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Clay Ripening Pilot Project, Delfzijl - NL




Location: Eems-Dollard Estuary – Delfzijl (The Netherlands)

Date: 01-01-2018 - (expected) end date: 1-1-2022

Involved parties: Rijkswaterstaat, Province of Groningen, Groningen Seaports, Water Authority Hunze en Aa’s , Het Groninger Landschap and EcoShape. The project is financially supported by the Waddenfonds and the Dutch national flood protection program HWBP.

Technology Readiness Level 3: Proof-of-Concept Demonstrated, Analytically and/or Experimentally

Environments: Estuaries, Ports & Cities

Keywords: soft sediment, sludge, clay, ripening, desalination, dike reinforcement, water quality, ecology, economic basis


Building with Nature designTraditional design

Collecting fine silty sediments from the estuary and converting this into clay soil creates a win-win situation: the water quality improves and more clay becomes available for reinforcing dikes, and raising land behind the dike.

In traditional dredging, fine sediments dredged from an estuary are disposed at sea or stored in large (land) disposal sites, without further application or use. (picture: Slufter disposal site – NL).




Excessive levels of sediment in the water column in the Eems-Dollard estuary have a negative impact on water quality and thus also on biodiversity. Large amounts of sediment accumulate in ports, making regular dredging necessary.

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On the other hand, clay soil is needed in the area to strengthen dikes and to maintain and improve agricultural land. Collecting sediment from the Eems Dollard and converting it into clay soil creates a win-win situation: the water quality improves and there is more clay soil for reinforcing dikes and raising farmland. There are plans for more dike upgrade operations in the future on the Eems-Dollard coast, and therefore clay soil will be needed there, too.


Rijkswaterstaat, the provincial authority of Groningen, Groningen Seaports, the Hunze and Aa's water authority, nature conservation organization 'Het Groninger Landschap' and EcoShape team up in the Clay Ripening pilot project to look at different ways of transforming sludge into clay soil applicable in dike strengthening operations. EcoShape researchers are engaged in practical experiments to see which approach to ripening works best.


Initiation Phase

The Clay Ripening Pilot is part of the subprogram ‘Useful apply of fine silty sediment’ of the so-called Eems-Dollard 2050, a program aimed at improving ecological values by reducing the turbidity in the Eems-Dollard estuary. All projects within this adaptive program share the same goal: “Balancing ecological values and economical advantages”.

The pilot is based on collecting silty sediments from the Eems-Dollard estuary and converting it into clay soil creating a double win situation: the water and ecosystem quality in the estuary improves, and more clay soil is available for reinforcing sea dikes along the Eems-Dollard estuary. The material may also be used for raising agricultural land to compensate soil subsidence.

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More specifically, the goal is:

To find out with which innovative methods sludge on land can be converted into clay in a useful and cost-effective way, whereby an economic basis can be laid under the desired sludge extraction from the Eems-Dollard.”

Rijkswaterstaat (part of the Dutch Ministry of Infrastructure and Water Management), the Province of Groningen, Groningen Seaports, Water Authority Hunze en Aa's, nature conservation organization Het Groninger Landschap and EcoShape team up in the Clay Ripening pilot to look at different ways of transforming dredged fine sediment into clay soil, applicable in dike reinforcement operations. EcoShape researchers are engaged in practical experiments to investigate which approaches to clay ripening performs best. This pilot forms a good example of how intensive and constructive stakeholder involvement can lead to a unique project, in which multiple stakeholder objectives, gathered around the reduction of freely moving silts in the estuary, can be merged effectively.

The questions below will be answered in the Pilot project:

  • Can clay-ripening through land farming contribute to a (feasible) solution for the turbidity problems in the estuary?
  • How can silts properly be extracted from the Eems-Dollard estuary for efficient clay-ripening?
  • Which clay quality will clay-ripening produce?
  • How can ecosystem services contribute to clay-ripening? For instance, by growth of vegetation to enhance evapotranspiration.
  • How can clay-ripening contribute to ecosystem services? For instance, by producing clay: a natural material for embankments
  • Will clay-ripening provide a positive business case?

The business case evaluates a scenario of large scale implementation of the concept. It takes into account the need for clay to strengthen dikes and the value of the quality improvement of the estuary. In addition, knowledge is lacking about the ripening process of clay. Although clay ripening itself is well known (with a lot of experience in the mid to late 20th century, in the Netherlands, but also especially on the German Eems-Dollard estuary), monitoring a (large-scale) ripening processes of marine sludge and methods to influence this ripening process (i.e. by removal of salt and organic matter) have barely been systematically researched.

The aim of the pilot is to convert 290,000 m3 of silty estuary sediment into roughly 105,000 m3 of clay which meets the requirements of clay for dikes. “Water Authority Hunze en Aa's” will be using 70,000 m³ of the produced clay to transform/strengthen one kilometer of dike into a Wide Green Dike, in a subsequent pilot project. This is a dike with a shallow slope covered by grass. If the produced clay proves to be successful in this test section, the remaining section (about 11.5 kilometers) of the flood defense structure it forms part of may also be strengthened with this method. In addition, the ripened sediment may be suitable as a raw material for the brick industry and for raising agricultural land.


