Determination of the drag coefficients of emergent and just submerged willows

Abstract

This paper presents results regarding the flow resistance and the drag coefficient of floodplain vegetation. According to the literature review, most of the investigations concerning the interaction between flow and woody vegetation are based on laboratory experiments where the vegetation is idealized with a high level of abstraction. However, the vegetation's resistance behaviour depends on many natural parameters, such as the flexural rigidity, the manner of contraction in the current, the degree of foliation and the vibration behaviour. Furthermore the varying shape of the vegetation elements due to the current forces has to be observed. The drag coefficients of three kinds of willow species depending on the flow conditions and the effect of the plants flexibility and foliation on the projected plant area and thus on the drag coefficient are presented in this paper. The results show that it is very important to distinguish between leafy and leafless condition as the drag coefficient is a result of different physical effects.

Keywords:

• Flow resistance
• drag force
• woody vegetation
• hydraulic modelling

Acknowledgements

The experimental research was funded by the Ministry of Science, Research and the Arts Baden-Württemberg and the European Social Fund (ESF) within the ‘Schlieben–Lange programme’ to whom the writers express their gratitude.

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