‘Ecosystem engineers’ are plants and animals that influence their surroundings. Marram grass helps to construct dunes, silt accumulates around saltmarsh plants, and mussels and oysters build reefs and bind sediment. These organisms can help to make flood defence systems more effective.
In light of the expected climate changes and rise in sea levels, the Wadden Area Delta programme has been seeking new ways of keeping the coast safe. The programme includes identifying the ecosystem engineers present in the Wadden Sea and how they can play a role in flood protection in the long term. There were already many ideas regarding their deployment but a complete overview was needed.
Strength of the defences
Ecosystem engineers can help to strengthen flood defence systems. Marram grass stabilises and raises the dunes in the Wadden Islands, which is why they have been used in coastal protection for centuries. Saltmarsh plants accumulate mud, allowing vertical growth in the mudflats. The extra layer of clay helps to reduce seepage and piping erosion, making a dike more stable. The importance of saltmarshes is well known but needs to be given more attention in formal evaluations.
During storms, wave damping occurs in shallow waters. Saltmarshes have this effect even during extreme storms. Banks of molluscs (shellfish such as mussels and oysters) reduce wave intensity if the water is not so high. They also trap sediment in the Wadden Sea with the result that shoals (sandbars) build up and the waves do less damage to the flood defences. The advantage of wave damping is that the flood defences do not need to be so high or so strong.
Trapping sediment in the Wadden Sea
By trapping sediment (sand and silt) from the water, saltmarshes and mollusc banks help the Wadden Sea development keep same pace with sea-level rise.
Development keeps pace with sea level, yet controllable
Saltmarsh and dune growth can, within certain limits, keep pace with sea-level rise. This means they are flexible in changing circumstances.
More is needed than just the ecosystem engineers and their products to protect the Wadden area against flooding. However, some ecosystem engineers, namely marram grass and salt marsh plants, are certainly able to contribute to flood protection. Dunes accomplish this the best; saltmarshes will perhaps require some human intervention (plenty of relevant experience is available), and molluscs are particularly sensitive to changes in storm frequency, temperature and salinity.
This research was carried out jointly by IMARES and Deltares