How oysters build strong coastlines

Wageningen Climate Solutions

An oyster reef as a nature-inclusive solution. Photo: Marijn Tangelder

BY Hanny Roskamp - September 2019


River deltas around the world are being sorely tested by the rising sea levels caused by climate change. That means that over the next 10 years, we need to work on defences for these vulnerable coastal zones. Now nature can take over the indefatigable work of humans and machines.

More than half the world’s population lives in river deltas or vulnerable coastal regions. The question of how to protect these areas from flooding, drought and salinization has become increasingly urgent due to climate change.

One effect that we expect in delta areas is the accelerated erosion of sandbanks and foreland (land on the exposed side of dykes). This will increase the vulnerability of the coast. The Wageningen researcher Marijn Tangelder is looking at ways to let the foreland grow again. Tangelder is a marine ecologist with a background in water management. She works in a multidisciplinary team of ecologists, economists and biologists studying the consequences for the ecology of water systems of interventions in water management. Tangelder: “At the same time, we are also investigating the development of habitats for birds and other coastal wildlife.”

Eco engineering is used to protect the coast of Bangladesh. Every year, floods wash away the earthen dikes along the Bangladeshi coast. A barrier could be built in the sea to break the waves. Natural processes could be used to help build this barrier, like oysters.

WHEN YOU BUILD WITH THE HELP OF NATURE, THE NATURE ITSELF BECOMES STRONGER

Nature does the real work

A magical word in all this is ‘eco-engineering’, a field of expertise at Wageningen University & Research. Eco-engineering is an approach to building in which technology plays a very minor role while nature does the real work. Building with the help of nature not only leads to more foreland but also strengthens nature and improves biodiversity. An additional advantage is that the extra land can be combined with recreation and food production.

One example of eco-engineering is an oyster reef in Bangladesh, the most densely populated river delta in the world. Each year, about 30 to 70 per cent of the land area floods due to excessive rainfall and cyclones. The Wageningen researcher Tom Ysebaert and his colleagues created an imitation mature oyster reef 70 centimetres high using 50-metre-long hollow concrete blocks.

One year later, the concrete structure had become covered with large numbers of oysters and the damping effect on the waves had led to sand accumulating behind the reef. The oysters also grew well; some had already reached four centimetres after one year. The researchers’ calculations show that this oyster reef could grow in height by two centimetres a year. If so, it will keep pace with rising sea levels.

Photography: Shutterstock

Mangrove forests bear the brunt

But an oyster reef off the coast is not sufficient to cope with heavy storms. At very high tides, the waves wash over the reef. In the past, Bangladesh had a lot of mangrove forests along its coast that provided protection, but they have largely been felled for timber.

If it is up to the researchers, mangroves will be replanted because of their protective function. Mangrove forests and oyster reefs would then form a symbiotic coastal defence system. That is because the mangrove forests absorb much of the brunt of storms and storm surges. As a result, the earthwork dykes behind them suffer less damage, and local people and their possessions are better protected against flooding. The reefs and forests would also attract a lot of fish and crabs, which could serve as food. That is particularly important in Bangladesh as there is a lot of pressure on food production there.

ONE YEAR LATER, THE OYSTER REEF HAD ALREADY BECOME COVERED WITH OYSTERS

Oyster reef made of shells and gauze

Ysebaert based the oyster reef in Bangladesh on a reef that he and his colleagues built in the Eastern Scheldt estuary. This estuary in the province of Zeeland can be completely closed off by a storm surge barrier in the event of a storm. But that prevents sand from the sea being added to the coast by tidal flows, while the wash of the waves still carries sand away.

On the edge of the tidal sand bank, which was eroding fast because of the waves, Wageningen researchers constructed an oyster reef consisting of the shells of Japanese oysters held together by gauze. This is because oysters need something firm to attach themselves to. Once the oysters had settled there, it became a robust structure that significantly reduced the erosion behind the reef. The reef also stayed in place after the gauze disintegrated.

An oyster reef constructed in the Eastern Scheldt estuary in the Netherlands. Photo: Brenda Walles

Bio-builders’ protect life in river deltas

Oysters are called ‘bio-builders’ because of the positive impact they can have on their environment. Other bio-builders that can be used to protect life in river deltas include plants such as marram grass or the plants that grow in salt meadows (foreland with vegetation). Every time the sea washes over them, they trap a little sediment and this lets them help the land to grow.

Tangelder: “The great thing about such solutions is that they are flexible and can grow in line with sea levels. That is not the case with rigid structures such as dykes. What is more, natural solutions are much cheaper to construct and maintain than technically complex structures made of concrete. That is a big bonus for low- and middle-income countries.”

This infographic shows how the oyster reefs are located off the coast for maximum effect.

THE GREAT THING IS THAT SETUPS WITH BIO-BUILDERS CAN GROW IN LINE WITH SEA LEVELS

‘Hard’ dykes are still needed

There are also parts of the coast where bio-builders cannot be used, mainly places without forelands. In these coastal regions, the sea would be able to flow into the hinterland if it were not for the water defences (dykes). There are an awful lot of such areas around the world. ‘Hard’ dykes are still needed for them, but the land behind the dykes could also become part of the coastal zone, for example by developing so-called ‘alternating polders’. These are areas behind dykes where the dyke is low enough for the water to flow over it or that has holes the water can flow through.

Tangelder: “Every time the land behind floods, sediment is left behind so that the land rises through accretion. It grows slowly along with sea levels. So you make coastal zones more robust by letting parts of the coast get flooded at times.”

Every time the sea washes over the reefs, they trap a little sediment and this lets them help the land to grow. Photo: WUR

The next 10 years are crucial

Tangelder believes the nature-inclusive development of coastal zones has numerous positive effects. “Nature provides us with a lot of services that are not always that evident at first. Think of the purification of the water and air, added value for housing and recreation, the conservation of habitats and biodiversity, and healthy fish stocks. We call these services provided by nature ‘ecosystem services’. That’s why it is so important to incorporate nature in coastal development.”

Which solution can be applied where depends on the characteristics of an area. But Tangelder warns that we do not have much time left to find out what works best. “The next 10 years are crucial: we will have to conduct studies, then take decisions and roll out solutions. Meanwhile, the places that are already encountering problems offer us an opportunity to try out new solutions. Let’s say a dyke needs to be reinforced or some area is suffering extreme salinization; we could investigate which nature-inclusive solution we are best off deploying. And whether we could make room for a combination with saline agriculture or aquaculture.”

COCKLE DIE-OFF

The erosion of sand banks is not the only problem in the Eastern Scheldt. The summer of 2018 saw cockles dying on a mass scale in the estuary, as well as in the Wadden Sea and Western Scheldt. Marijn Tangelder and her colleagues are now investigating whether this was due to heat stress in that hot summer. “When the water recedes at low tide from the sand banks where cockles live and the sun shines on them, these molluscs can be badly affected if temperatures rise too high.”

Previous research showed that cockles die if they are exposed to temperatures of 35 degrees Celsius for six hours or more. It is possible that the cockles in the Eastern Scheldt estuary were exposed to such temperatures during low tide.

For more information about WUR’s research on building with nature, please visit our website.

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