Polders that (also) serve nature conservation: Thanks to the Masterplan Ems 2050, they are being created on the Ems, but they also exist on the Scheldt in Belgium and the Netherlands. Responsible for this is the Sigma Plan, originally a large-scale anti-flood project that began in 1977.

How to do it on the Scheldt, where the similarities and differences lie: A delegation from the Lower Saxony State Agency for Water Management, Coastal and Nature Conservation (NLWKN) has now got an idea of this. The employees who travelled with them work in the fields of hydraulic engineering and coastal protection, hydrology and nature conservation as well as biology and regional planning for the Ems master plan and work on the creation of tide-influenced habitats, polder projects as well as on the planning and management of meadow bird areas and tidal control with the Ems barrage.

Scheldt and Ems

The Scheldt is a river that at first glance has little in common with the Ems, especially regarding the dimensions: Antwerp, Europe's second-largest port, is located on the river. The largest container ships in the world sail 80 kilometers up the river from the North Sea to unload their goods there. The fairway is kept correspondingly deep. At the same time, several refineries and many other industrial plants line the quay walls in the port. Everything seems more powerful.

 Nevertheless, the two rivers have something in common: a silt problem, a huge tidal range – and over the centuries they have recorded a great loss of habitats that should be typical of tidal rivers. And like the Germans on the Ems, the Belgians and Dutch are trying to restore these habitats to the river through polders on the Scheldt – through a further development of the Sigma Plan. He initially worked with a focus on storm surge and flood protection, then gradually nature conservation aspects and the recreational value of the polders were added.

The Kruibeke polders

Upstream of Antwerp, where the Scheldt suddenly becomes narrower and the water masses have to squeeze from the wide into the narrow riverbed, used to be one of the hotspots of flooding. Here, more than 100 kilometers from the estuary, people struggled with storm surges from the sea and floods from the interior. Dike breaches and flooded villages were not uncommon. One of a total of 14 structures intended to prevent flood hazards is the Waal-Zwijn polder near the municipality of Hamme, which is still under construction in parts and consists of several areas: The area closest to the Scheldt was undiked and left to the free influence of the tide. Since the opening a year and a half ago, extensive freshwater mudflats have been created here on 25 hectares, crossed by tidal creeks and overgrown from the sides: this will create a river marsh in a natural way. The main dike was moved far back by the river to give the river space and now directly protects the villages. An overflow polder was built between the mudflats and the dike; it is surrounded by a dike with a lower height, which is overflowed during floods and thus lowers the river level. The area is 140 hectares in size. When the river level drops, the water is let out again via outlet sluizes.

Today, the overflow polder presents itself as a living area with a tidal creek, extensive reed beds, perennial meadows and willow floodplain forest - all biotopes that can cope with regular flooding and offer a variety of habitats, which can be experienced by a footpath and cycle path on the ring dike. This fulfils all three aspects: flood protection, nature conservation and experiencing nature, as Max Verdonck, press spokesman for the Sigma Plan, explains to the guests during the tour of the dike around the area. Above all, the technically intelligent and simple inlet and outlet structures gave the planners of the NLWKN many suggestions for further projects on the Ems.

A little closer to Antwerp are the polders of Kruibeke.  More than 600 hectares of river mudflats, alluvial forests, reed beds and a large area for meadow breeders extend here. The former protective dike on the banks of the Scheldt was reduced in  height on a stretch of eight kilometres, so that it is overflowed at flood water levels and thus helps to lower the level in the river. On the other side of the area, a ring dike was built over a length of six kilometres to protect the neighbouring settlements from the water in the event of flooding.

At normal water levels, the polder in the northern area is connected to the tide by an inlet structure, which creates a tidal range of around one metre in the polder – in front of the dike in the Scheldt it is around five metres. The structure also provides a very special attraction for flat Flanders: the so-called Kruibeke waterfalls. In order to avoid rapid silting up of the polder, only the top layer of water is allowed into the polder at high tide; there is the least amount of suspended solids. Therefore, a few meters have to be overcome to the polder level, and this is achieved via three stages, over which the water temporarily foams and roars, bringing a lot of oxygen into the polder channel.

In the south of the area, the tide does not flow in. Here, flooding is only carried out during floods, and only on the meadow bird areas. The alluvial forests are decoupled from the floods. In flood situations, however, a large part of the area can be used as a relief polder.

The project was not without controversy, reports Max Verdonck. Above all, safety concerns arose, but there was also a fundamental rejection of the restoration of natural habitats on such large areas. Those responsible built an information centre for the debate. "Sometimes you have to explain the same thing 45 times," said the press spokesman. However, the involvement of local conservationists as "guides" for guided tours has also contributed a great deal to understanding. As in Wal-Zwijn, however, the protests were also about the loss of agricultural land for the polders. And because the Sigma Plan focuses on flood safety, land was also procured through expropriation: "Of course, for appropriate compensation." In the case of the nature conservation project "Master Plan Ems", on the other hand, expropriations are excluded.

