Flow resistance of emergent rigid and flexible floodplain vegetation

Abstract

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.

Keywords:

• Environmental fluid mechanics
• hydraulic resistance (drag coefficients and friction factors)
• leaf area index
• river restoration
• streams and rivers
• vegetated flows

Acknowledgements

This research was supported by Deutsche Forschungsgemeinschaft (DFG, contract AB 137/3-1), the Academy of Finland, and Maa- ja vesitekniikan tuki ry. The authors are grateful to the members of the DWA-Working Group 3.1 Hydraulische Berechnung von Fließgewässern mit Vegetation and three anonymous reviewers for comments and suggestions.