Numerical study of solitary wave interaction with a vegetated platform

Climate changes and increase in the sea levels have led to the occurrence of extreme sea events such as tsunamis, coasting erosions and flooding. Being a global emergency, it requires urgent and effective solutions. Research has shown that coastal vegetation systems can be effectively used in the management of the coastal ecosystem due to their ability to attenuate the incident wave energy attributed to good wave damping efficiency. For example, the recently developed Martin Ecosystems BioHaven Floating Breakwater, comprising of a simple platform, vegetation stems, and vegetation roots, has been effective in protecting the shoreline and stabilizing the banks. Coastal vegetation can be classified as suspended, emergent and submerged vegetation.  Wave attenuation by submerged and emergent vegetation has been studied numerically. However, the interaction between the vegetated platform and extreme sea events has not been fully explored.

In a recent paper, Yanxu Wang (Ph.D. student), Professor Zegao Yin, and Professor Yong Liu from the Ocean University of China investigated the solitary wave interaction with vegetated platforms. In particular, a two-dimensional numerical model based on the microscopic approach was developed. The feasibility of the model was validated using data obtained from the existing literature. The research work is currently published in the research Ocean Engineering journal.

Numerical simulations based on the OlaFlow solver was conducted to evaluate the solitary wave interaction with the vegetated platforms. The wave damping efficiency of the vegetated platform being greater than that for the simple platform, the simple platform played a significant role while the vegetation played a supportive role in the wave damping. On the other hand, the vegetation roots exhibited better performance than stems for reducing wave transmission. However, the stems always contribute to diminishing the wave elevation oscillation and sub-peak in the undulating tails.

The individual drag coefficients for vegetation stems and roots were determined by investigating the velocity distributions of the green water and the underflow above and below the platform. This required a better understanding of the effects of various parameters: incident wave height, vegetation density, stem height, root height and platform width on the hydrodynamic coefficients. For green water, a self-similar velocity behavior is observed along the platform. The maximum velocity occurred at the green waterfront while the minimum velocity occurred at the seaside edge of the platform. However, the velocity of the underflow maintained almost constant along the platform and the maximum water velocity of the underflow occurred earlier than that for green water, especially for larger platform widths. Generally, an increase in the platform width led to an increase in the maximum velocity of the green water while that for the underflow followed the opposite trend.

In a nutshell, Ocean University of China scientists developed empirical formulas for predicting solitary wave transmission and reflection over simple and vegetated platforms. Overall, even though the study insights are limited to mainly solitary waves, it provides a useful approach for further numerical investigation regarding the effects of irregular waves on the vegetated platforms. Therefore, Professor Zegao Yin, in a statement to Advances in Engineering, observed that the study will advance the coastal environmental management strategies.