Erosion, an important cause of mangrove loss in the Sundarbans

As officials assess the damage caused to the Sundarbans mangroves by cyclone Amphan, a new study says the ecologically fragile region in India and Bangladesh has lost 24.55 percent of mangroves (136.77 square km) due to erosion over the past three decades.

Mangrove forests, nature’s buffer against disasters, are imperiled by unregulated coastal development, shrinking of deltas and climate change linked extreme events. The degrading health of mangroves affects their resilience and recovery potential against climate change consequences like sea-level rise.

In the Bay of Bengal delta, reduction in sediments due to upstream dams (human-induced) and increased wave action due to storms (natural) are the two major drivers of permanent loss of land to water in the Sundarbans, said study author Radhika Bhargava.

“The loss of land due to erosion is leading to a direct loss in the mangrove forests,” Bhargava told Mongabay-India, adding most of the erosion is permanent in the Sundarbans (24.55 percent) as compared to smaller temporary (seasonal and ephemeral) losses.

“This number (24.55 percent) only refers to losses due to erosion, there might be other causes of mangrove cover change during that time (like deforestation) which these statistics do not account for,” said Bhargava of Department of Geography, National University of Singapore.

Cloud computing can be applied to track mangrove loss from Amphan

The study is based on the use of a cloud-based platform that uses machine learning techniques which makes the specific classification of permanent versus temporary losses of the shoreline more efficient and accurate.

And the cloud-computing based method can be used to map mangrove loss due to erosion caused by the extreme weather event (cyclone Amphan). “We can easily run the method on new sets of satellite images taken after Amphan to estimate change. The loss would be in terms of mangrove loss due to erosion caused by the extreme weather event.”

The Sundarbans comprise a cluster of small low-lying islands (less than five metres in height) in the Ganga-Brahmaputra-Meghna River delta. Home to the royal Bengal tiger, it is the world’s only mangroves where tigers reside.

According to India State of Forest Report (2019), West Bengal has 42.45 percent of India’s mangrove cover, followed by Gujarat (23.66 percent) and Andaman and Nicobar islands (12.39 percent). The mangroves in Bengal are spread over an area of 2112 square km across South 24-Parganas (2082 sq km), North 24-Parganas (25 sq km) and Purba (East) Midnapore (4 sq km). The report records a loss of 1.89 sq km in mangrove cover with respect to the 2017 assessment.

The Sundarbans region in India is administered by two West Bengal districts: North 24-Parganas and South-24-Parganas. G. Santhosha, Divisional Forest Officer (DFO) of South 24 Paraganas Forest Division, West Bengal said the entire 1660 square km of the forest has been destroyed in that area by cyclone Amphan.

In Bangladesh, the Sundarbans sustained three times the damage by cyclone Amphan as compared to cyclone Bulbul, officials said. The mangrove species ‘goran’ (Ceriops decandra) were the most impacted by the cyclone.

“Based on my fieldwork experience in Satjelia island right after Cyclone Bulbul in November 2019, I interviewed a few community members who lost their houses to erosion overnight. They showed where the coastline was five years ago and a day before the cyclone and then we were standing at the edge of the shore a week after the cyclone had passed, the shoreline retreat overnight after the cyclone was equal to the amount of retreat in five years,” recalled Bhargava.

Such changes, she said, can be monitored using the methodology presented in their paper. Some of these changes can be permanent losses while some can be temporary flooding which can be differentiated using their method.

Bhargava observes that several islands in the entire Sundarbans are eroding- to the point that some islands are completely lost to erosion. The losses are happening in both India and Bangladesh.

“Apart from permanent loss in land, there are also seasonal and ephemeral losses which can signify long term flooding of islands. The pattern of erosion is along the channels in the central and northern parts of the Sundarbans but in the southernmost parts the erosion is happening in large amounts along the shoreline. In the eastern and western parts, progradation (gain in land) can be seen,” she said.

As for progradation, the study finds that 12.52 percent of land area increased over the past 30 years due to progradation or settlement of sediments by the shoreline.

“Although erosion leads to a direct loss in mangrove forests, we cannot say that progradation leads to an increase in mangrove forests (although it leads to an increase in land). It is important to understand that although progradation forms new land, it takes several years for mangroves to settle and grow on the new land. So, the gain in land can or cannot promote an increase in mangrove forests,” Bhargava said.

Ecosystem, if undisturbed, can balance periodical erosion and accretion

In deltaic mangroves like Sundarbans periodical erosion and accretion are internal dynamics and it can be balanced by the ecosystem itself if the system is undisturbed said mangrove scientist P. Ragavan of Council of Scientific and Industrial Research (CSIR), National Botanical Research Institute.

