Hybrid shoreline protection – It Soal, IJsselmeer

Operation and Maintenance

  1. Results related to morphology
  2. Effects of rising water levels on morphology
  3. Results related to ecology
  4. Effects of rising water levels on ecology
  5. Conclusion

1.Results related to morphology

At It Soal there is a considerable wave- and current-induced north-bound longshore sediment transport. The breakwater shelters the sandbanks from this transport process. Fifteen years after construction, the sandbanks still exist, with approximately the same size. This rather stable situation indicates that the sandbanks are not part of the longshore transport process. The project protects the shore from heavy storms, but other influences on the morphodynamica in the area have not been observes.

2.Effects of rising water levels on morphology

Since the mean water level in the IJsselmeer is stable since the construction of It Soal, there is no experience with higher water levels. The project is an example of how valuable nature can develop in sheltered areas with bare sandbanks. Higher water levels will permanently submerge the sandbanks and lead to more wave and current action, hence more morphodynamic activity, until a new equilibrium has been reached.

3.Results related to ecology

  • Compared to more recent projects, It Soal has contributed more to reaching specific nature and water quality goals. This indicates that nature development takes its time after implementation of the engineering works.
  • It Soal includes a number of habitats, such as sandbanks, stoneworts and pondweed. The breakwater itself is a habitat for bullheads and zebra mussels. As compared with the rest of the IJsselmeer, the number of waterfowl has remained the same or decreased less rapidly. The project has resulted in habitats of both larger diversity and size; from a nature development perspective the project is therefore a success.
  • For breeding birds the artificially regulated water level at It Soal is a problem since the banks are submerged during the breeding season and shoreline vegetation hardly develops. Breeding higher up on the banks is not always an option because of the higher vegetation density there.
  • The project also had some negative effects on fauna, which manifests in a decrease in southern pochards, fish eating birds and specific waterfowl.

4.Effects of rising water levels on ecology

If the mean water level is raised and no extra measures are taken (heightening the sandbanks and breakwater), the sandbanks are likely to be permanently submerged. Aquatic plants will only survive if the bank height is increased along with the rising water level; if not, the aquatic vegetation will probably move to higher parts of the land.  As the water level rises, aquatic plants have a decreasing ability to trap sediments in between their roots and to stimulate sedimentation by decreasing the flow velocity. As a result, the flow velocity will increase and erosion processes will be enhanced.


Sand nourishments along the Frisian IJsselmeer coast are valuable if they:

  1. Do not lead to the destruction of valuable existing nature
  2. Contribute to the creation of a larger area of sheltered shallow zones and related vegetation
  3. Lead to higher diversity of land- water gradients
  4. Lead to a higher amount of bare emerged sandbanks which are important to a variety of bird species.
  5. Exist long enough to achieve an almost stable situation with the above features.

The project was successful in reaching its objectives. The breakwaters helped to retain the sediment and the project contributed to Nature2000 (bird species) and the EU Water Framework Directive (macrofauna, macrophytes and fish).