Based on existing wave dissipation model by vegetation, a kind of forced vibration with damping model is used to take care of the vegetation swaying in the water waves with different wave parameters. By introducing the factor D as the concession to the wave force, the drag coefficient CD is replaced with an equivalent coefficient CDf in flexible vegetation, and then CDf=|1-D|D, where D is the relative value of the velocity of plants swaying and water particle. Therefore, according to the theory applied in this paper, the original SWAN-VEG model is modified, in order to simulate the attenuation of wave energy well in the numerical wave tank. In addition, an experiment with artificial flexible materials is carried out to calibrate the calculated results, and the results of the improved SWAN-VEG model yield the experimental results much better than those from the original model. It indicates that for a flexible case the swaying of vegetation should not be neglected, and it relates to the natural circle frequency, damping ratio, height of plants, and the incident wave parameter. On the basis of the comparison between calculated and experimental results, it can be concluded that the way of using the equivalent drag coefficient CDf in the modeling of flexible vegetation fields such as kelps, reeds, seagrasses and so on are valid.

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