Author(s): Richard Sharpe

Linked Author(s): Richard Sharpe

Keywords: Vegetation drag; Flow resistance; Drag coefficient;

Abstract: A substantial body of literature exists on the influence of vegetation on flow resistance in rivers and floodplains. Research has shown that the quadratic drag force-velocity relationship used in boundary layer theory does not hold when flexible vegetation bends under the influence of drag. As such, a small number of drag force models that aim to account for reconfiguration of tree foliage have been developed. Drag force measurements for testing and parameterizing thesemodels are limited, especially for Australian riparian tree species. This paper presents work that is being undertaken to measure drag forces on Casuarina cunninghamiana (river she oak). The research aims to measure drag forces across a range of tree sizes and velocities to test published drag force models, and ultimately, to model flow resistance through monocultural stands of C. cunninghamiana. Initial experiments were undertaken on an estuary. A drag resistance frame was designed and constructed and attached to the side of a boat to measure drag forces. Since drag on rigid smooth cylinders is well established, initial experiments used rigid cylinders to test the experiment design. The results replicate expected drag coefficients for velocities greater than 1.0 m/s. At lower velocities, drag coefficients were high and substantial noise was encountered due to lateral vibration of the frame. This suggests that the drag resistance frame requires adaption to overcome vibrations that may have been amplified by resonance. Methods for estimating tree area and flexural rigidity are also presented. Near field LiDAR survey and photographic survey were used to estimate the frontal area of a sample tree. The relationship between pulling force and tree deflection for an in-situ tree was also recorded. The results suggest that, for the velocities tested in the drag experiments, drag forces on the sample tree would cause substantial bending of the tree stems. It is expected that the reconfigured tree frontal area will influence the drag force-velocity relationship, which will be investigated in future experiments measuring drag forces on trees.

DOI: https://doi.org/10.3850/38WC092019-1649

Year: 2019