Cyclones are a regular occurrence in the region and contrary to the commonly held perception, the mangrove ecosystem is quite resilient to momentary disturbances. However, when combined with other threats, most of which have an anthropogenic origin, the sustenance of the ecosystem can indeed be found wanting.
The south of Bengal has been decimated by the category 5 super cyclone Amphan during the night of 20 May and the early hours of 21 May. This was the strongest cyclone ever to strike the eastern state in the recent past, leaving behind a trail of destruction. This has potentially created a scene of disaster like never before — with the coupling of an existing pandemic with a high-intensity super cyclone. Until Saturday, the death toll lay at 86 with lakhs of people left homeless while the authorities continue to assess the quantum of devastation that was unleashed. As the winds wreaked havoc and the administration looked helplessly at an unfolding natural calamity, the mangrove ecosystem of the Sundarbans could have absorbed a part of the force to considerably reduce the intensity of the storm.
Amphan began making landfall near Sagar Island on the western periphery of Sundarbans in the late afternoon of 20 May. The wind speed was in the range of 155 -165 kmph gusting to 185 kmph. At the time of landfall, the diameter of the eye was about 40 km while the front and rear side had a width of 120 kms each. The impact of the cyclone manifested in two ways — high-velocity wind (which is self-explanatory) and a storm surge induced flooding. A storm surge is an abnormal rise in sea level generated by a cyclone or other intense wind formation, which is over and above the predicted or normal or astronomical tide. According to the storm surge estimation based on IMD track forecast at 03:24 IST on 20 May, parts of the South and North 24 Parganas districts, Howrah and East Medinipur districts were expected to see a storm surge of 0.5 to 4.6m with the extent of inundation being up to 15 kms inland from the coast.
On the night of Wednesday, the cyclone approached land from the south-southwest direction and moved in a trajectory that was headed to north-northeast direction. This meant that the cyclone, by and large, passed tangential to the forested area of the Sundarbans. However, despite the cyclone not directly hitting the wall of the mangrove, the extended region comprising of spiral bands of the cyclone could have interacted with the shield to its right, thereby facing resistance. The bottom right map shows the air circulation in real-time from modeled data (sourced from windy.com) at 11:25 pm on 20 May. It can be easily inferred that despite a largely favourable pressure gradient, the wind speed is visibly lower (shown with a green shade) in the region immediately north of Sundarbans. Similarly, based on the map to the left, the force of the storm surge could have also been partly reduced by the expanse of mangroves in the Sundarbans. At present, this data can only be indicative and it would definitely require a detailed study to further substantiate this claim.
Mangroves provide a wide range of ecosystem services, one of which is the regulating service of acting as a barrier against an incoming cyclone, thereby reducing intensity of the wind and the extent of inundation from storm surges. Studies have pointed out that a wider zone of mangrove vegetation between the coast and the habitations in Odisha helped in restricting the damage and deaths that were caused during the super cyclone of 1999.
The mechanism of this protection to moderate the impact of powerful winds and the relentless storm surges is both direct and indirect. Directly, mangroves provide additional drag through their trunks, leaves and pneumatophores which reduce the wave energy, while dense canopies help to attenuate the energy of the wind. Indirectly and over a considerably longer period, mangroves’ roots aid in sediment accretion and counteract the erosional forces at work to ultimately expand the coasts and add to the net stock of buffer against cyclones. This storm protection service of a mangrove forest is often undervalued and ignored. According to a research paper that empirically assessed the damage avoided during cyclone Sidr in southwest coastal Bangladesh in 2007, loss incurred per household was reduced by half in areas protected by mangrove and polder.
The indomitable human spirit will surely reign supreme and it is only a matter of time before the restoration work begins in full capacity. However, as this process is initiated, it is important to actively consider the agenda of conserving this natural shield against cyclones, the intensity of which is likely to increase with warming oceans.
Cyclones are a regular occurrence in the region and contrary to the commonly held perception, the mangrove ecosystem is quite resilient to these momentary disturbances. However, when combined with other threats, most of which have an anthropogenic origin, the sustenance of the ecosystem can indeed be found wanting.
Several local, as well as regional challenges, exist that threaten the health of the ecosystem. Local challenges in the Indian Sundarbans include widespread poverty with low human development measures — 34 percent of the population is below the poverty line and 47 percent live with some food shortage. The dependence on forest has also increased and biotic pressure along with unsustainable exploitation of forest resources has led to further degradation. Regional and global issues include sea-level rise (6 times more than of the global average) with certain parts experiencing rapid erosion of land, human-induced hydrological alterations in the upstream and consequent ‘sediment trapping,’ discharge of untreated domestic and industrial effluents and so on.
Relief and restoration works have already begun. The chief minister of West Bengal has already announced Rs 1,000 crore fund for restoration work. However, in the long run, the development of the region should be attuned to ensuring the ecological security of the mangrove ecosystem. The policies for redevelopment should be aligned towards the larger goal of reducing stress on the ecosystem, as the infrastructure is rebuilt. The shield of mangroves that continues to take the pounding of incoming cyclones, can only recuperate if the existing threats are reduced at all levels.
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Sayanangshu Modak was a Junior Fellow at ORFs Kolkata centre. He works on the broad themes of transboundary water governance hydro-diplomacy and flood-risk management.Read More +