Oyster reefs and mangrove forests help to prevent coastal erosion and flooding. At the same time, the reefs and mangroves attract fish and crab, creating a source of food for the local human population. This is demonstrated by an international eco-engineering project in Bangladesh, a country that regularly faces severe tidal floods and cyclones.
Shellfish such as oysters can create reefs off the coast which influence the tides and the waves. In this way, the reefs prevent coastal erosion. Experiments carried out in the Netherlands and the United States indicate that humans can construct artificial oyster reefs as a means of protecting the coast. Bangladesh, which is highly sensitive to flooding, can also benefit from artificial reefs. Researchers from Wageningen Marine Research and Wageningen Economic Research studied the ecological and socio-economic possibilities, while engineers from Royal Haskoning DHV examined the technological aspects.
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First of all, the researchers studied where oysters can grow in Bangladesh and what the best material is for a reef. Oysters only attach to hard surfaces. In a trial project, the researchers used a concrete structure measuring 50 metres long and approximately 70 centimetres high to imitate a fully-grown oyster reef. 'This proved to be the best, technologically and financially. The oysters attached themselves to the hollow concrete structures, which also form a habitat for crabs and fish,' says Tom Ysebaert of Wageningen Marine Research. After one year there were large numbers of oysters, while the wave-dampening effects resulted in sediment being deposited behind the reef. 'The oysters showed good growth. After one year we already had oysters measuring four centimetres,' says Ysebaert. According to calculations with models, the oyster reef could grow in height by about two centimetres per year. This will allow it to keep pace with the rising sea level.
But when a genuine tropical storm hits, an oyster reef is not enough. The waves reach higher than the reef and will simply crash over it. Each year, approximately 30-70% of the land area of the densely populated country of Bangladesh is covered by flooding. In the future, climate change will result in even more storms and cyclones in addition to a rapidly rising sea level. There used to be a great many mangrove forests along the coasts of Bangladesh; these have largely been cleared. The researchers would prefer to plant new mangroves because of their ability to protect the coastline. 'We expect that the mangrove forest will survive better and spread faster if there is an oyster reef present. In order to protect the coast, a strip of mature mangroves measuring at least 250 to 500 metres wide is needed,' explains Ysebaert. 'During storms and flooding, the mangrove forest is the first line of defence and therefore takes some damage. It helps to protect the earthen dikes behind it from damage, and the local people and their property are better protected against floods.'
It is important that the oyster reefs and mangrove forests are properly managed, in order to keep people from harvesting all the oysters or wood, for instance. 'Management requires the cooperation of the coastal communities. The local population needs to realise that in the long term, they benefit from ensuring that the reef is protected and repaired,' stresses Arie van Duijn of Wageningen Economic Research. As well as oysters, the reefs and mangrove forests provide a habitat for a great many fish and crabs which can serve as a food source. The researchers are examining the possibilities. 'The coastal residents do eat oysters, but it is not yet a high-demand product in Bangladesh. Crab, on the other hand, did turn out to have export value and could be found more rapidly,' says Van Duijn.
Not every sea floor location along the coast is suited for mangroves and oyster reefs. Ysebaert says, 'This method can be applied at a great many locations. It's impossibly expensive to construct concrete dikes everywhere, and the earthen dikes alone are often too vulnerable. This form of eco-engineering is sustainable, durable and relatively inexpensive. In the long term, it provides a self-repairing coastal defence system of oyster reefs and mangrove forests.'
The Bangladeshi television channel Channel i broadcast a talk show about eco-engineering and the project in December 2014. The Bangladeshi news also broadcast an item about the project in late November. Follow-up research is currently taking place, and the Bangladeshi government is interested in larger pilot projects.