From inside the embankment
to outside the embankment

written by Wietse van de Lageweg
Coordinator of the research group Building with Nature

Since 25 June 2015, saltwater from the Westerschelde has been flowing into the nature reserve at Perkpolder twice a day. On that date a bulldozer pushed away the last remains of the sea embankment, creating a breach that connected the area to the estuary. The penetration of saltwater, the import of sediment and the creation of creeks and the mud flats as a result of erosion and sedimentation have since been the designers of the area. This dynamic environment offers opportunities for birds, benthic communities and vegetation. HZ researchers and students investigate the developments in Perkpolder and contribute to the knowledge about depoldering. To follow this dynamic environment, they make recordings of the altitude, vegetation, benthic communities and birds every six months.

Erosion and sedimentation

Perkpolder is located next to the former ferry port in Zeeuws-Vlaanderen. It is a low-dynamic and silt-rich area. For the largest part of the area, height differences between the beginning and the end of the measurement period are between 0 m and 0.5 m, but especially the deep well at the inlet and the southern, smaller and higher-lying creeks show a growth of a few meters. It is striking that the northern, larger creeks show erosion in the order of decimetres up to a meter locally. This shows that the natural processes are in full swing to adapt the creeks dug with bulldozer and cranes, so that the natural character of the area returns more and more.

Vegetation

No plants have taken root in Perkpolder yet. Monitoring shows that seed supply is not the problem. It is mainly the low location that prevents plants from taking root. The constant silting up will eventually lead to plant growth. The trenches created will contribute to this, as experiments show that seedlings survive well in well-drained soil.

Why depolder?

The Westerschelde is a Natura 2000 area with a unique ecology that is of international importance for a large number of birds. The mud flats and sandbanks that fall dry at low tide are essential foraging areas for tens of thousands of waders. These so-called intertidal areas are very productive environments where the majority of shellfish, crustaceans and worms live. At the same time, the Westerschelde is an important economic vein that provides access to the ports of Antwerp and Vlissingen. Because the ships are becoming larger and larger and because sand trenches naturally form in the channels of the Westerschelde, the navigation channel must be deepened regularly. By deepening the channels in the Westerschelde, the flow velocity increases and the edges of the sand slabs become steeper: shallow water areas and low intertidal areas decrease as a result. Depoldering is one of the options for restoring intertidal areas and thereby creating opportunities for nature and society.

Benthos and birds

The development of a benthic communities following the tidal recovery in Perkpolder happens faster than anticipated. A biologically active mud flats area has formed in a short time. Within three years the benthic communities show a development similar to that of the natural mud flats and banks in the Westerschelde. It is expected that a stable community will emerge within a few years instead of decades, as originally predicted. The area is also often visited by birds that forage at low tide and during high tide rest on the surrounding embankment.

Groundwater and seepage system

New sea embankments have been built around the area and the existing ones have been reinforced to protect the hinterland against flooding. A system has been installed behind the embankment to collect the salt seepage that seeps under the dike. Groundwater measurements show that the seepage system is effective. This means that it is able to protect agriculture in the hinterland against unwanted saltwater. Three years after the opening of the area no changes in the fresh-salt water transition are seen. Moreover, it has been shown that the seepage system can also be used to increase the amount of fresh water in the soil during periods with heavy precipitation.

Finally
Current monitoring provides important insights into the design of the inlet, the dimensions of the tidal creeks dug, the topography of the mud flats and how these influence morphological and ecological developments after tidal recovery. In addition, unique knowledge has been obtained about the effectiveness of a seepage installation to protect the surrounding freshwater agricultural areas. The cooperating partners intend to continue monitoring in Perkpolder with the aim of gaining a better insight into the medium (4-10 years) effects of tidal recovery on abiotic and biotic factors.Perkpolder offers a unique opportunity to follow and study the physical and ecological changes in an area that is transforming from an agricultural area to a tidal area. Not much is known about this transition. Perkpolder thus forms a blueprint for future restoration projects such as the Hedwige-Prosperpolder.

More information about Perkpolder can be found here.
Pictures: © HZ / Edwin Paree