Mangroves as a coastal protection from waves in the Tong King delta, Vietnam

Study Number:

34

Author:

Y. Mazda, M. Magi, M. Kogo & P. N. Hong

Abstract:

The wave reduction (wave period; 5-8 sec.) was investigated in a mangrove reforestation area (Kandelia candel) close to aquaculture ponds in the Tong King delta, Vietnam.
On one site where only young mangrove trees grew, the wave reduction due to the drag force on the trees was harldy effective. On the other site where mangroves trees were sufficiently tall, the rate of wave reduction per 100m was as large as 20%. Due to the high density of vegetation distributed throughout the whole water depth, the effect of wave reduction was large even when the water depth increased. These results demonstrate the usefulness of mangrove reforestation for coastal protection.

Main Results and Conclusions:

• The study sites consisted of a 1.5 wide x 3 km long strip of reforested mangroves (Kandelia candel) in the coastal area of Thuy Hai (split up into three areas of study: A,B and C), while an additional site was located northeast on the coast of Thuy Truong (Sonneratia caseolaris) (128).
• In terms of high-frequency sea level fluctuation: “The wave height of the swell at all stations increases with increasing tidal level or water depth, and decreases with increasing proximity to the coast. These findings suggest wave energy loss caused by bottom friction and resistance to flow due to mangrove vegetation”(130).
• The study created an equation that calculated the rate (r) of wave reduction per 100m in the direction of wave propagation. “The difference in the value of r between Areas-A, B and C (Figure 4) correspond to the differences in the degree of growth of the vegetation between those areas, demonstrating the effect of the drag force due to the trees”(131).
  • “In Area-A, the value of r decreases with increasing water depth (Figure 6b), because, with little vegetation, the wave energy loss is caused by bottom friction only. On the other hand, in Area-C, r is about constant, or independent of water depth because the wave energy loss in Area-C is caused by the drag force on the submerged plants which increase with increasing water depth”(133).
  • “Our finding that in a well grown mangrove area the effect of wave reduction does not decrease with increasing water depth, has important practical implications. Indeed, tropical depressions, storms and typhoons often occur in the central and northern coasts of Vietnam, and include the mean sea level rise (Hong and San, 1993). Furthermore, the sea level may rise by 0.6m within the next century due to global warming. In such larger water depths, the high density of vegetation distributed throughout the whole water depth will still work effectively to reduce wave and protect the coast from wave erosion”(133-134).
• In conclusion: “The plantation of Kandelia candel at the coastal area of Thuy Hai has the effect of significantly reducing swell with the periods of 5-8 sec. and protecting the coast. A six-year-old mangrove trees strip 1.5 km wide will reduce 1 m high waves at the open sea to 0.05 m at the coast. Due to the high density of vegetation distributed throughout the whole water depth, the effect of wave reduction is constant even when the water depth increases”(134).
   

Works Cited:

Hong, P.N. and San, H.T. 1993. Mangroves of Vietnam. IUCN, Bangkok, Thailand, 173 pp.