Observations of Beach-Dune Interaction in Man-Made Trough Blowouts

Dunes act as an indispensable natural safety barrier against marine flooding, are valuable natural environments, serve for the production of drinking water, and offer recreational opportunities. The safety function has dominated dune management in the Netherlands, as well as on other developed shores, for decades. Measures to minimize the seaward loss of foredune sand under storm wave attack have resulted in over-stabilized foredunes and, accordingly, have impoverished natural beauty and degraded biodiversity. Stabilized foredunes are nowadays increasingly reactivated by removing vegetation and by digging trenches that resemble natural trough blowouts as corridors for aeolian transport from the beach into the backdunes. Crucially, such measures see the beach-dune system as an integrated landscape, reconnecting the beach, foredune and backdunes through aeolian transport. This raises questions on how much sand is actually blown into the dunes, which factors affect this amount, and how effective the large-scale dune-restoration measures are.

This contribution addresses the evolution of five trenches that were dug through the 20-m high foredune at the Dutch National Park Zuid-Kennemerland. The width of the trenches was 50 - 100 m, their cross-dune length was 60 - 100 m, and the heighest part of the valley floor was 9 m above mean sea level. Since their construction in the 2012/2013 winter the trenches have been surveyed approximately 3 times per year using airborne laser scanning or UAV photography, resulting in a multi-temporal data set of Digital Elevation Models (DEMs) with a 1x1 m resolution. Difference maps illustrate that the sidewalls of the trenches have steepened during the first two years after construction, but that their width and the height of the valley floor have remained largely unaltered. Landward of the trenches large sedimentation lobes have formed, which with time have grown both laterally and vertically. Locally, the lobe thickness now exceeds 5 m. Priliminary analysis of the DEMs further indicates that the sand volume of the lobes exceeds the sand loss from the trenches, implying that part of the deposited sand is wind-blown beach sand. Our observations will help validate aeolian transport modules that are currently being developed to build coupled dune-erosion and growth models.