Drag forces and flow resistance of flexible riparian vegetation
ABSTRACT: Hydraulic resistance of riparian vegetation depends on the drag exerted on the plants and hence on plant specific parameters and foliage. However, until today, most studies related to the estimation of flow resistance of riparian emergent vegetation have been carried out under idealised conditions using cylindrical roughness elements, regular element spacing, etc. Although these studies contributed significantly to the current understanding of the relevant processes, there are still many open questions with regard to the influence of flexibility and foliage on drag forces, flow resistance and the flow field. This paper discusses various issues related to these questions using data from experiments carried out with flexible artificial and natural small scale vegetation elements in a laboratory flume and with natural trees in a towing tank facility. The comparison of drag forces on artificial and natural plants is used to introduce a concept for the comparison of the resistance behaviour of flexible artificial plants with their natural counterparts. The data are also used to discuss the spatial variability of the drag forces within a vegetation array and to highlight the effect of foliage on drag forces. It is shown and that foliage contributes significantly to total drag predominantly at low velocities while the wooden parts of the trees contribute more to total drag at larger velocities. The differences between the drag force - velocity relationships for flexible plants and rigid bluff bodies are evaluated in terms of the recently defined Vogel exponent. It is shown that this exponent is implicitly incorporated in an existing approach for the determination of flow resistance of emergent riparian vegetation, which is validated using the acquired data.