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19 November, 2019 00:00 00 AM

Sundarbans: A natural shield against natural disasters

Mangroves contribute to reducing the loss of life and damage to property from storms and cyclones as they reduce the impacts of waves, storm surges, and high winds
Ranjan Roy
Sundarbans: A natural shield against natural disasters

Climate-related natural disasters—floods, cyclones, and tornado—have been on the rise worldwide. Global damage from natural disasters has been steadily increasing, reaching about $142 billion annually in the last 10-year period. Scientists have been exploring optimal ways of protecting natural disasters. Evidence confirms that, in some cases, well-established forests (the mangroves) and tree plantations have offered an effective physical barrier against natural disasters and helped save both lives and property. The world’s largest mangrove forest, the Sundarbans, provided an effective attenuation buffer during Sidr, significantly reducing the impact of the storm surge. Of late, the Sundarbans acted as a natural shield against the cyclone ‘Bulbul’ and captured the electronic and print media headlines.

Due to the Sundarban’s interruption, losses and damages by cyclone Bulbul have largely been reduced, as weather experts and forest men say. This cyclone hits the coastal regions adjacent to 24 Pargana of the West Bengal with the wind forces between 115 and 125 kilometers per hour. However, the wind speed of this cyclone was around 93 km per hour whenever it reaches to the Bangladeshi coastal belts.

When cyclone Bulbul arrived at Khulna, it was much weaker. This is because of the mighty Sundarbans that decreased its violent wind speed. As ramifications, the devastating effects are lesser than the forecasted, although all effects are not estimated yet. If this cyclone beats the coastal belts directly, the losses and damages are expected to much higher, several reports state.

Moot points are how a mangrove forest can protect against natural disasters, and how to protect and enhance the capacity of this forest? Mangrove acts as a natural shield in various ways. First, mangroves reduce the height and energy of wind and swell waves passing through them, reducing their ability to erode sediments and to cause damage to structures such as dikes and sea walls. It also reduces winds across the surface of the water, and this prevents the propagation or re-formation of waves. Mangroves with a complex structure of dense aerial roots and low branches, with various species of different age and size, are most likely to be effective at reducing wave heights.

Second, mangroves contribute to reducing the loss of life and damage to property from storms and cyclones as they reduce the impacts of waves, storm surges, and high winds. During relatively large storm surges, the leaves and branches of the forest canopy help to reduce wave energy providing the trees are tall enough. Mangroves are extensively able to reduce storm surge water depths as the surge flows inland. The dense mangrove forest canopies also reduce wind speeds locally.

Third, evidence indicates that mangroves reduced tsunami impacts by reducing the destructive energy of water flowing inland. Wider mangrove forests are more effective at lowering tsunami height, as well as the speed of the water and the area flooded by the tsunami. Dense forest vegetation also helps to reduce tsunami depth and area of flooding. However, massive tsunamis can damage mangroves.

Fourth, mangroves reduce erosion and bind soils together. The dense roots of mangroves help to bind and build soils. The above-ground roots slow down water flows, encourage deposition of sediments, and reduce erosion. Complex aerial root systems help slow water flows, allowing sediment to settle and causing sediment to accrete rather than erode.

And, fifth, there is growing evidence that mangroves may keep up with sea-level rise (SLR). Over time mangroves can actively build up soils, increasing the height of the mangrove soil surface, which may be critical as SLR accelerates. However, healthy mangroves are a prerequisite for all aspects of coastal protection. Healthy mangroves require sufficient sediment and freshwater supply and connections with other ecosystems. Conversely, pollution, subsidence, and unsustainable use jeopardises mangroves.

There is no denying that mangrove serves purposes of disaster management, for example, the Sundarbans serve as dike systems to protect coastal areas from extreme natural disasters, such as tsunamis and cyclones, studies reveal. Protecting and enhancing the capacity of the Sundarbans is, therefore, an urgent issue. The mangrove has been systematically deforested under the very noses of the authorities. Around 8.3% of the total area of this forest was deforested by individuals and businesses. Most of the Chakaria Sundarbans have been deforested due to the expansion of shrimp and salt farms. Protection of the Sundarbans must stop deforestation.

Coastal erosion and accretion are continually reshaping the total area of Sundarbans. The average rates of erosion for the eastern and western parts are 14 m/year and 15 m/year, respectively. Reducing coastal erosion requires an assemblage of ‘soft’ and ‘hard’ structural/engineering options: riparian revegetation and offshore breakwater.

A long-term investment in the mangrove plantation for coastal protection is crucial. Planted mangroves protect riverbank erosion and save the households near the riverside from different natural calamities. Understanding the underlying science between the Sundarbans and natural disasters is very important to sustainably planning and managing this forest. Immediate actions are needed to implement the three strategies of the Delta Plan for expanding the Sundarbans (a) planting in all the layers of a forest at the same time, (b) not being competitive against Nature rather being collaborative, and (c) maintenance of perennial tidal flow.

Moreover, three issues are critical to managing mangroves for coastal defense. First, integrating mangroves into coastal defense strategies since mangroves don’t always provide a stand-alone solution. They may need to be combined with other risk reduction measures to achieve a desired level of protection.

Second, mangroves have to be incorporated as a part of coastal zone management. For mangroves to optimally contribute to risk reduction, their conservation needs to be assimilated into broader coastal zone management policy instruments, e.g., Coastal Zone Policy.

And, third, investments are required in bringing the mangroves back. Mangroves and their coastal risk reduction function can recover in most places where appropriate ecological and social conditions are present or restored.

There is compelling evidence that the Sundarbans is an essential part of the survival of the coastal people and property. Stringent mangrove governance measures are, therefore, a sine qua non of minimising the impacts of natural disasters and protecting Bangladesh as a whole.

The writer is an Associate Professor in the Dept. of Agricultural Extension and Information System, Sher-e-Bangla Agricultural University, Dhaka.



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Editor : M. Shamsur Rahman
Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

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