J. Bulens1*, J.H. Nienhuis1, F. Dunn1, M. Nussbaum1, M. Haasnoot1,2
1 Department of Physical Geography, Utrecht University, Utrecht, the Netherlands; 2 Deltares, Delft, the Netherlands
* Corresponding author: j.bulens@uu.nl
Introduction
Nature-Based Solutions (NBS) can help to increase elevation and limit the pressure of sea-level rise (SLR) on the Wadden Islands. In this presentation, we will show our new statistical approach to extract elevation trends from recently merged TrilaWatt topography and bathymetry data (2016-2022, Lepper et al., 2025). With this approach we have assessed the geomorphic effect of NBS across the Dutch, and German barrier islands.
Objective and Methods
We mapped three types of implemented NBS across Wadden Islands: (1) the creation of dune notches that aim to restore dune , (2) removal of sand drift dikes to restore overwashing, and (3) salt marsh restoration. For each NBS, we extract gridded annual elevation data from 2016-2022 (Lepper et al., 2025). Then, we used empirical distribution functions and generalized additive models with random effects in R (mgvc-package, Wood, 2011) to map elevation trends in time at different sites for each grid cell. We compare NBS elevation trends against island volumes, island means, and elevation change of selected conventional management analogues.
Results
We find that all three types of NBS lead to increased elevation gains. (1) Dune notches are effective in elevating lower-lying dunes areas, at average a rate of ~2 cm/year (2) Sand drift dikes prohibit sedimentation and have caused a cumulative backbarrier elevation deficit of 0.73 m. Reinstating overwashes through sand drift dikes is therefore likely to be an effective NBS. (3) Wadden Island salt marshes, including those restored from previously poldered areas, are currently keeping pace with SLR.
These NBS elevation gains occur on top of islands that are already increasing in volume. On average, the Wadden Islands grow at a rate of 0.3% per year. Furthermore, the extracted islands' sedimentation rates generally outpace sea-level rise: out of the 12 growing islands, 11 grow faster than present-day SLR.
Nevertheless, NBS implementation remains limited in scale. Although dunes, that cover 37% of the area, contain 60% of the islands' volume, restoration and notch projects are limited. Additionally, only 13 % of barrier island surface is salt marsh, compared to a poldered 26%.
Finally, we can conclude that NBS have the capability to increase the barrier island elevation. This will help to maintain the islands despite projected sea-level rise.
References
Lepper, R., Reinert, M., Gundlach, J., Weber, J., & Kösters, F. (2025). A hydrographic dataset of the Wadden Sea as a foundation for a digital twin of the coastal ocean. Scientific Data, 12(1), 1746. https://doi.org/10.1038/s41597-025-06211-1
Wood, S. N. (2011). Fast stable restricted maximum likelihood and marginal likelihood estimation of semiparametric generalized linear models. Journal of the Royal Statistical Society, 73 (1), 3–36. https://doi.org/10.1111/j.1467-9868.2010.00749.x
