One Ring To Save Them All: Terracotta Rings For Mangrove Regeneration

In the fantasy novel, The Lord of the Rings, the inscription “One Ring to rule them all, One ring to find them, One ring to bring them all, and in the darkness bind them” described the malicious golden one ring that shook the Middle Earth.

But the one ring does not exist in the reality of India’s eroding wetlands, and instead of orcs, humans are responsible for much of the destruction.

Nonetheless, there is hope that rather than a golden one ring, a humble terracotta ring could change the fate of the Sundarbans wetlands by being “A ring to trap the silt and soil, A ring to green them, A ring to bring them all and in the embankments bind them.”

Arrangements of large terracotta rings, ranging from two to four feet in diameter and standing at about a foot or two in height, may help save sinking islands in the Sundarbans Biosphere Reserve. These rings, installed in grids measuring 92 metres in length along the embankment, and about 6.5 metres wide along its slope, are silt traps.

Over a 16-month period, these silt traps have had some interesting impacts on the embankments as reported in this study that recorded observations of the experiment conducted between 2023 and 2024. The rings have captured and retained sediment, reducing the erosion of the embankment. The net sediment accumulation in these traps was two to three times the rate of annual sea level rise, which makes them an effective and relatively cheap solution to the problem of island sinking in the Sundarbans. As an added bonus, the trapped silt was found to support the growth of local mangrove plants and oysters, raising hopes that these structures could aid mangrove regeneration and form a nature-based living shoreline to protect embankments from toe-line erosion.

Why terracotta rings?

“Originally, the idea was to set up vertical tubes in the foreshore area. During high tide, these tubes should get filled with sediment and settle into the slope of the embankment to naturally reinforce the embankment,” says Anamitra Anurag Danda, Director, Sundarbans Delta Programme, World Wide Fund for Nature (WWF)-India. “So, we tried bringing in large-diameter iron pipes, used for laying water pipelines, and installing them in front of the embankments,” he says.

Previously, vertical rods 10-25 metres in length, known as surface elevation table marker horizon set or rSET-MH rods, have been planted in the Sundarbans to measure surface elevation changes. However, inserting hollow pipes of large diameters is very different from planting thinner pipes like the rSET-MH rods.

“Installing those wrought-iron pipes vertically was a challenge we could not surmount. That was one of our first failures in this project,” says Danda. “But then, Sugata Hazra, who was formerly the Director of the School of Oceanographic Studies in Jadavpur University, came up with a wild idea – he asked, why not use terracotta rings? These rings have been used since ancient times to line wells and sewage systems and can be easily and locally made,” he adds.

Ancient ring wells, unearthed during archaeological digs, have been used to store water, where they functioned as silt traps to keep sand out and filter water. In this project in the Sundarbans, the concept of silt trapping by the rings has been employed to retain sediments that can naturally reinforce embankments and offset sea level rise.

The team also drew inspiration from a project in Bangladesh, which used concrete rings that promoted oyster colonisation to form breakwater reefs. These artificial reefs have helped reduce erosion, stabilise the adjacent mudflat, trap sediment, and aid seaward expansion of the area’s saltmarsh.

Preliminary observations and a surprising find

The terracotta silt trap instalments were placed in seven sites within the Sundarbans Biosphere Reserve – three in the lower estuary and four in the middle estuary between June 2022 and February 2023. All these sites were near human habitations but lacked any vegetation.

The silt deposition in the traps was checked every month on the day of the highest high tide, for 16 months (between May 2023 and August 2024). The traps designated to be monitored had tin sheets placed at their bottoms, and the height of the deposited sediments was measured suing a steel ruler inserted into the trap till it touched the tin sheet. Sediment deposition on average across all the sites over this period was 22 cm. However, these values varied hugely, ranging from four cm to 42 cm between sites and across seasons, with peaks during the pre-monsoon and drops in the post-monsoon periods.

Net sediment accumulation at the end of 12 months of observation ranged between 1.4 cm to 16.5 cm. The lowest sediment accumulation was observed at sites 1 and 4, which were shielded by a sandbar and islands, respectively. The highest sediment accumulation was observed in site 2 in the lower estuary. Fairly high rates of accumulation were observed at site 3 and the easternmost sites (sites 5-7). Granulometric analysis of the accumulated sediment showed that the sites near the estuary mouth (sites 1-3) had more sand than clay or silt, whereas those areas farthest from the estuary mouth had clay-dominated sediments. These patterns of sediment composition in the silt traps were similar to those observed in the natural surroundings.

An unexpected but welcome result of this project was the colonisation of the silt traps by natural mangrove vegetation. The sites with sediments rich in clay (sites 3–6) showed natural growth of  Porteresia coarctata, Sueda maritima, and Avicennia marina. The best growth was seen in sites 4–6 in the middle estuary. Site 1 was colonised by oysters, predominantly Saccostrea cuculata, along with some Crassostrea cuttackensis. No colonisation by either mangrove plants or oysters was observed in sites 2 and 7.

