Mangroves as Bio shield

By: Aritro Dasgupta
Mangroves have traditionally been part of the coastal ecosystem, acting as a natural barrier against tidal waves. Studies have now established that mangroves did indeed play a crucial role in minimising damage during the December 2004 tsunami. Certainly the benefits of mangroves are manifold and they must be nurtured and conserved.
Ecology

Six years ago the islands of Andaman and Nicobar and a large part of the coastline in India were engulfed in a single stroke. The tsunami, caused havoc in all the countries situated around the epicentre, including India. Thousands of habitations were ripped apart, over two hundred thousand people lost their lives and countless were left homeless. However, on closer inspection it was revealed that the humble jungle of mangroves could have saved hundreds of lives.

Traditionally various types of mangrove forests have dominated the Indian coastline. Mangroves are a floral species which effectively reduce the impact of tidal waves and at the same time enhance the natural ecosystem of an area, helping in carbon fixation and providing a safe nursery – essential for robust coastal food webs. However, increasing industrial, agricultural and fishing activities have resulted in a steep decline of these highly sensitive coastal ecosystems. In 2004, unfortunately for numerous unguarded villages these degraded coastal mangroves could not provide an effective barrier against the surging waves. On the other hand, there are cases where pockets of healthy mangrove forests did help protect. A case in point are three villages in Cuddalore district of Tamil Nadu, that survived the tsunami with minimal damage guarded behind the vegetation.

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Image: Mangrove restoration carried out in Tamil Nadu. Pic 1: Canal digging; Pic 2: First year plantation; Pic 3: Second year plantation; Pic 4: Restored mangroves.

Species selection

Recognising the importance of mangroves as a cheap and natural barrier against tidal waves, several tsunamis affected countries started a blanket coastal plantation drive in the wake of the disaster. One such programme was by the Tamil Nadu government in collaboration with the World Bank. Casuarina saplings were planted along the State’s coastline to act as barriers. Unfortunately, this resulted in the Olive Ridley turtles loosing their nesting grounds and the programme had to be modified. Also, while casuarina is considered an effective tool for reducing cyclone impact, its usefulness in case of a tsunami is uncertain. Therefore, not all plantations are suitable along the coast and due care must be taken to select the species.

The views of two eminent scientists shed further light on the dynamics involved in developing mangroves as bio shields and the immense benefits that can be derived from them. Dr V Selvam, head scientist at the Coastal System Research, M S Swaminathan Research Foundation, Chidambaram and Prof K Kathiresan a senior scientist at the Centre of Advanced Study in Marine Biology, Annamalai University, Chennai, share their views about developing such bio shields.

Both scientists agreed that mangrove plantations along the coastal areas are effective in reducing the impact of cyclones and tsunamis. Citing an example from the 1999 super cyclone that hit Orissa and the other coastal states, Dr Selvam said that in the areas where mangroves were present the wind was directed upward leading to a significant drop in subsequent speed levels. Prof Kathiresan pointed out that Japan, a tsunami affected country has undertaken extensive mangrove plantation along its coastline as a protective measure. He also opined that the destruction in Myanmar during Cyclone Nargis in 2008 could have been checked with the presence of mangroves.

Elaborating on a suitable variety of mangrove to act as a barrier, Dr Selvam said that the Rhizophora is best suited for developing plantations. It grows 300 to 400 roots from a single plant and thus helps in holding the soil together and is also suitable for water retention. While Prof Kathiresan added that the muddy substratum is ideal for mangrove plantations.

 

Replanting mangroves

For specific dimensions of a mangrove plantation, the scientific community in India agrees that normally 20 to 30 per cent of the beach from the ocean should be left out of the purview of a plantation. Therefore, if the breadth of a beach is 100 m then the mangrove plantation can be done at 20 to 30 m from the water. However, this distance varies with the slope of the beach, on a steep sloped beach the distance should be less and on a smooth sloped beach the distance should be more. The distance between each sapling should ideally be 1.5 to 2 m, which means that an area of 100 sq m should hold about 30 plants. Inter cropping is not feasible as the mangrove is a fast regenerating species which does not allow other crops to flourish.

 

Endnote

In India 60 per cent of the mangrove forests are found along the eastern coast as it has a smoother slope compared to its western counterpart. Mangrove based eco tourism is fast catching up in this part of the country. The Bhitarkanika in Orissa and the Sundarban in West Bengal are both popular with tourists – former for the endangered salt water Crocodile (Crocodile porosus) and the latter for its near extinct Royal Bengal Tiger (Panthera tigris). Therefore, while disaster risk reduction is one of the biggest benefits of mangrove plantations, there are several other dimensions to encouraging mangroves reforestation. Cyclones are not an everyday phenomena and tsunamis even less so, but the mangroves can be a source of unlimited benefits to the local communities in the present and act as a bio shield from marauding waves in the future.

 

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