Mangrove ‘greenbelts’ to prevent coastal erosion

Tan Cheng Li, writing in the August 30, 2005 edition of the thestar.com.my, comments that nations hit by the December 26 tsunami are now planting mangrove trees along coastlines to create ‘greenbelts’ or vegetated strips of land, in the hope of preventing further erosion of damaged shores as well as to shield them from future giant waves.

Mangroves are something of a rarity – they cover a mere 0.04% of Earth’s surface and 0.12% of its land area, and whatever mangrove resource that remains should be conserved. There is an economic benefit to this as well.

In Malaysia, it costs RM15,000 to replant one hectare of mangroves. The government has identified 4,016ha of degraded mangroves requiring rehabilitation at a cost of RM110mil. Some 151ha have been replanted so far.

Impact of the tsunami
The December 26 tsunami left behind extensive environmental damage across the region. It totally changed coastal landscapes and ecosystems. The impact included:
# Loss and degradation of mangrove and seagrass beds
# Silting and degradation of coral reefs
# Change in tidal flats and coastal lagoons
# Uplifting of the seafloor in some areas
# Turbid coastal water
# Scouring of coastline
# Inland deposition of sand
# Salination of coastal land
# Impact on fisheries

The role of mangroves during the tsunami:
# Reflect and resist tsunami energy, thus reducing the inundation depth and area.
# Trap driftwood and other debris, thus reducing human injuries and property damage.
# Prevent people from being washed out to sea.
# Reduce erosion of beaches and sand dunes which also act as barriers to tsunamis.

Research has been done into integrating mangroves and aquaculture into sustainable aquaculture-silvoculture systems.

Silvofisheries is a form of integrated mangrove tree culture with brackishwater aquaculture. It is a form of low input sustainable aquaculture. This integrated approach to conservation and utilization of the mangrove resource allows for maintaining a relatively high level of integrity in the mangrove area while capitalizing on the economic benefits of brackishwater aquaculture. Further extension of aquaculture to meet the needs of the rural poor may be tolerable provided it is carried out in a controlled manner outside those areas already heavily exploited and environmentally sensitive in an integrated program of conservation and utilization, such as silvofishery methods.

Research has also been conducted into Disused Shrimp Ponds: Options for Redevelopment of Mangrove. N.J. Stevenson describes that associated with the rapid increase in the production of cultured marine shrimp, has been large scale conversion of mangrove to shrimp ponds. Production in many regions has proven to be unsustainable, largely due to inappropriate construction methods, poor environmental conditions, overstocking and disease problems. A number of shrimp ponds are consequently unproductive and lie idle. Accurate assessments of pond disuse are difficult to obtain, however, unofficial estimates have suggested that as many as 70% of ponds may be disused after a period in production. Pond construction, shrimp culture and pond disuse lead to alterations to the physical and chemical properties of soil, hydrological conditions and the flora and fauna composition of the pond area. The case for restoration, or rehabilitation to a sustainable use, is strong. Consideration must be given to the causes of production failure, the environmental conditions remaining following disuse, the needs and preferences of pond owners and coastal managers, and technical constraints.

 

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