“However, an upstream anthropogenic activity like damming diversion of the waterways leads to an imbalance in erosion/accretion cycle and subsequently causes collateral damage to the ecosystem. Increasing erosion in Sundarbans is attributed to reducing sediment supply as most rivers are drained or do not supply required inputs,” Ragavan told Mongabay-India.

“Construction of macro to micro-level dams retain maximum sediments and water flow which causes the shrinking of river beds. It slowly destroying the river morphology and ecological settings over Ganga Brahmaputra Meghna delta where Sundarbans mangroves lie,” said Subha Chakraborty, GIS specialist at Indian Institute of Engineering Science and Technology.

“Climate change impacts such as the continuous increase of sea level, changes of coastal longshore current, changes in wind speed are another key factor of high erosion activity over the area,” said Chakraborty.

In the case of extreme events like cyclones and tsunamis mangroves are highly resilient to the damage caused.

In 1999 Odisha supercyclone, mangroves significantly reduced the number of human deaths, according to a study on Kendarapara, one of the worst-hit coastal districts in Odisha. According to a 2018 study on storm protection service of the Sundarbans mangrove forest in Bangladesh, villages that were not protected by mangrove and polder (low-lying area reclaimed from a body of water by the construction of dikes) incurred the highest loss per household from 2007’s cyclone Sidr.

However, the frequency and intensity of extreme weather events (typhoons, cyclones, and hurricanes) are predicted to increase in the future and the Bay of Bengal region has seen a 26 percent of intensification in cyclonic activity in last century.

“Rapid sediment input and nutrient pulses that occur during intense storms have the potential to maintain soil elevation in the face of sea-level rise and stimulate productivity and growth. However, physical damage caused by the increasing frequency and intensity of extreme events has the potential to offset the resilience and recovery of mangroves,” Ragavan said.

A  satellite-based study last year also pointed to the evidence of a decline in the health of about 25 percent of the mangrove trees in the Sundarbans. Authors said those areas will be more exposed to harm in the future, especially if extreme events such as cyclones become more common.

Basically, increased wave action due to increased storminess (like in the case of Amphan), sea-level rise (due to global climate change), and coastal squeeze (land cover change from construction, aquaculture ponds etc.) that reduces the width of the mangrove forest leaving less space for mangrove forests to grow inwards leads to increased erosion, iterated Bhargava.

“This erosion leads to degradation of mangroves. But if the mangrove stretch is already degraded due to reduction in sediment inflow (upstream dams), tree felling, and pollution then an already eroding shoreline goes through further cycles of erosion causing excessive loss.”

“Now, in the case of Sundarbans, some of the ‘plausible’ causes of permanent loss of land to water can be attributed to increased storminess (natural) and reduction in sediments due to upstream dams (human-induced). Although other drivers as mentioned above also contribute, it is likely that these two drivers cause the majority of the pressure,” explained Bhargava.

Restructuring protected areas of mangroves

Sundarbans mangrove is under tremendous pressure and despite the legislative protection of the major portion of Sundarbans, it is obvious that the sustainability of Sundarban mangroves is decreasing, stressed Ragavan, adding that concept of protected areas of mangroves needs to be restructured and even a small patch of mangroves should also be strictly protected.

“Particularly, the rivers – the roots for the stability of coastal wetlands – must be declared as protected areas and their peripheral bank areas should be declared as an eco-sensitive zone/buffer zone for immediate action to halt further developmental activities and minimise the adverse effect of existing activities.”

For collaborative management of Sundarbans, uniform methods are needed

Sundarbans in a single ecosystem divided by a political border. But it is managed as two separate units by India and Bangladesh.

Erosion is an issue faced by both sides of the Sundarbans and the large-scale drivers of erosion (like extreme weather events, upstream dams) are also affecting both sides.

“For collaborative management, uniform methods are needed for the evaluation of the ecosystem and the estimation of losses. So far, the methods used to estimate erosion in the Indian Sundarbans were not uniform with the methods that other studies have used to monitor erosion in the Bangladeshi Sundarbans,” noted Bhargava.

“This discrepancy causes further gaps in the implementation of uniform policies by two different countries. Our paper provides a method that estimated erosion threat for the entire Sundarbans in a uniform manner over a long time. Thus, the outputs of our paper can inform cross border management of erosion losses in the Sundarbans,” she added.

Banner image: Mangroves line the islands in Sundarbans. Photo by Kartik Chandramouli/Mongabay.