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A large proportion of flow resistance can be due to vegetation in open-channel flows. Classic methods are unable to estimate flow resistance in the presence of vegetation. In this study, drag coefficient was estimated as a parameter to represent the flow resistance in emergent and submerged vegetation patches with different densities. Emergent vegetation was simulated by wooden cylinders 8 mm in diameter and 0.35 m high. Four emergent patches with different densities, arrangements, tandem, random, and circular, were examined. Also, submerged vegetation was simulated by nails 4 mm in diameter and 4.5 cm in height. The phenomenon of dip occurred throughout the dense patches because of secondary flow and the drag force generated by vegetation. Four methods were selected to estimate the drag coefficient and the balance approach-blockage factor method was used as the main method to estimate the patch drag coefficient. Results showed that velocity profiles were S-shaped and the drag coefficient of random vegetated patches was greater than the coefficient of tandem patches. The drag coefficient revealed a direct relation with vegetation density. In all patches, the initial rows had the highest contribution to flow resistance, so the drag coefficient decreased through vegetation patches.

Estimation of drag coefficient of emergent and submerged vegetation patches with various densities and arrangements in open channel flow