Wave reduction in a mangrove forest dominated by Sonneratia sp.

Study Number:

37

Author:

Y. Mazda, M. Magi, Y. Ikeda, T. Kurokawa & T. Asano

Abstract:

Based on a field observation at the Vinh Quang coast in northern Vietnam, the characteristics of wave reduction due to the drag force of one mangrove species, Sonneratia sp., were quantitatively analyzed. The reduction rate of sea waves in this area changed substantially with the tidal phase, due to the unique vertical configuration of Sonneratia sp. At the shallow range of water depth, since the shape of pneumatophores of Sonneratia sp. tapers off upward, the effect of drag force by these roots on the wave reduction decreased with the increase in the water level, resulting in a decrease in the rate of wave reduction. On the other hand, when water levels rose above the height of thickly spread branches and leaves of these trees, the rate of wave reduction increased again with an increase in the water level. Further, at this high range of water level, the rate of wave reduction depended strongly on the incident wave height. These results indicate that the thickly grown mangrove leaves effectively dissipate huge wave energy, which occurs during storms such as typhoons, and protect coastal areas. Referring to the past studies, our results suggest that the hydrodynamic knowledge in various mangrove conditions such as the vertical configuration of mangrove species, their vegetation conditions, water depth, incident wave condition and the relationships between these factors should be further accumulated and then quantitatively formulated in order to protect coastal areas from severe sea waves. 

Main Results and Conclusions:

• The presence of mangroves helps attenuate waves: “According to Figure 6, in the area without mangroves the rate of wave reduction decreases with the increase in the water depth from 0.002 m–1 to 0.001 m–1”(372).
• Mangrove pneumatophores, vertical, cone-shaped, respiratory root structures, aide in wave attenuation: “Figure 6 the rate of wave reduction in an area with mangroves varies substantially with water depth, and the magnitude of the rate is as large as approximately 0.006 m–1 at shallow water depths. This is indicative of the nature of this area particularly dominated by Sonneratia sp. The great change in the rate of wave reduction with water depth in a shallower range is considered to be due to the resistance caused by vertically erect pneumatophores. Since the pneumatophores of Sonneratia sp. taper off upwards (Figure 2), the drag force of these roots decreases sharply with the increase in the water depth, resulting in a sharp decrease in the rate of wave reduction”(374).
• Wave reduction is caused by the combination of both water depth and thickness of mangrove vegetation: “When the water depths are 0.6 and 0.2 m in the mangrove area, the rates of wave reduction are 0.003 and 0.006 m–1, respectively (Figure 6). In these cases, the wave reductions within a distance of 100 m are therefore calculated as 26 and 45%, respectively, from Equation (2). These values emphasize the significance of wave reduction due to mangrove vegetations and the strong dependence of wave reduction on the water depth, compared with that caused by bottom friction”(374).
• In conclusion: “The reduction rate of sea waves in this area changes substantially with tidal phase due to the unique vertical configuration of Sonneratia sp. This behavior is in contrast to the effect due to bottom friction. At the shallow range of water depth, as the shape of the pneumatophores of Sonneratia sp. tapers off upward, the effect of the drag force of these roots on the wave reduction decreases greatly with the increase in the water level, resulting in a huge decrease in the rate of wave reduction. On the other hand, when the water level reaches the height of the thickly spread branches and leaves of these trees, the rate of wave reduction increases again with the increase in the water level. Further, at this high range of water level, the rate of wave reduction depends strongly on the incident wave height. This fact suggests that the thickly vegetated mangrove leaves have a significant effect on the wave energy dissipation, especially during storms or typhoons which occur so often in these areas”(377).