Assessment of the Ability of Mangrove Structures for Attenuation of Coastal Wave Energy: A Case Study in Bac Lieu Province, Vietnam

Mangrove forests play an important role in coastal disaster mitigation as a natural measure by dissipating wave energy, which performance would change according to mangrove structures, including mangrove density, tree height, root height and tree diameter. This study was aimed at assessing the wave energy dissipating function in terms of mangrove structures and determining the necessary thickness of the mangrove forest to reduce the impacts of waves on the Bac Lieu coastal zone, Vietnam. Standard plots with the unit distance of 20 m mangrove were set for each transect, ranging from the seaside edge of mangrove to 100 m distance inland. Properties of mangrove structures were measured at three transects to ensure the representation of the study area. Waves were measured along the transects using wave gauges and characteristics of wave attenuation due to the drag force of one existing mangrove species, Avicennia sp., were analyzed. The density, height and diameter of mangrove trees were found to be distributed relatively evenly, with a slight increase of the number of trees per standard plot from 183 at the seaside edge to 218 at the 100 m distance inland. Regarding mangrove root, while its density was relatively uniform along transects, its height varied significantly. The amplitude of wave in the Bac Lieu coastal zone changed substantially with the tidal phase ranging from 0.09 to 0.22 m. The wave reduction between the seaside edge and 20 in and 100 in distances inland from the edge, respectively, increased from 32 % to 91 %. A number of mangroves characteristics influenced the rate of reduction of wave height per unit distance, most notably the physical structures of mangrove trees, especially their densities. The wave reduction coefficient was positively correlated with the thickness of mangroves (r = 0.96). When the distance of wave energy transmission through mangroves was greater or equal to approximately 76 m, the wave energy reduction reached 80 % or above, which assures the wave energy would not influence the coastal zone. Therefore, in Bac Lieu with dense mangrove trees, a thin hand of mangroves could provide an adequate defense to protect the coastal area in this tidal range under normal wave conditions. However, for the case of storms with high waves and surges, more thickness of mangroves would be required to guarantee the protection function, which should be analyzed in future studies.