The polder is located between the Scheldt and the village of Kruibeke. And perhaps the situation has calmed down because the polder has become a popular local recreation area. Cyclists, skaters and pedestrians are on the dike, enjoying the summer day and the changing nature impressions that the polder offers. A path made of wooden planks including a bridge leads over the tidal creek and through the reeds, waterfowl are observed through binoculars, local recreation is important here. Because in Kruibeke – as in many other places on the Scheldt – it becomes clear how close people, nature and industry come to each other on the Scheldt. While on one bank the polder areas glisten in the sun, on the other bank of the Scheldt there is a power plant, a shipyard and other sites of the manufacturing industry. From the Coldemüntje polder you can also see the paper mill in Weener, but again you come across a completely different dimension here.

Incidentally, there is a plaque directly at the waterfall that reminds us that the principle of operating polders only with a reduced tidal range via specially designed inlet structures was developed and first implemented here on the Scheldt by Professor Patrick Meire from the University of Antwerp. This idea is also the inspiration for the polder constructions on the Ems.

Burchtse weel

The Lower Saxons once again experienced how important this principle is for the operation of polders on silt-bearing tidal rivers at Burchtsee Weel, a polder directly opposite the city centre of Antwerp. The area was created to compensate for the temporary loss of mudflat biotopes during the construction of another car tunnel near Antwerp. Initially, the area was connected to the Scheldt by deep, open channels, which opened up the flow of liquid silt from the deeper layers of the river into the polder. The result: four metres of silt per year.  Since that was not the goal, according to Max Verdonck, the inlet and outlet structures were redesigned – according to the Meire model. Since then, sedimentation has been almost zero.

Bankbusters

Endless pipes, tanks and cracking plants of refineries, the approach leads through endless port area with warehouses, container gantry cranes and stacks, through a network of highways and tracks, smoke rises into the sky everywhere, it smells of gasoline and silt: hardly anyone would suspect a nature conservation project in this totally reshaped landscape. And yet: Right next to the Waterbus stop, a fast catamaran line through the harbour and directly opposite a refinery, scientists from the University of Antwerp have created a test field. Professor Tom Maris from the University of Antwerp explains the purpose: The aim is to test the ecological effect of fresh clay soil.

If deposited silt is permeable, it can extract nitrate from the water flowing through it and enrich it with silicate, which stimulates and promotes the growth of diatoms in the water – an important function of marsh soils. However, the researchers have observed that deposited silt quickly solidifies and the water does not flow through it. This also means that the biochemical effect is lost. Here, the soil, which is protected against erosion by wickerwork, has been mixed with wood chips in order to enrich it with organic material and thus increase the pore volume. Numerous measuring devices and probes are now to determine whether the porosity achieved in this way is permanent.

Maris also discusses the proximity to the refinery; the management is planning expansions, but must prove, among other things, that the additional nitrate quantities will be absorbed. This is one of the reasons why the industry is watching the research closely and is pinning its hopes on soil analyses. At the same time, the willow reinforcements are intended to test how the soil can be kept on the shore. Despite its good ecological balance (because it replaces country bus lines and car traffic), the fast catamaran waterbus causes such a high wave that ecologically important marsh waters erode.

Hedwige and Prosper polder

Just a few kilometres downstream from Antwerp are the polders Hedwige and Prosper; 495 hectares of floodplain completed in 2023; you can see very fresh tidal flats, whose silting up is just beginning at the edges, algae and salt-resistant plants are the pioneers, islands are slowly forming in the silt. In the middle of it all is the Belgian-Dutch border; on the Belgian side, a canal through the dike provides the opening to the tide, on the Dutch side the dike was completely razed. The material that was produced during the lowering of the site was used to build a nine-metre-high dike around the polder and a viewing hill for "seers".

15 years passed from the initial planning to completion. This was partly due to the great resistance in the Netherlands. According to Tom Maris, this had something to do with the fact that in the Netherlands, dikes and poldering had been the means of choice against the forces of water since the 1950s. To open up an area now contradicted all experience. The amount of water now taken from the river protected above all the upstream residents of the river - i.e. the Belgians. In addition, polder construction cost a lot of agricultural land.

There has already been cross-generational experience here. To the north of Hedwige and Prosper lies "Das Verdronken Land van Saeftinghe". This area was once poldered, there was agriculture and many settlements. In the 16th century, during the Dutch War of Liberation against Spanish occupation, the Dutch rebels pierced the dikes of Saefthinge for military reasons. All new attempts at poldering failed.

The area, today the largest salt marsh area in Europe, covers 3580 hectares and consists of mudflats and salt marshes with numerous tidal creeks. The difference between high tide and low tide at the highest point of the terrain is an average of 4.80 meters, with spring tide the difference can be up to seven meters. The tidal difference is the largest in the Netherlands. An information centre and boardwalks make parts of the area accessible to people. The creators of the Sigma Plan also see the Sunken Land as a huge spring biotope from which the new polders are settled.

The information and visitor centre for these three areas is located in the abandoned village of Doel, south of the areas, in a historic mill. The village was supposed to make way for a port expansion, but this is on ice. Nevertheless, most of the residents have left their homes. Between the information centre and the polders is the Doel nuclear power plant. As everywhere else, the economy and nature around Antwerp are close together.