Part One: ‘Living Shorelines’ and Other Plans To Protect the World’s Coasts

A living shoreline project at the Trinity Center in Pine Knoll Shores, North Carolina
The Wire Science
Australia
Part One: ‘Living Shorelines’ and Other Plans To Protect the World’s Coasts

About 40% of North Jakarta sits below sea level, with some models estimating that 95% of North Jakarta could be fully submerged by 2050. Bangladesh loses 34 square kilometers of land annually, which may increase further with sea level rise.

In Odisha, India, the advancing sea has turned the people of Kendrapara district into coastal-erosion refugees. More villages are at risk of submersion.

A 2018 national assessment on shoreline changes by the National Centre for Coastal Research found that nearly 33% of India’s coastline is under erosion, while 29% is accreting (growing). Erosion is highest in West Bengal, with 63% of the state’s coastline eroding, followed by Pondicherry (57%), Kerala (45%) and Tamil Nadu (41%).

Coastal erosion was one of the major issues that an international group of scientists and ecologists discussed at the 6th International EcoSummit, held in June at Gold Coast, Australia.

In particular, the focus was on nature-based defense. While nature-based methods of tackling coastal degradation such as planting mangroves is not new—Bangladesh’s mangrove afforestation program dates back to the 1960s—in recent years, a newer approach combining ecological engineering with nature-based solutions has come about. These hybrid solutions strengthen natural habitats with hard elements such as living shorelines or artificial oyster reefs.

Living shorelines

In the United States, nearly all coastal states encourage living shorelines, said Donna Marie Bilkovic from the William & Mary Virginia Institute of Marine Science.

Unlike a hard structure like a concrete seawall, a living shoreline is made of natural materials such as plants, sand or rock. The primary advantage is that these do not impede the growth of plants and animals, allowing these shorelines to grow over time. The earliest adopters of living shorelines were the states of Maryland and Virginia in the 1980s.

Bilkovic said that living shorelines perform as well as marshes in supporting fish, grass, shrimps, blue crabs, terrapins and shorebirds. In particular, living shorelines serve as excellent primary nurseries for young juvenile fishes.

Living shorelines can also aid the growth of mangroves.

While planting mangroves effectively slows coastal degradation, they may not survive the planting stage at all coastal sites. So, methods to protect newly-planted mangroves have been attempted.

In New South Wales in Australia, living shorelines with rock fillets—a rock wall built in front of the eroding bank—have been attempted to reduce wave energy and facilitate the growth of mangroves along the shorelines.

Sophie Cheuk Yan Chan from the University of Melbourne is studying living shorelines more than a decade old. She looked at historical aerial images to measure shoreline and mangrove cover changes. She found that mangroves successfully established themselves at most living shorelines. However, erosion was more variable and highly dependent on site characteristics.

While the Indian government has been pushing hard on mangrove afforestation programs—the 2023 to 2024 budget announced a new scheme for mangrove plantation along the coastline—some of these programs have been plagued by failure as mangrove saplings are unable to survive after the planting stage. A 2020 expert committee report on plantation programs in the Sundarbans concluded that "mangroves cannot germinate and hold ground in open, eroding sections of the coast."

A layer of protection using hybrid solutions to protect mangrove saplings could work here, as the experience of Australia shows. One pilot project between 2014 and 2018 in which native salt-tolerant grasses were planted in the Sundarbans did manage to stabilize the soil for mangrove plantations.

But the success of these programs is highly site specific.

A team of researchers for the University of Melbourne School of Biosciences used pods made of concrete—one in a hexagon shape and another shaped like a waist—to protect the mangroves from wave actions. In trials at three sites with no existing mangroves, seedlings had higher survival in pods, but ultimately survival was meager at most locations. There was some success at one place in Singapore.

Amanda Hsiung, one of the researchers involved, said, “we find that mangroves are really vulnerable to different conditions at different life stages. It is really hard to grow mangroves where they don’t want to grow.”

Artificial oyster reefs

Another nature-based solution that is being used increasingly are oyster reefs. These reefs, quite numerous in India, are natural breakwaters that can protect the shoreline from erosion. By combining with hard engineering elements, these natural habitats can sustain a broader range of energy conditions.

The effectiveness of reefs in preventing erosion can be significant. They are also adaptable to sea-level change.

In one example, a team from the Wageningen Marine Research in the Netherlands found that erosion behind artificial oyster reefs was only six centimeters in nine years. In comparison, erosion around the reefs was 25 centimeters—a nearly four-fold increase.

Location is key to the success or failure of artificial oyster reefs, said Brenda Walles, who was part of the Wageningen research team.

“Location determines the time that the reefs spend underwater as the tides change, and this can affect growth. In one example, of two nearby reefs, the higher one did not survive while the lower one developed healthily.”

Oyster reefs have also shown promising results in Bangladesh for mitigating coastal erosion. In collaboration with the Wageningen Marine Research, a team from the University of Chittagong, Bangladesh, constructed three oyster breakwater reefs on Kutubdia Island. They placed 80-centimeter-high concrete rings in the intertidal zones, which get occupied by oysters.

The team found that the reefs successfully weakened waves and halted erosion. Moreover, these reefs also facilitated the expansion of salt marshes, which further stabilized the shoreline, said Walles.

