Restoration measures for salt marshes in the Dutch Wadden Sea
In recent years, the salt marshes of the Wadden Sea have slowly been expanding again. However, the ecological quality of these areas is under pressure. There is decreasing variation in vegetation and development stages. This is problematic, as European and international agreements aim to improve the condition of these dynamic coastal ecosystems.
Deltares was commissioned by the Morphology Knowledge Programme of Rijkswaterstaat and in collaboration with Marinka van Puijenbroek (Wageningen Marine Research), to investigate where and under what conditions two restoration measures could be effective:
- Local excavation of salt marshes to make room for vegetation rejuvenation.
- Stopping maintenance of brushwood dams to give natural processes more space.
Deltares contributed to this study by analyzing abiotic boundary conditions, such as soil composition, water levels, wave exposure and sediment availability. Based on measurement data and model outputs, a qualitative assessment was made of where these measures are most promising without structurally compromising the surface area development of the salt marshes in the long term.
The study focuses on the Dutch Wadden Sea, excluding the Ems-Dollard area, due to its fundamentally different dynamics.
Key abiotic conditions for healthy salt marshes are:
- Elevation of the salt marsh
- Frequency and duration of inundation
- Availability of sediments
- Wave energy
- Soil composition
- What type of soil there is there is
Key findings
The study shows a relationship between the sediment composition of the intertidal flats in front of the marshes and the vertical development (elevation change) of the marshes. Marshes adjacent to mud-rich flats show greater year-to-year variability: in some years, substantial sediment accumulation occurs, while in others, substantial erosion is observed. In contrast, marshes in sandier environments tend to have lower but more stable development. Other environmental factors did not show a clear relationship with field measurements.
The experts have created a map indicating where the proposed measures could be effective. The most suitable site for local excavation is Noord Friesland Buitendijks. There, enough space and sediment are available to allow the salt marshes to recover. Other potential areas are Peazemerlannen and the Groningen salt marshes, although these sites require further investigation. One important point of attention is ensuring adequate connectivity to the Wadden Sea to promote sufficient sediment supply.
In other marshes, such as the remaining Frisian salt marshes, Balgzand, and the island marshes, surface lowering is not advised due to space constraints or limited expected effectiveness.
Noord Friesland Buitendijks is also the most suitable site for discontinuing maintenance. In some areas, such as Wierum, this is not a good idea due to erosion.
Next steps
This study is an initial exploration of suitable locations. Further location-specific analysis and design development are necessary before implementation can be recommended. Before measures are taken, further elaboration, consultation with local residents and other stakeholders in the area are necessary, and a decision must be made on what to do with the excavated clay. It is not recommended to implement the two measures simultaneously, as it would be difficult to distinguish the effect of each respective measure.
