Monitoring of a coastal foredune restoration project: Noordwest natuurkern, The Netherlands

Gerben Ruessink 1, Jasper Donker1, Sebastiaan Arens2, Marieke Kuipers3
1Utrecht University, Utrecht, The Netherlands, 2Bureau for Beach and Dune Research, Amsterdam, The Netherlands, 3PWN Drinking Water Company, Velserbroek, The Netherlands

Coastal dunes form an indispensable natural safety barrier against marine flooding. Therefore, dune management along developed shores typically comprises measures to stabilize the foredune, for example, through marram grass planting. Dune stabilization, however, negatively affects dune biodiversity and geomorphological diversity. In response, stabilized foredunes are nowadays increasingly reactivated by removing vegetation and by digging notches, resembling natural trough blowouts, as corridors for aeolian transport from the beach into the backdunes. Crucially, such large-scale measures see the beach-dune system as an integrated landscape, reconnecting the beach, foredune and backdunes through aeolian transport. This type of foredune restoration, however, has a high 'learning-by-doing' character, as the evolution of man-made nothces and their long-term (> years) effectiveness are not understood.

The objective of this contribution is to analyze the evolution of five notches that, in the 2012/2013 winter, were dug through an 750-m long stretch of the 20-m high foredune at the Dutch National Park Zuid Kennemerland (Noordwest natuurkern project). The initial width of the notches was 50 - 100 m, their cross-dune length was 60 - 100 m, and the highest part of the valley floor was 9 m above mean sea level. In total, 170,000 m3 of sand was removed in digging the notches. Since February 2012 the region has been surveyed with airborne laser scanning and UAV photography, resulting in a multi-temporal data set of 10 Digital Terrain Models with a 1x1 m resolution. Topographic difference maps illustrate that the width and depth of the notches have remained largely unaltered, but that their sidewalls have steepened. This has caused the cross-section of the notches to develop from an initial V-shape into a more U-shape. In total, about 8,000 m3 of sand was eroded from the five notches after the first year, and 24,000 m3 after the second year. Landward of the notches large depositional lobes have developed, which with time have grown both laterally and vertically. Locally, lobe thickness now exceeds 7 m and lobe length 50 m. Additionally, some sand has been deposited on the foredunes between the notches. The total volume of sand gained was about 16,000 m3 after one year, and 44,000 m3 after two years. This implies that about half of the deposited sand was eroded from the notches, with the second half presumably originating from the beach. The latter would equate to roughly 12.5 m3/m/year of wind-blown beach sand. We believe that our observations will help validate aeolian transport modules that are currently being developed to build quantitative beach-dune interaction models.