In the Delta Flume of the Deltares knowledge institute in Delft, The Netherlands, researchers are testing the design of a so-called Wide Green Levee with storm-force waves. Hunze en Aa’s water authority wants to strengthen their levee in the far northeast of the province of Groningen, following a new design that makes it possible to use circular clay. This Wide Green Levee will have a shallow slope with a grass cover and consists of locally extracted and dried sediment from the Eems-Dollard estuary.
The trials are part of an extensive study into the suitability of circular clay as a material for dyke reinforcement. Already the strength of the circular clay (without grass) has been tested in the first series of trials in April. In the current second series of tests, the design of the levee with grass cover is being tested.
Two Series of Tests
Project manager Erik Jolink: “In the first series of tests, we tested the strength of two types of clay: clay from the so-called clay ripener, in which we let locally extracted sediment dry into clay, and clay that was extracted from the nearby salt marsh. The levees with this clay held up well in the Delta flume. The clay must be able to withstand a superstorm for 15 hours, but after 22 hours, the levee still held up. In this second trial, we test the design of the levee with a grass cover. We combine the data from both tests to be able to optimize the design.”
Clay with Grass Cover
The 150 tons of grass-covered clay that is required for the trial, comes from an existing levee in the province of Groningen. The same amount of clay comes from a levee in the province of Friesland.
In May, excavators removed 23 blocks of clay measuring two by two meters and one meter thick from each of the two levees. The levees were immediately restored to ensure safety. Two 8-metre high levees of these blocks have been recreated in the Delta Flume. The reason that the Frisian clay is also being tested is to discover how long this section of levee can still last. Paul Buring of Wetterskip Fryslân, the Frisian water authority: “In the future, we need to strengthen our levee as well. This test is actually an exam for our dyke.”
Wide Green Levee
The results of the trials will be known this fall. The next step is the construction of one kilometer of Wide Green Levee in 2022. If this reinforcement proves to be satisfactory in practice after a few years, the entire 12.5-kilometer levee will be adjusted in this way. Approximately 1.7 million cubic meters of locally sourced and ripened clay will be used for this. If the trial is successful, it will also provide many benefits for the entire dyke improvement works in the Netherlands because the clay doesn’t have to be transported from far away.
Hunze en Aa’s water authority is the initiator and lead of the research. The EcoShape partners Deltares, Van Oord and Boskalis support and carry out the research. Advice is provided by the Clay Ripening project group led by the Province of Groningen and The Frisian Water Authority, Wetterskip Fryslân. The Dutch Flood Protection Program is the primary financier and advises on the research.