While restoration attempts in degraded mangrove ecosystems through dense planting and site-specific strategies has shown good results, early attempts at planting saplings for this project had failed.

“This really was quite a pleasant surprise for us, because initial attempts to plant mangrove saplings in the rings were failures. None of them survived. At any of the sites. So that was money, time, and effort that was spent unwisely. All the vegetation we see now has arisen naturally,” says Danda.

Hurdles to success

When asked about the problems in the execution of the project, Danda points out that the idea of using terracotta rings as embankment protection is entirely novel. Therefore, the project faced many challenges.

“Whenever you’re doing something that hasn’t been done before, you’ll make mistakes and there’s constant learning. One of the first problems we encountered was in transporting of the terracotta rings. They tend to be brittle and bringing them from the manufacturer in Howrah, to the islands in the Sundarbans, was a logistical challenge,” says Danda. Nevertheless, this challenge was successfully overcome and now the loss of rings in transit has been minimised.

“Another challenge was the human element. People were extremely sceptical of this idea and getting the required permissions to install these rings was an uphill task,” he adds.

Danda also explains that this is only a pilot study and is still at the very initial stages of validation. “These installations were an elevation gain experiment. We wanted to see if silt trapping could offset the sinking of the islands in the delta because of rising sea levels, and if the accumulated sediment could support mangrove growth. While our results are encouraging, longer-term observations are still required to test the viability of this model. Also, this model hasn’t really been tested under extreme conditions as we’ve not had a major cyclone since the rings were installed. So, we don’t really know how robust the system is when faced with an extreme weather event,” says Danda.

“If we zoom in on the seven experiment sites of the work on Google Earth using the coordinates given in the paper, all of them are located on mudflats, with no evidence of coastal erosion or embankment retreat. This means that the ring structures will work well in stable banks of the estuary to promote sedimentation,” says Sunando Bandyopahyay, Professor of Geography, University of Calcutta. “These will probably not work in the eroding and retreating sectors of the tidal islands, where the marginal embankments are under real threat. This limitation of the method – that the rings are not likely to promote sedimentation along the eroding sectors of the estuary banks where it is really needed – cannot be resolved easily. This idea will work fine on stable coasts but will get obliterated if that particular stretch turns erosional in future,” he adds. Bandyopahyay further warns that although mangroves may delay coastal or creek-bank erosion, they cannot completely stop the process.

In addition, the variability of sediment accumulation and composition in the lower estuary due to high-energy sea waves and the tidal bore, adds a large element of uncertainty to any undertaking aimed at preserving embankments.

Krishna Ray, from the Ecological Restoration Alliance and the Department of Botany, West Bengal State University, who has worked extensively on restoring degraded mangroves, agrees. “In the Indian Sundarbans, I have seen that a hybrid approach involving engineering and nature-based approaches involving ecological restoration of degraded mangrove habitats works well in protecting shorelines. For example, natural mangrove belts existing in front of concrete embankments are successful. However, when it comes to eroding shorelines, the Sundarbans have many exceptions due to their extremely dynamic nature; the success of a project is entirely site-specific. Something working in one site may not always work in another site and no definite rule of thumb exists for this region,” she says.

Cost implications and what the future holds

Currently, the Irrigation and Waterways Department (IWD) uses soil-filled gunny bags and Eucalyptus poles to protect eroding parts of islands in the Sundarbans. The IWD personnel estimate that the cost of creating such structures measuring a kilometre in length and two to three metres in width would be approximately $100,000, with an additional 12% cost for annual maintenance. But these structures do not support silt retention, and the soil-filled gunny bags often get washed away.

On the other hand, constructing a one km-long and 6.5-metre-wide terracotta ring-based silt trap costs around $50,000, which is roughly half that of the budget that is now being spent to protect embankments.

“The cost of the terracotta silt traps is considerably lower and requires no maintenance. Despite a 6-12% breakage rate of the rings after instalment, there is no need to replace these rings as they function just fine within the structures. Now what remains to be seen is if these structures can extend the lifespans of existing embankments, which currently last for an average of 10 years,” says Danda.

While monitoring of the current silt traps will continue, new ideas for other installations using these silt traps are already being explored.

“When we highlighted the success of our installations in elevation gain, one of the local foresters asked us why this idea couldn’t be used to hold on to a piece of land that is eroding away? So, we created a bamboo frame within which we put in these rings. This could work as an anti-erosion structure. Right now, we have two such installations of bamboo and terracotta rings – one in the lower estuary and another in the middle estuary – the idea is to stop or at least slow down erosion,” says Danda.

(Published under Creative Commons from Mongabay India. Read the original article here)