Safe dykes with geosynthetics
More than 300 kilometres of dykes on the coast of Zeeland (NL) have been reinforced this year with the use of geosynthetics, thus making the sea wall completly safe for at least the next fifty years. TenCate has supplied geosynthetic materials for those dykes along the Oosterschelde and the Westerschelde that would not provide sufficient protection in the event of a ‘superstorm’.
Using materials like TenCate Bidim®, TenCate Geolon® and TenCate geocomposite a total of 325 kilometres of dyke were reinforced and adapted in 2015 to meet current standards.
A dyke must offer protection against high water levels and powerful waves. In Zeeland a safety standard of 1: 4000 applies, which means that a dyke must be able to withstand a super storm surge that occurs on average once every 4,000 years. After the flooding disaster in 1953 all the dykes were raised. Most slopes are covered with stone cladding; concrete and stone ensure the solidity of the dyke, but its strength turned out not to be sufficient everywhere.
A dyke body is made of sand. In order to prevent this sand and the foundation being washed away by erosion, geotextile is installed as a filter layer between the sand and the concrete Hillblocks and stones. Hillblock is a dyke stone or slope block that ensures a significantly stronger braking of waves and less wave run-up than the existing shoreline protection. The use of the Hillblock means that dykes need to be raised less or even sometimes not at all. Thanks to its shape and weight the Hillblock is more stable and easier to put in place.
Placing the Hillblock elements at the Noordwaard Polder (photo ©Hillblock)
The work was carried out between 1 April and 1 October, so was completed before the start of the storm season. Work on a dyke may only take place during these six months of the year. During this period plants flower, birds nest and people spend their leisure time near the dykes. Work like this requires a great deal of time, consultations and good collaboration with all the parties involved. The owner of a camping site would of course prefer to see all the work completed before the beginning of the tourist season. It is, however, sometimes not possible to start work in early spring because of breeding birds. For the conservation of both nature and the environment permits and exemptions are required.