Laboratory Study of Broken Wave Force Damping by Submerged and Emergent Long Tree Canopies

IntroductionForests as Green Belt reduce the height and energy of waves passing through them, reducing their ability to erode sediments and to cause damage to structures such as dikes and sea walls, (Mclvor and et all.,2012). It is well-accepted that wave attenuation by emergent and submerged vegetation is a function of plant characteristics as well as hydrodynamic conditions, (Augustin et al., 2009). (Hirashi and Harada, 2003) showed that the pressure difference on the sides of the green belt is mainly due to drag resistance. this study a new and unique method based on the principle of momentum and direct force measurement has been used to measure wave energy decay simulated by the Green Belt.Most previous studies have been conducted to analyze the wave forces on emergent structures and obstacles. Relative submergence is considered to be the relative roughness of trees and is a factor influencing the resistance of the forest to the passage of waves, (Davoudi et al., 2016). So the resistance of the canopies in the submerged state is one of the important factors in wave damping. Also, in previous studies, variable still water depth has been considered to create different ratios of submergence, which will change the characteristics of the wave. Therefore, in the present study, decided that the resistance of the canopy against broken waves in the submerged state would be determined by changing the height of the obstacles, which will be examined in this study (Figure 1). Figure 1-- Schematic view of the Rigid vegetation with different relative depth of submergenceMethodologyExperiments were conducted in the Hydraulic Modeling Laboratory of the Faculty of Water Engineering, Shahid Chamran University of Ahvaz, in a 8.3×0.8×0.55 rectangular flume called Knife Edge Flume. Flume has Plexiglas sidewalls and bed and designed to measure the force exerted by the wave force on the barriers at its shore by means of a dynamic load cell installed between the movable and fixed parts of the flume. At the beginning of the experiments, wooden circular cylinders with 1 cm in diameter, fixed parallel arrangement were placed in the moving part of the flume with constant slope of zero. By changing the height of obstacles and generating breaking waves with constant height the absorption force by the canopies was monitored via an electrical display connected to the dynamic load cell.Results and Discussion The submerged ratio of tree canopies is equal to height of inundation depth to height of trees. Therefore, in the submerged state, the ratio is greater than one, and in the emergent state it is less than one. What is important in this study is that different ratios of immersion with different heights of barriers have been created. Fig.2 Variation of drag force to the inundation height of barriers to still water depth at different ratio of submergenceThe force when the submerged ratio is greater than one, which means that the cylindrical barriers are immersion, the variation of drag force are greatly increased and the amount of absorbed force is greatly reduced, which is well illustrated by the diagram. In submerged conditions, the water level is above the barrier. Therefore, there is no obstacle to the passage of higher speed particles and the drag force is reduced. Fig.3 - Variation of drag force by submerged vegetation against the ratio of submergence of broken waves At the fixed depth of the reservoir, the amount of wave force absorption decreases as the still water depth increases. Fig 4- Variation Drag Coefficient for relative depth of submergence with constant still water depthAs the height of barriers decreases, the effective cross-section of the canopies decrease. Although the absorption force also decreases, the effect of cross-sectional reduction is greater, which increases the drag coefficient at a specific wave with different ratios of plant submergence.Conclusions- In cases where the canopies are emergent, the amount of force absorption and wave damping is higher than in submerged canopies.- When the submerged ratio increases from 1 to 2.5, the drag force reduction for reservoir water depths of 40, 34 and 28 is 41.63, 29.87 and 19.89%, respectively. The highest force reduction is equivalent to 41.63%.- When the submerged ratio increases from 1 to 2.5, the drag force at the different still water depth of 8, 12 and 14 cm decreases by 29.35%, 41.63% and 41.56%, respectively.- At a constant still water depth, the drag coefficient decreases with increasing submerged ratio (Hs), resulting in less wave loss and damping.- As the depth of the still water increases, the drag coefficient decreases due to the reduction of the force absorbed by the obstacles.AcknowledgementThis study funded by the Shahid Chamran University of Ahvaz, Iran. The support of this organization is appreciated. Keywords: Green belt, submerged ratio, wave absorption force, wave attenuation