Experimental study on drag coefficient of flexible vegetation under non-breaking waves

Laboratory experiments of wave propagation over rigid and flexible vegetation fields, with the same configurations, were conducted to understand the effect of vegetation flexibility on the drag coefficient (C_D). The direct method and the least squares method (LSM), based on force and flow measurements, are applied to calculate the C_D in the experimental conditions. The formulations of both methods are extended to estimate the C_D for flexible vegetation cases. A video analysis was performed to account for the swaying motion. Typically, wave dissipation is lower for flexible than for rigid vegetation of the same configuration, under the same flow condition. Therefore, a proportional effect in the corresponding C_D results, obtained from common C_D calibration to wave dissipation without considering vegetation motion, is usually observed. However, the present results show that although the wave dissipation was 34% lower for flexible relative to rigid vegetation, the respective C_D values were close. C_D estimations considering vegetation motion and inertia suggest that C_D of flexible vegetation was up to 13% higher relative to rigid vegetation. Accounting for inertia reduced the C_D for rigid vegetation up to 7%, while raised the C_D for flexible vegetation up to 13%.