Effect of mangrove forest structures on wave attenuation in coastal Vietnam

Study Number:

30

Author:

T. Q. Bao

Abstract:

This paper analyses wave attenuation in coastal mangrove forests in Vietnam. Data from 32 mangrove plots of six species located in 2 coastal regions are used for this study. In each plot, mangrove forest structures and wave height at different cross-shore distances are measured. Wave height closely relates to cross-shore distances. 92 exponential regression equations are highly significant with R2 > 0.95 and P val. < 0.001. Wave height reduction depends on initial wave height, cross-shore distances, and mangrove forest structures. This relationship is used to define minimum mangrove bandwidth for coastal protection from waves in Vietnam.

Main Results and Conclusions:

• Forest structure has a large impact on the ability of mangroves to attenuate waves: “The results show that wave height decays exponentially and is significantly related to distance (Figure 3). All 92 exponential regression equations of five research areas with different mangrove forest species are highly significant with P values of < 0.001 and R2 > 0.95. The exponential reduction of wave height in mangroves can be explained by the dense network of trunks, branches and above-ground roots of the mangrove trees, increasing bed roughness, causing more friction and dissipating more wave energy (Quartel et al. 2007)”(811). (Extinction).
• A mangrove structure index was created to classify forest type based on forest structure (i.e. height and density), to determine the required band width (width of the forest from the coast line) of a forest to successfully attenuate waves: “The mangrove structure index is classified into 5 levels of wave prevention based on its relation to wave height (Figure 6; Table 2). The required mangrove bandwidth decays exponentially with the vegetation index (V). When the mangrove forest is tall and dense, and the canopy closure high (i.e. a high V index), a narrower forest band is required. When the mangrove forest is short, the tree density and canopy closure low (i.e. a low V index), a wider mangrove band is required”(814).
• The average maximum wave height observed was 300 cm, and the safe wave height behind the forest band was determined to be 30 cm (817).
• To summarize the V index (mangrove structure index), the smaller the V index, the lower the level of protection provided by the mangrove forest, and therefore the wider the mangrove forest width needs to be to most effectively attenuate waves. The five levels of V index’s data is a follows:
  • “Level I has a V index of <0.005 with a required band width (in meters) of >240, which would offer very weak prevention.
  • “Level II has a V index of 0.005–0.010 with a required band width (in meters) of 120–240, which would offer weak prevention.
  • “Level III has a V index of 0.010–0.015 with a required band width (in meters) of >80–120, which would offer moderate prevention.
  • “Level IV has a V index of 0.015–0.028 with a required band width (in meters) of 40–80, which would strong prevention and finally.
  • “Level V has a V index of >0.0280 with a required band width (in meters) of <40, which would offer strong prevention”(815).
• An example of how this study used the V index is as follows: “As the southern mangrove forests of Vietnam protect waves better than the northern mangrove forests do, they have a higher V index”(817).

Works Cited:

Quartel S., Kroon A., Augustinus P. G. E. F., Van Santen P., and N. H. Tri. 2007, Wave attenuation in coastal mangroves in the Red River delta, Vietnam. Journal of Asian Earth Science 29 (4): 576–584. doi:10.1016/j.jseaes.2006.05.008.