Spatial and Temporal Evolution of the Foredune Restoration Project at Terschelling, The Netherlands

Abstract

Coastal management practices intended to strengthen established foredunes have contributed to the stabilization of coastal ecosystems. As a result, the biodiversity within these ecosystems, as well as their ability to grow with rising sea levels, declined. To counteract these adverse trends, some coastal management projects nowadays aim to restore dynamics in coastal ecosystems through the excavation of foredune notches, gaps in the most seaward dune that resemble naturally occuring trough blowouts. It is essential to understand how these restoration measures effect the spatio-temporal evolution of the coastal ecosystem. However, knowledge regarding the physical-ecological interactions that influence the evolution of restored coastal ecosystems, especially after initial developments, is limited. Using annual lidar data and freely available Landsat and Sentinel-2 satellite imagery, this study examines the development of a coastal dunefield at Terschelling, the Netherlands, where sand trapping fences were removed and notches were excavated through a 5-km long unvegetated foredune in 1995. The results reveal that the restoration measures initially led to remarkable dynamism, with sand surface areas gaining ~2.85 ha/yr on average and absolute sand volume change rates reaching up to ~7.1×105 m3/yr. The unvegetated rolling foredune eroded and new foredunes and transgressive sand sheets were gradually formed. As sand deposition on and seaward of the new foredunes limited sand transport towards the hinterland, aeolian erosion formed deflation basins between the new foredunes and transgressive sand sheets. Vegetation primarily re-established in the deflation basins, at elevations below 3 m mean sea level, near the water table, where depositional and erosional rates were below 0.3 and 0.1 m/yr, respectively. Once established, vegetation spread to higher elevations. The results indicate that vegetation growth within the dunefield is mainly regulated by the geomorphological evolution, dynamism and topography of the dunefield. Furthermore, the findings indicate that the dunefield will restabilize completely. Proactive measures such as repeated excavation may be required to reintroduce dynamism.