This paper summarizes current practices for the estimation of flow resistance caused by floodplain vegetation in emergent flow conditions. The current state-of-the-art for the parameterization of vegetative form drag and associated flow resistance was explored with a view on practical applicability. Specifically, the dissimilar resistance behaviour of simply shaped rigid elements and foliated natural vegetation was emphasized by compiling and reanalysing data published by the authors’ research teams as well as others. It was shown that describing the key hydraulic properties of plants, geometry, and flexibility, with species-specific parameters is superior to the rigid cylinder analogy commonly used in hydraulic engineering practice. The discussion on the limitations of many existing approaches for the determination of vegetative flow resistance is intended to advance the use of modern practices such as the parameterization of vegetation density with the leaf area index, a parameter that can be derived using remote sensing techniques.

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.

[1] Both the foliage and stem essentially influence the flow resistance of woody plants, but their different biomechanical properties complicate the parameterization of foliated vegetation for modeling. This paper investigates whether modeling of flow resistance caused by natural woody vegetation can be improved using explicit description of both the foliage and stem. For this purpose, we directly measured the drag forces of Alnus glutinosa, Betula pendula, Salix viminalis, and Salix x rubens twigs in a laboratory flume at four foliation levels, parameterized with the leaf-area-to-stem-area ratio A_L/A_S. The species differed in the foliage drag but had approximately equal stem drag. For the foliated twigs, increasing A_L/A_S was found to increase the reconfiguration and the share of the foliage drag to the total drag. The experiments provided new insight into the factors governing the flow resistance of natural woody vegetation and allowed us to develop a model for estimating the vegetative friction factor using the linear superposition of the foliage and stem drag. The model is novel in that the foliage and stem are separately described with physically based parameters: drag coefficients, reconfiguration parameters, and leaf area and frontal-projected stem area per ground area. The model could satisfactorily predict the flow resistance of twig to sapling-sized specimens of the investigated species at velocities of 0.05–1 m/s. As a further benefit, the model allows exploring the variability in drag and reconfiguration associated with differing abundance of the foliage in relation to the stem.