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

• Published in: Ocean engineering 2024-03, Vol.296, p.117002, Article 117002
• Main Authors: Reis, Rui A., Fortes, Conceição J.E.M., Rodrigues, José A., Hu, Zhan, Suzuki, Tomohiro
• Format: Article
• Language: English
• Subjects: Drag coefficient; Flexible vegetation; Inertia; Vegetation motion; Wave dissipation
• ISSN: 0029-8018; 1873-5258
• DOI: 10.1016/j.oceaneng.2024.117002

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 (CD). The direct method and the least squares method (LSM), based on force and flow measurements, are applied to calculate the CD in the experimental conditions. The formulations of both methods are extended to estimate the CD 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 CD results, obtained from common CD 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 CD values were close. CD estimations considering vegetation motion and inertia suggest that CD of flexible vegetation was up to 13% higher relative to rigid vegetation. Accounting for inertia reduced the CD for rigid vegetation up to 7%, while raised the CD for flexible vegetation up to 13%. •Methods to calculate the drag coefficient of flexible vegetation are provided.•Data of the vegetation motion obtained through a video analysis is considered.•The wave dissipation was 34% lower for flexible relative to rigid vegetation.•Only 13% difference in CD of flexible relative to rigid vegetation is suggested.•Inertia reduced CD for rigid vegetation (up to 7%) while raised the CD for flexible vegetation (up to 13%).