Following the success of this study, the Bangladesh government is looking to scale up the use of oyster reefs to protect newly built dams being washed away by tidal waves.

Oyster reefs have also been combined with living shorelines. However, design plays a vital role in hybrid living shorelines, said Rebecca Morris, a researcher from the University of Melbourne, who was part of a study of oyster reefs on the east coast of the United States, ranging from New Jersey to Louisiana. Here, very few oyster reefs could attain both their ecological and engineering goals, she said.

“In some cases, they were good for wave attenuation but not suitable for oysters. In other cases, they were not good at attenuating waves but provided a good habitat for oysters.”

“The main take home is that the height of the reef compared to water level is important,” said Morris.

One of the significant challenges is that shellfish take time to establish and, in some cases, struggle to attach to the hard substrate, said Brendan Lanham, a researcher from the University of Melbourne School of Biosciences. “If we want to grow an oyster reef in the bay, we can’t just put a substrate into the water and hope it will get covered in oysters. They do recruit naturally but in very low numbers.”

Lanham was part of a team of researchers from the University of Melbourne School of Biosciences looking to design concrete reefs that can increase shellfish numbers. One set of trials looked at changing the surface roughness by sandblasting with regular aggregate as well as oyster aggregate.

The team found that regular sandblasting increased mussel settlement by 2.5 times, while oyster shells increased settlement by four times. Oysters also showed a similar increase, with a 3.5 times increase with sandblasting and five times increase with oyster shells.

Another set of trials looked at increasing the complexity of the substrate by introducing more ridges and crevices. While mussels showed no change, the oysters showed eight to 20 times increased settlement.

Reversing man-made changes

Other forms of nature-based solutions involve partially reversing man-made changes.

In Nova Scotia, Canada, dykes drain salt marshes and convert them into agricultural land. Initially built in the 1600s by French settlers, most contemporary dykes are from the 1950s. But dykeing has caused enormous damage to tidal wetlands.

Dykes disrupt the geomorphological processes that shape the coast, said Lara Cornejo, from Dalhousie University, Canada, who studies dyke lands in the Bay of Fundy.

“The consequences of these disruptions are increasing with climate change. Coastal squeeze, land subsidence, and dyke overtopping due to storm surges are becoming more common.”

Strengthening dykes by making them taller is one option. However, there’s an increased push for a nature-based strategy called managed realignment. In this method, a new dyke is constructed behind the agricultural dyke, and the old dyke is breached. Water and sediments enter the farmland to create a new tidal wetland.

Cornejo, who assessed the trade-offs involved in this decision-making, said that one positive outcome was that Indigenous communities see wetland restoration as an opportunity for reconciliation. “Maintaining and restoring wetlands will protect important cultural landscapes significant to Indigenous communities.”

India’s fight against sea erosion

Home to close to 170 million people, the human cost of coastal erosion along India’s shoreline —nearly 5,400 kilometers along the mainland, and 2,000 kilometers along the Andaman, Nicobar and Lakshadweep islands—could be tremendous if climate change targets are not met.

While mangrove plantations have been the mainstay of fighting coastal erosion in India, the other nature-based solution adopted in India is Casuarina plantations. This form is quite prevalent on the east coast, particularly in the states of Odisha, Andhra Pradesh and Tamil Nadu. But some researchers have warned that these plantations are not particularly useful in combating erosion, and can have adverse effects on the local ecosystem.

But the overwhelming way to fight coastal degradation in India are still hard engineering structures. A report by the Central Water Commission to deal with coastal erosion in 2020 notes that seawalls have been the predominant method of protection. West Bengal began constructing rubber mound seawalls to fight erosion back in the 1970s. In Kerala, 378 kilometers of seawall have been constructed along the coast. These have been the only coastal protective structure adopted by the state since 1964. Other states like Karnataka (34 km), Tamil Nadu, Goa and Odisha have also used seawalls on several stretches of the coast.

One alternative to seawalls that has been attempted is groynes in a handful of states like Tamil Nadu, Pondicherry and Kerala. Groynes are rigid structures built perpendicularly to the ocean shore and seawall constructed from rock to catch as much sand as possible to prevent erosion. For instance, following the construction of a new harbor, the Puducherry government built groynes to tackle coastal erosion. But their utility has been questioned. One report found that these measures were not providing any protection at all, and in fact they were making the problem worse.

Puducherry also experimented with artificial reefs in 2018 by submerging a 900-ton steel cassion (pipes) 2.5 meters below sea level to reduce wave activity.

But outside of mangroves, attempts with soft-engineering solutions like nature-based solutions are few. The CWC report notes the lack of technical input to resort to such alternatives for state governments. “Hard engineering usually result in long term changes in coastal morphology, particularly erosion alongside protected areas,” the report says. "Soft engineering if designed properly is an environmentally friendly approach which works towards providing a dynamic equilibrium at the coast whereby erosion is kept to minimum."


This story was supported by Internews' Earth Journalism Network. It was first published in The Wire on July 17, 2023 and has been lightly edited for length and clarity. 

Banner Image: A living shoreline project at the Trinity Center in Pine Knoll Shores, North Carolina / Credit: North Carolina Coastal Federation.

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