Study on vegetation drag coefficient in combined wave with following and opposing currents

Drag coefficients (CD) are a key metric in quantifying the vegetation effect on riverine and coastal modeling. However, drag coefficients determined using various approaches under combined wave-current flows have not been comprehensively explored. In the paper, a three-dimensional (3D) numerical model was developed to study the wave dissipation induced by submerged canopies in both following and opposing currents. The results reveal the characteristics of variation of vegetation drag force under pure wave and combined wave-unidirectional flow, as well as the relationship between drag force and flow velocity. The calibration and direct measurement methods were applied to conduct the drag coefficients under various wave-current combinations. The temporal variation in horizontal velocity U, drag force F, and the drag coefficient within the aligned canopy and staggered canopy considering the combined effects of wave and current show different patterns. Moreover, the empirical relations between drag coefficients derived using different methods with Reynolds number (Re) and Keulegan-Carpenter number (KC) are proposed. Comparison of the CD-Re and CD-KC relations would provide insight into the understanding of wave dissipation by vegetation under combined wave-current flow conditions.