Explanation Clay Ripening in Dutch (English Subtitled) 

If successful, future scale up of clay ripening on land could significantly reduce the amount of sediment, freely moving with the tides, in the Eems-Dollard estuary. The aim regarding the ecological rehabilitation is to sustainably extract 1 million tonnes (dry mass) of sediment from the Eems-Dollard annually by 2022 and clay ripening on land could contribute to this goal by offering a suitable material application. This is expected to have a positive effect on the unique ecological value of the estuary.

The pilot will generate knowledge about transforming dredged material into clay soil and the usefulness of this process. This knowledge could also be very valuable elsewhere, where building materials are scarce and intensive flows of fine sediment lead to high dredging costs and/or loss of ecological value, such as in Singapore or the Western Scheldt estuary (NL).


Planning and Design Phase

Activities in the field started in 2018 by setting up two depots and filling these with fine sediments by pumping of dredged material , (see figure 2). One depot is for ripening dredged sediments from the Zeehavenkanaal, the channel leading to the port of Delfzijl. This depot is located just behind the sea dike (figures 1.1 and 1.2). The other depot is located further east, outside the sea dike on the salt marsh, and will be filled with sediment dredged from the Breebaart polder near Termunten (figures 1.1. and 1.3), a polder that inundates with the tides and is thus filled with sediment continuously, as a kind of silt-trap. The sediments in both depots are transformed into clay soil by means of processes such as dewatering, desalination and oxidation of residual organics.

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Each depot will contain 10-15 cells (see figure 3) that allow for testing different ripening approaches. The clay soil is predicted to have ripened sufficiently by 2021, at which time it will be suitable for application.



Figure 1.1: Locations for the Clay Ripening pilot project in the Province of Groningen (NL): near Termunten and the Breebaart polder. The areas in red and white show where the sediment will be collected (Zeehaven Kanaal and Breebaart polder); the green areas show the sections of the clay ripening site near Delfzijl and alongside the Dollard dike.

Figure 1.2: Clay Ripening location Delfzijl (left) with location of source of the sediment (right) (picture: Satelliet data portaal, 2018)

Figure 1.3: Clay Ripening location Kwelder (left) with location of source of the sediment (right) (picture: Satelliet data portaal, 2018)

The location of the depots along and outside the Dollard sea dike are carefully chosen, because this section of the dike needs to be reinforced in the near future. Hence, transport costs will be reduced to an absolute minimum here. The two locations together consist of 25 cells of ca 100x100 m in which different ripening strategies will be executed.

Construction Phase

The construction of a depot for clay ripening can roughly be distinguished in six steps:

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  1. Preparing the sites, incorporating leveling, removal of existing structures and construction of containment dams
  2. Building separation dams to create compartments
  3. Excavation of a ditch around the depot to drain the water originating from the sediments
  4. Install equipment for water management such as drains, the so-called water boxes (in Dutch: 'stortkisten' )
  5. Monitoring equipment for meteorological information, soil level, water content and -related- soil-density
  6. Install pressure pipelines to transport the sediment-water mixture from the dredging vessel to the compartments


Operation and Maintenance Phase

The marine sediments are dredged relatively close to the depot location and are brought into the depots by pressure pipes (Figure 2). The location of the dredging is chosen carefully based on exploratory research of the seabed, relating to the environmental quality and physical characteristics of the sediment (high density, suitable for future application).

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In the various depot compartments different techniques or circumstances for ripening are tested. These include amongst others:

  • Mixing of the sediment – salt water mixture with fresh water
  • Variation of placement layer thickness
  • Biological methods (e.g. plant seeding)
  • Frequency and intensity of machines strokes using several land farming techniques
  • Adding a sand layer for bottom-drainage purposes


Figure 2: Sludge mixture entering the depot in Delfzijl.

Essential for the pilot is monitoring the clay ripening process, to find out what the most efficient methods are to convert fine sediment into clay in a useful and cost-effective way. The research strategy (and the choice of the sludge) is based upon the parameters which are routinely applied for evaluation of clay soils for use in embankments:

  • Erosion resistance class (a Dutch clay characteristic based on Atterberg-limits)
  • Consistency index

  • Sand (>63 µm) and lutum (<2 µm) content

  • Calcium carbonate content

  • Salt content

  • Environmental quality

  • Organic matter matter

Besides composition and environmental quality, another relevant parameter is time. The time of the ripening process is unknown and depends on the consolidation behavior and on the weather, especially the evaporation. The knowledge gained in the project will be used for pilot evaluation and creating a business case for clay ripening on large scale.




   Figure 3: Filled depots


Lesson Learned

As of end 2018, the project is expected in the next 2-3 years to show valuable information regarding the transformation of clay-water mixtures into clay soil. The following lessons have so far been learned from clay ripening in the first depot, at location Delfzijl.

It is possible to create a pilot which investigates estuary quality improvement, production of geo-technically suitable construction materials as well as economic stimulation. By tackling those goals simultaneously, a diverse group of stakeholders can participate.

Other findings are:

  • Ripening of dredged fine sediment mixtures on land does not need to be associated with environmental pollution if sufficient care is taken in the choice of the sediment dredged.
  • Transforming marine fine sediment into clay soil requires removal of water and salt, oxidation of organic matter as well as enough time for the relevant processes to occur.
  • The depots have been filled in two rounds: a first batch is placed in April 2018, followed by a second batch on top in July 2018 (in most of the compartments). One of the advantages of working in two rounds is the possibility to adjust the volume, for example when expected fill densities are not reached. Consolidation takes place during the two filling rounds resulting in a higher (dry) soil storage volume.




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