Breakwaters to stave off erosion in the Western Scheldt

HZ studies the growth of low-dynamic intertidal area

Knuitershoek and Baalhoek are the imaginative names of the locations where, since 2016, a consortium has been conducting research into the repair and restoration of the nature outside the dikes in the Western Scheldt. The project - with the less imaginative but very apt name Outside the Dikes - runs until September 2022. “We are now finalising, but to a large extent it is already clear that the measures are working”, researcher Wietse van de Lageweg says.

Eugène de Kok, editor HZ Discovery

The nature restoration at Knuitershoek and Baalhoek, on both sides of the ‘hump’ directly above Hulst and Kloosterzande, is part of Nature Package Western Scheldt. The package should generate 600 hectares of additional estuarine nature (salt marshes and mud flats) in the Western Scheldt. The well-known and much-discussed projects, such as the Hedwigepolder, Perkpolder and Waterdunen are also part of this package. The measures are needed because the mud flats and salt marches have shrunk significantly due to deepening of the shipping channel in the Western Scheldt, land reclamation, and natural processes. This threatens the loss of a lot of rare flora and many important foraging areas for birds.

Wave action

In 2016, in Baalhoek and Knuitershoek, three existing breakwaters were raised, and two new ones created. These slow down the current velocity and affect wave action. This results in more sediment being deposited, so that new mud flats and salt marshes are created, and existing ones are restored. “The expectation was that, by creating breakwaters, good prerequisites were created to allow for the low-dynamic intertidal area to return”, says Wietse van de Lageweg of the HZ lectorate Building with Nature. Together with Wageningen Marine Research, NIOZ, and Deltares, the HZ is part of a consortium that conducts research into the working of the breakwaters. The HZ is looking into whether and how the quality of the intertidal areas has improved by the construction and whether this is linked to the interventions.

The researchers focus on the biodiversity on and around the breakwaters. It is clear that species diversity has increased, but a clear trend is difficult to identify. “With birds it is always a challenge”, Van de Lageweg explains. “Our project locations are small compared to the large Scheldt system they are based in, so birds have a lot to choose from. If in a certain location there are suddenly more people about, the birds will fly to a different, quieter, location. These types of effects are difficult to exclude.” In the areas, species like widgeons, shelducks, oystercatchers, and redshirts appear to be more prevalent.

Education

Outside the Dikes has also been incorporated into the education, for example in a minor. As part of this, students can develop research skills, for example in the area of data collection in the field.

Several minor students have helped with taking measurements. “In a challenging environment”, Van de Lageweg knows. “It is not just sand, you also run into oyster banks and other rough terrain.”

'It's not just sand, but you also come across oyster beds and other rough terrain'

The project is also featured as a case study in the module Ecological Engineering of the Water Management programme. Over the coming months, fourth-year student Water Management, Charlotte Pons, is studying a small area that is deviating from the rest. “Contrary to the other locations, where more sediment is being deposited, we are seeing that this area has remained very stable and has even eroded recently. That is something that stands out, of course. We want to understand why this is happening.”

Height measurements

Van de Lageweg and his colleagues are incorporating the obtained knowledge into their project Outside the Dikes II. Near Bath, eight breakwaters were created with the same objective as in Knuitershoek and Baalhoek. The consortium wants to do research into the ecological consequences in that area as well. Although Outside the Dikes will continue until September, the researchers are already positive about the results. “In the past few years we have taken a lot of height measurements and ecological recordings. These showed promising results. We are seeing a decrease in the current and increase in the low-dynamic area.” According to Van de Lageweg, the differences between Knuitershoek and Baalhoek are remarkable. The prior area is situated lower, causing a small area to fall dry each time. “So, that area requires more time to accomplish the same as in Baalhoek. You can use that knowledge by selecting areas for these types of measures where you can quickly make a lot of progress. Meaning you can more accurately estimate your chances of effectively intervening.”