. The procedure is novel in that it uses sound hydraulic principles and methods that are available but incorporates some adjustments based on the knowledge on mechanical design of trees and deformation of foliage in a flow. The procedure is able to account for the natural branched structure in determining area or volume of a woody plant. This makes the prediction of resistance caused by plants more accurate than if they were treated as arbitrary cylinders. The accuracy of the approach to estimate f and U was somewhat better for the leafless condition (mean error of f was −5% to +4%) compared to the leafy condition (mean error of f was −9% to −3%). The presented procedure is intended as a practical tool for estimating the relationship between plant characteristics and flow resistance for flows over floodplains and wetlands growing woody vegetation.