News - June 4, 2015

Oyster reef is weapon against erosion

Rob Ramaker

Creating artificial oyster beds may help protect nature and save money on coastal protection. Oyster beds prevent the erosion of sand flats behind them by the waves

They protect not only the place they cover themselves, but also the area behind them. These are the findings of Brenda Walles, a PhD graduate from the Sustainable Shellfish Culture chair group, in the thesis she defended on 1 June.

Worldwide, erosion exposes millions of coast dwellers to risks of flooding. What is more, natural ecosystems such as mangrove forests, swamps and coral reefs are disappearing. One possible solution is to raise dykes. That is very costly for the Netherlands, and is totally beyond the means of a country such as Bangladesh. So engineers are looking for new methods of protecting coastlines.

The concept of ‘building with nature’ has been gaining ground in recent years. In this philosophy instead of holding back water with stones we get nature to work with us on making coasts safe. ‘Such solutions are less damaging to nature,’ says Walles, ‘and they are self-sustaining, whereas a dyke has to be maintained.’ One example is the ‘sand motor’, a peninsula created off the Dutch coast from dredged up sand. Scientists are looking at whether the sand motor allows the coastline to expand and at the same time harbour new natural vegetation.


In the Oosterschelde, Walles measured the amount of sand behind a couple of natural beds of Japanese oysters. By doing this at several locations, she could show three-dimensionally how reefs influence sand levels. She also measured over a longer period of time the sand level behind artificial oyster reefs. These measurements revealed that the oyster beds effectively protected the sand flats. ‘Where at first 2 centimetres of sand was disappearing every year through erosion,’ says Walles, ‘now a centimetre of sediment was being added.’

The artificial reefs were made by filling cages with - mainly dead – shells. Walles studied several squares of 25 by 25 centimetres to see how quickly living oysters established themselves. The artificial reef turned out to renew itself quickly and probably sustains itself over a longer period without any human intervention. What is more, the reef also appeared to ‘grow’ upwards, so that it can keep up with rising sea levels.

The research took place in the Oosterschelde precisely because there is a lot of erosion here. The creation of storm barriers significantly changed conditions such as the speed of currents and the differences in water level between high and low tide. Since then the shipping channel has been slowly filling up with sand and it is getting smaller. Given that rivers and the sea hardly deposit any more sand here, the sand must be coming from erosion of the sand flats.