Salt marsh restoration at a rapidly retreating site: a multie-scale assessment of interactions between hydrodynamics and biogeomorphic development

Many salt marshes around the world are currently retreating or at risk of retreat as a result of edge erosion. The salt marsh of Wierum is an example of a rapidly retreating marsh in the Dutch Wadden Sea (northwestern Europe), with limited landward accommodation space due to a landward seawall with a village immediately behind, i.e. it is a site experiencing coastal squeeze. In August 2024, marsh restoration was implemented by building brushwood dams seaward of the marsh, to 1) prevent further marsh retreat and 2) form sedimentation fields.
To gain insight into the local effects of this intervention on marsh retreat, hydrodynamics and biogeomorphic development, we assessed at different scales how the brushwood dams affected the hydrodynamics, morphodynamics, sediment properties, vegetation establishment, and the interactions between them. Utilising the opportunity to collect data both before and after brushwood dam construction, we periodically collected data on wave conditions, surface elevation, sediment characteristics (shear strength, organic carbon content, and nutrient availability), and vegetation presence.

Our results show that the brushwood dams strongly attenuated wave energy, reducing energy by up to 80% (55% average). During the initial 5 months after restoration, 10-25 cm of sediment was deposited within the sedimentation fields. Vegetation establishment seaward of the marsh edge was observed as early as spring/summer 2025. Nonetheless, the sediment within the sedimentation fields remained relatively soft compared to the marsh, with low shear strengths, indicating that rapid vertical accretion resulted in low sediment stability.

In conclusion, first-year observations show that brushwood dam construction is a suitable intervention to attenuate wave energy and enhance sediment accretion at the Wierum marsh. However, it may impede the rapid development of stable sediment beds. This highlights the importance of multi-year evaluation of trade-offs between rapid sedimentation, vegetation establishment, and the development of sediment-driven ecosystem services in marsh restoration projects.