Drowning forests

The rising water levels in the mangroves of Sunderbans has created an imbalance in the area which is otherwise blessed with a unique species diversity. The reason for this is unplanned aquaculture which needs to be immediately checked, writes Atula Gupta.

The impermeable floating jungles on the seashores of India and Bangladesh, Sunderbans, are both a blessing and a challenge for life to exist in the region. With an intricate network of interconnecting waterways criss-crossing the area, land has been moulded into patches of innumerable big and small islands. 

But it is also the richness of the soil and the sustainability of the ecosystem that has blessed the area with species diversity second to no other mangrove habitat in the world. The eternal battle of the sea and rivers, however, that plays out in this mangrove forest shaping its character, is lately tilting in the sea’s favour. So much is the imbalance that scientists fear the rising sea level could soon gobble up the entire eastern end of the Sunderbans.

Sunderbans, the world’s largest estuarine forest and delta covered by mangrove forests and vast saline mudflats, is situated on the lower end of Gangetic West Bengal. A land of 54 tiny islands, Sunderbans is bound on the west by river Muriganga and on the east by rivers Haribhanga and Raimangal. Other major rivers flowing through this ecosystem are Saptamukhi, Thakurain Matla and Gosaba.

A recent study by the World Bank and the Institute of Environmental Studies and Wetland Management (IESWM) has revealed that sea level rise in these areas is 3 mm annually and the sedimentation is not keeping pace with this rise.

It may look like a small number but if the deposition of sediments does not happen at the same pace as the rising sea level, the sea water may soon spread to parts of former land.

The upper Bidya and Raimangal regions in the Sunderbans are important tiger habitat, home to local villagers as well as popular tourist destinations. The upper Bidya region is hardly a 15-minute cruise from Godkhali and known for the fishing communities of Gosaba village. 

The Raimangal region — a seven-hour boat ride from Sonakhali — is one of the favourite tourist haunts for spotting a tiger. Therefore, it is easy to infer the mammoth loss of life and local economy the catastrophic sea-level rise can bring. As a precaution, scientists have suggested de-populating parts of some islands along the fringe areas. 

This will allow the existing embankments to be moved back and allow a wider tidal channel to develop. When this happens, gradually, the deposit of sediments will increase and the land rise will be at the same pace as the sea-level rise.

Threat from the sea

The physiography of the Sunderbans is distinctive and ever-changing because of the land and water interaction. Rivers here tend to be long and straight, a consequence of the strong tidal forces, and the clay and silt deposits which resist erosion. Easily eroded sand collects at the river mouths, and form banks and chars. Finer silts are washed out into the Bay of Bengal, but, where they are protected from wave action, mudflats form in the lee of the dunes.

These become overlain with sand from the dunes, and develop into grassy midden. This process of island building continues for as long as the area on the windward side is exposed to wave action.

But it is not just the water-land interaction, but the types of water that is a major influence in defining the shape and life forms of Sunderbans. The most biologically-rich areas are in the east where, because of the numerous rivers, freshwater influences are greater.

The confluence of saline and freshwater is a harbinger of life, breeding point of many marine species including fishes, and the rare Olive Ridley turtles.

However, the bad news is, with the sea level rising and the river banks not wide enough to accommodate the rising water, the land that is moulded wave after wave will soon be underwater completely.

The rivers on upper Bidya and Raimangal — Pathankhali and Jhila — need to be at least 340 and 420 metres wider respectively to withstand the impact of sea level rise, warn researchers. 

As that is not the case presently, life and homes of the villagers and the 57 Royal Bengal tigers living in the forested areas of eastern Sunderbans, namely the jungles of Jhila, Arbesi and Khatuajhuri, are both endangered.

Unplanned development

According to IESWM scientist Somenath Bhattacharya, who conducted the research between 2009 and 2012 with geologist Kakoli Sen Sharma and World Bank consultant John Pethick, the reason for this growing threat from the sea is unplanned aquaculture.

He says aquaculture has gobbled up more than 550 sq km area on the Sunderbans in the last 30 years. Also, because these involve low-intensity management techniques, more sea water has entered the creeks than was required, causing them to erode much faster.

 “We need to widen these rivers by at least 300 metres on both sides to accommodate this huge flow of sea water. And for this, there will be loss of both agricultural land and settlements,” warned Bhattacharya. Conversely, the western end of the Sunderbans portray a different picture. The western parts comprise forests of Chulkathi and Dhulibhashani, according to Sharma, and rivers like Banstala and Ghughudanga on the Saptamukhi estuarine system here are not only developing extensive mangroves, but are also capable of accommodating a future sea-level rise of over one metre without any impact on the embankment integrity.

She adds that like the popular dyke model of the Netherlands, here too, dykes or flap sluices have been made by the inhabitants to regulate the water levels. Also, the populated areas of Patharpratima and Bakkhali are almost free of aquaculture, giving them much more chances of surviving well even if the sea levels rise.

At present, 3.5 million people live within 20 km of the mangroves’ northern and eastern boundaries. This includes farmers, fishermen, and wood and honey collectors whose daily sustenance is entirely dependent on the forest. If they do want to stop their precious little land from being snatched away by the sea gods, re-embankments, dyke building, planned aquaculture and recalibrating nature’s balance in these parts are the only solutions.