Laboratory investigation of solitary wave interactions with mangrove seedlings in hexagonal planters

Abstract

The integration of engineered and natural elements, known as hybrid solution, combines physical protection with ecological benefits for coastal defense. This study evaluates the performance of a hexagonal concrete planter designed to shelter mangrove seedlings under wave attack. Laboratory experiments measuring wave heights, fluid velocities, and forces on seedlings were conducted to investigate the mechanical behaviour of this hybrid solution. Results show that the concrete planters effectively dissipate wave energy, with the contribution of mangrove seedlings to wave attenuation is negligible due to their small density. Among various parameters, water submergence is identified as the most important factor influencing wave transmission. The planter's presence alters the surrounding flow field by increasing horizontal flow velocity near the seedlings' upper portions; however, force measurements demonstrate that the planter significantly reduces the hydrodynamic force exerted on the seedlings by lowering flow velocity within its interior. To account for the altered velocity field, a modification coefficient is proposed to predict the depth-averaged velocity near the hybrid solution. This modified velocity approach yields accurate force predictions under low submergence conditions, but significant deviations occur at high submergence, mainly due to the bending of mangrove seedlings. Overall, this study highlights the planter's effectiveness in reducing forces on seedlings and attenuating wave energy. The findings provide valuable insights into hydrodynamic interactions between hybrid solutions and incoming waves. Additionally, the presented results offer practical guidance for designing and deploying concrete planters, bridging the gap between academic research and engineering applications.