M.W.J. Smit1*, W.P. Bodde1, A.H. Verheijen1, J.K. Leenders2, C.W. Wegman2
The mega nourishment of foreshore, beach and dunes (36 Mm3) at the Hondsbossche and Pettemer Sea Defence in The Netherlands was monitored for 3 years from 2015-2018. The aim of this Ecoshape innovation project was to increase understanding of how aeolian processes shape a constructed dune and how this can be applied to optimise the design and create additional value such as coastal safety, ecological development and optimal spatial integration.
For this purpose a monitoring program was set up which included 9 laser altimetry measurements and aerial photographs of the zone above the waterline, covering the entire project area, including some of the adjacent coastline. Also, half-yearly site visits were conducted and the ecological development was monitored. Dune growth rates, beach erosion rates and wind data were analysed for this period.
It was found that the average dune growth rate over the first three years is 33 m3/m1/year. Important sources of the sand that blows into the dune area are the shallow foreshore and the intertidal beach as over 50% of the sand originates in those areas. The dune growth rates vary along the nourishment which is explained by the orientation of the dune foot with respect to the dominant wind direction, the beach width and the sediment grain size.
It was observed that most sand is captured in the dune foot and on the seaward side of the dune. This area quickly evolves in a natural looking area with fresh and healthy vegetation. It is this dynamic zone where, in future projects, the aeolian evolution can be directed by planting variable patches of vegetation, applying a variable geometry or adding willow screens. This can prevent dynamics if required for coastal safety or to reduce hindrance, but it also offers opportunities to stimulate dynamics at locations where this is desired.
In combination with the Dutch coastal policy of maintenance the dune growth means that the coastal area becomes safer and is able to keep up with sea level rise.
Figure 1 Aerial photographs of central part with dune valley, showing evolution of dune area over two years. The view is Northward, with the North Sea (not visible) and beach on the left side.