Case studies: Twin Dikes
A different approach
Along the fringes of the Waddenzee, the dike needed strengthening between Eemshaven and Delfzijl. The conventional solution is strengthening the revetments and, where necessary, increasing the height of the existing dike. The Twin dike adopted a different approach and, in a joint effort, was developed and facilitated by the Regional Water Authority Noorderzijlvest and the Province of Groningen.
Method
By constructing a second inland dike, a combined flood protection system was created that requires less strengthening of the existing dike. Behind the existing dike, a new smaller and lower dike was built with clay, excavated from the area between both dikes.
The area between both dikes will be converted to a brackish area with brackish agriculture and an area intended for the controlled sedimentation of fine sediment. This sediment can then be used to strengthen dikes.
Results
The costs for building the second dike are lower than conventional dike improvement. Moreover, it creates a brackish water landscape. These new forms of land-use are a potential new development that addresses long-term and regional challenges and fit within a wider long-term coastal strategy. The low-lying land behind the existing dikes will face increasing problems with brackish groundwater. Saline agriculture and aquiculture may form a structural and economic viable solution to this problem. The concentrations of fine sediments are presently too high at the Eems-Dollard estuary and form a major hindrance to good ecological functioning. These sediments can however be converted into high quality clay, which is an important building material for dike strengthening.
Local context
The dike section is situated in an intertidal area, the Waddensea. The local tidal amplitude is in the order of 1 to 1,5 meters. This part of the coast is partly sheltered by salt marshes. It is an area with historic land reclamation areas, so there is a set of older dikes further inland. The newest dikes are closest to the Wadden Sea, and encompass agricultural land that is situated higher than previous impoldered areas. In the case of the Twin dikes project, a new dike will be built.
Future research directions
- Improved see protection concept
- Operational management that balances water protection goals and economic exploitation goals
- Application of caught sediments as useful base material
Documents
All documents of the BwN project can be found at publications, below you find the documents belonging to this specific case study:
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