Hydraulic resistance of vegetated flows: Contribution of bed shear stress and vegetative drag to total hydraulic resistance

Abstract

Hydraulic resistance of floodplain flows depends on both drag forces exerted by vegetation elements and bed friction. Until today, the contribution of bed shear stress to total hydraulic resistance has often been neglected in hydraulic flume investigations. This has been justified with the dominance of form drag over surface friction for densely vegetated flows. However, in riparian forests where shrubs and bushes are predominant, vegetation density can be relatively low. Therefore, neglecting bed shear stress contribution can result in a significant underestimation of total flow resistance. The objective of this paper is to investigate the contribution of bed shear stress to the overall flow resistance for vegetated flows. Drag forces acting on up to 10 flexible vegetation elements were measured directly and simulta- neously with specifically designed drag force measurement sensors in laboratory experiments. The mea- surements were carried out for three different vegetation densities, two vegetation patterns, mean flow ve- locities ranging from 0.1 to 0.7 m/s, and just submerged flow conditions. The hydraulic and drag force data are used to estimate bed shear stress and to assess its contribution to total hydraulic resistance depen- dent on vegetation density, vegetation pattern and hydraulic conditions. Existing relationships for the hy- draulic resistance of vegetated flows are tested and the significance of plant specific parameters such as streamlining is highlighted.