Quantifying Drag Coefficients Due to an Isolated Circular Vegetation Patch

The role of a circular vegetated patch size that emerged in stream flows has not yet been fully investigated. In this study, a circular patch was investigated to explore the influence of the patch diameter in terms of vegetation density and cylinder configuration in terms of blockage impact on the calculated drag coefficient C-D. Four patch diameters (25, 50, 75 and 100 mm) and two cylinder configurations, i.e. Type A (staggered) and Type B (linear), for a constant number of cylinders were investigated. Under uniform flow depth, the conditions in the flume are subcritical with a Froude number Fr of less than 0.52, a stem Reynolds number of 841-1,813 and a patch Reynolds number of 5,993-30,333. Results showed that the distribution pattern of the vegetation elements has a considerable effect on the rating curve. The C-D-Fr relationship exhibited a regular exponential decline with an R-2 of more than 0.95. The reported data showed an excellent agreement to the drag coefficient equation used in Cheng et al. Results showed that the smallest patch diameter (2.5 cm) model has the highest drag coefficient in comparison with other patch diameters (5.0, 7.5 and 10.0 cm). In addition, results showed that the values of the bulk drag coefficients have not influenced the two vegetation arrangements. Finally, in the staggered model, sheltering was numerically trapped between the wakes around the cylinders. By contrast, in the linear arrangement, sheltering does not occur.