Terracotta silt-traps being installed by WWF-India at a site in the Sundarbans

Living Shoreline

A nature-based solution to protect the Sundarbans

By 2050, even if the global community curbs their greenhouse gas emissions and meets their mitigation pledges, slow-onset climate-induced stressors are projected to force 4.5 crore people across India [1] to migrate from their homes. If all the people being pushed to migrate made up a country, it would almost be equivalent to emptying out the entirety of Spain ^[2].

India makes up less than 0.25% of the world’s total coastline, however about a tenth of the world’s total coastal population lives on these Indian coasts. Parts of this 7,500-kilometre-long coastline, particularly the northern parts of Bay of Bengal, are at the forefront of experiencing the gradual and incremental impacts of climate change such as sea levels rising, reduced agricultural yields, water stress, and ecosystem loss.

The Sundarbans’ coasts are frequently subject to extreme weather events. Video by WWF-India

A dynamic coastal ecosystem

One such vulnerable ecosystem characterised by a fragile ecosystem, but also by a resilient people, flora, and fauna, is the Sundarbans—a delta, a forest, and a cluster of islands.

Photographs by Shuvarthi Guha, Sunit Kumar Das, and Debmalya Roy Chowdhury

Continuous deposition of silt brought down by the Ganges, Brahmaputra, and Meghna rivers flowing into the Bay of Bengal have created the world’s largest delta. The region is also known as the world’s largest contiguous mangrove forest.

A Sanctuary for Vulnerable Wildlife

Known for its diverse habitats on land and water, the Sundarbans house hundreds of aquatic, terrestrial and amphibian species. Among them are the endangered and critically endangered species, like the Irrawaddy dolphin, Ganges river dolphin, Royal Bengal tiger and Northern river terrapin, which are the most susceptible to changes in their unique home.

Sources:
Northern River Terrapin Returns to Sundarbans by Shailendra Singh & Jones Justin S., All India Tiger Estimation - 2022 by MoEFCC, Irrawaddy and Ganges river dolphins’ populations from River Dolphins at Risk by WWF International

The Sundarbans Biosphere Reserve is divided into mangrove forests and human inhabited areas. While the forests are home to diverse species of mammals, reptiles, birds, and plants, the inhabited areas house 45 lakh people. They form a microcosm of coastal communities vulnerable to climate change.

Threats amidst the beauty

The inhabitants of Sundarbans primarily depend on natural resource-based livelihoods, like agriculture, aquaculture, honey collection and fishing. The slow-onset processes, like sea level rise and processes of erosion and accretion, and rapid-onset events, cyclones, can lead to the loss of lives, land due to inundation, livelihoods, assets and food and water security. Thus, due to their exposure to such events, the local inhabitants of the Sundarbans are vulnerable.

The warming of the Indian Ocean has resulted in increasing extreme events along the Indian coastline, with intensifying cyclones. In the past five years, three cyclones namely Bulbul (2019), Amphan (2020) and Yaas (2021) have occurred. As per the India Meteorological Department (IMD), all three cyclones fall in the category of severe and above.

A 2022 study found that South 24 Parganas district in West Bengal, where much of the Sundarbans is located, faces the most frequent cyclones among all Indian districts. The India Meteorological Department in Pune analysed cyclone patterns and determined that, on average, cyclonic storms affect this district every 1.67 years.

The imperceptible threat of rising sea levels, and thus increasing salinity in the ground makes life difficult to sustain.

Sadly, the last beneficiaries of the carbon economy will face the sharpest brunt of climate change.

A geographically fragile zone

Banking on embankments

It is embankments that let human life thrive adjacent to a dramatic sea. Embankments are concrete or earthen structures that make human settlements possible in the Sundarbans. Such structures protect inhabitants from the menace of floods and “brackish water ingress”. The embankments provide security to life and all agriculture -based livelihoods.

Embankments offer protection through prevention. The breach of an embankment starts a downward spiral for the people of the Sundarbans. Farmlands become uncultivable, forcing people to look to the surrounding forests for resources putting stress on the forest and increasing the risk of human-wildlife negative interactions. It also creates a dependency on informal labour migration. Both have adverse effects in the long run.

The triple threat of cyclones, erosion, and the rising sea looms on these life-sustaining embankments, while time-induced decay takes an additional toll on the 1,800-kilometre system of embankments that protect the Sundarbans.

Living shoreline

In this light, WWF-India, in May 2022, started working on a way to enhance the longevity of the embankments in human-inhabited islands. This took the form of creating a living shoreline to offer protection to existing embankments.

Silt-trapping Stability

To foster sediment deposition, 8,820 terracotta silt-traps were installed foreshore of the embankment, in a total area of 40,903 square feet. These structures capture and hold the sediment. Such structures, thus, have the potential to allow vegetation to take root and contribute to the eventual stabilisation of the shoreline. Terracotta was consciously used to ensure that the natural ecosystem remains undisturbed.

Installation of terracotta silt-traps. Video by WWF-India

WWF-India designed a monitoring plan to check the success of sediment capture. The strategic implementation has demonstrated success.

Increase in deposition is beneficial for coastal ecosystems

Recorded rate of sediment deposition at seven living shoreline sites in the Sundarbans

Source: WWF-India

SEDIMENT DEPOSIT AND VEGETATION GROWTH AT LIVING SHORELINE SITES

Indrapur, G-Plot

Rakhal Majhir Ghat, Kumirmari

Source and image credits: WWF-India

This nascent-stage scientific study is being carried out at seven different pilot sites, where silt deposition will continue to be monitored. Future plans include collaboration with the Irrigation and Waterways Department, GoWB, to evaluate the model for efficacy in terms of elevation recovery and stabilization of the embankment across the seven pilot sites. Depending on the efficacy, this Nature-based Solution can be adopted at large and incorporated into the Sundarbans Master Plan for Integrated Delta Development.

Sedimentation Outpaces Sea Level Rise

In the battle against coastal erosion and the threat of sea level rise, sediment deposition along the embankments offers a promising solution, outpacing the increase in sea levels. With effective sea-level rise in the Sundarbans measuring at approximately 6.4 mm per year, and silt accumulation (which provides elevation) at a slightly higher rate of around 6.92 mm per year, sedimentation can be a vital counterbalance.

The first vegetation to take root on the accumulated silt is Dhani (Oryza coarctata), a wild species of rice that thrives submerged in high saline water. Once this, and other grasses, manage to steady the soil, it makes room for bigger, sturdier mangroves.

By successfully retaining sediment, natural vegetation growth is expected to encourage and foster the development of a resilient living shoreline along human-inhabited islands. Through the strategic fostering of sedimentation, coastal communities can be protected from the relentless onslaught of rising sea levels, ensuring the security and stability of their habitats, hopefully, for generations to come.

Sources:

^[1]Costs of Climate Inaction: Displacement and Distress Migration, Climate Action Network South Asia

^[2]Population, total - Spain, The World Bank