Preprints
https://doi.org/10.5194/hess-2021-188
https://doi.org/10.5194/hess-2021-188

  15 Apr 2021

15 Apr 2021

Review status: this preprint is currently under review for the journal HESS.

A study on the drag coefficient in wave attenuation by vegetation

Zhilin Zhang1,2,3,4, Bensheng Huang1,2,3, Chao Tan1,2,3, Hui Chen1,2,3, and Xiangju Cheng4 Zhilin Zhang et al.
  • 1Guangdong Research Institute of Water Resources and Hydropower, Guangzhou, 510630, China
  • 2State and Local Joint Engineering Laboratory of Estuarine and Hydraulic Technology, Guangzhou 510630, China
  • 3Guangdong Provincial Science and Technology Collaborative Innovation Center for Water Safety, Guangzhou 510630, China
  • 4School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, 510641, China

Abstract. Vegetation in wetlands is a large-scale nature-based resource providing a myriad of services for human beings and the environment, such as dissipating incoming wave energy and protecting coastal areas. For understanding wave height attenuation by vegetation, there are two main traditional calibration approaches to the drag effect acting on the vegetation. One of them is based on the rule that wave height decays through the vegetated area by a reciprocal function and another by an exponential function. In both functions, the local wave height reduces with distance from the beginning of the vegetation depending on a damping factor (Eqs. (1) and (4)). These damping factors α' and k' are linked to the drag coefficient CD and measurable parameters (Eqs. (3) and (5)). So there are two methods to predict CD that quantify the effect of vegetation. In this study, a new equation is derived that connects these two damping factors (Eq. (12)). The different relations and methods to predicting the drag coefficient CD have been investigated by 99 laboratory experiments. Finally, different relations between CD and relevant parameters (Re, KC, and Ur) have been analyzed. The results show that α' approximately equals k' only for fully submerged vegetation, while the new equation can be used for both emerged and submerged canopy. It appears that the methods for predicting CD by Dean (1979) and Kobayashi et al. (1993) are consistent with the well-recognized method by Dalrymple et al. (1984) for submerged vegetated canopy. But when the vegetation emerges, only the new method based on Eq. (12) leads to almost the same results as Dalrymple et al. (1984). Hence, Eq. (12) has built a bridge between these two approaches for the wave attenuation by vegetation and has proved applicable to emergent conditions of vegetation as well.

Zhilin Zhang et al.

Status: open (until 10 Jun 2021)

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Zhilin Zhang et al.

Zhilin Zhang et al.

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Short summary
This work studies the reduction of wave height and the drag effect by the vegetation in wetlands. Local wave height through the vegetated area is well described by reciprocal function and exponential function. New relations have been derived by comparing these two methods and verified by 99 cases, building a bridge between different methods to better understand the drag effect.