Effects of flume characteristics on the assessment of drag on flexible macrophytes and a rigid cylinder

Previous research, using rigid objects, showed that the magnitude of mean drag on submerged bodies can be influenced by the blockage ratio (ratio of the frontal projection area of an object to the wetted flume cross-sectional area). In addition, drag measurements, under similar flow conditions, are likely to be affected by other flume characteristics. These factors lead to some uncertainty in drag measurement comparisons between identical objects for a given reference flow velocity. We measured drag on three pairs of replicate objects (two types of flexible artificial macrophyte and a rigid cylinder) in two flumes differing in width (0.25 and 0.75 m). Owing to different flume widths, the blockage ratios differed between the flumes for the macrophytes (c. 7% and c. 22% at intermediate flow rates) and the cylinder (2.5% and 7.5%). For theoretically identical mean cross-sectional velocities, the between-flume difference in mean centreline channel velocities across the range of flows used in this study was 6.4%, indicating similar bulk flow conditions for the two flumes. We found no between-flume differences in drag-velocity relationships for the cylinder, whereas one macrophyte had a distinctly higher drag in the narrower flume at low approach velocities (c. 0.2 m s(-1)). This difference of drag on the macrophyte likely indicates blockage effects in the narrower flume at low velocities, when plant bending and streamlining was low. At higher approach velocities, between-flume differences in drag on the macrophytes were minor, except for one type of macrophyte that had a higher drag at the highest velocity in the wider flume (possibly related to plant reconfiguration and fluttering).