S. IJff1*, B. Smits1, V.T.M. van Zelst1, B. Arens2
1 Deltares, 2 Arens bureau voor strand- en duinonderzoek
Dune management has traditionally stabilized foredunes to protect the hinterland against flooding. As a result of these static ‘sand dikes’, however, the biodiversity of the dune landscape was limited. Dynamic dune management allows and stimulates ‘dynamic foredunes’, where sand transport towards the backdunes is restored leading to a more diverse dune landscape and long-term coastal safety, because dunes can grow with a rising sea level. Both coastline management (nourishment) and foredune management potentially influence foredune dynamics, but the underlying relations are not fully understood. Here, we evaluate the role of nourishments and foredune management on dune volume and foredune dynamics. This research is part of the ongoing programme Natuurlijk Veilig, initiated by Rijkswaterstaat (www.natuurlijkveilig.nl).
We calculated sand budgets of the foreshore, the intertidal zone (including the beach) and the dune area along the entire Dutch coastline using coast-normal transects (Jarkusraaien) based on EO data from 1965 to 2017. Next, we categorized foredune dynamics based on the five ‘response types’ (RT) from stable (1) to dynamic (5) (Arens et al. 2010) using surface elevation (differences) maps and aerial imagery for the periods 1988-2008, 2008-2013 and 2013-2017. Foredune management data (Löffler and Veer, 1999; Löffler and Van der Togt 2018) and nourishment data (Rijkswaterstaat) are projected on the coast-normal transects. The relationship between nourishment and dune management on one hand, and foredune volume and dynamics on the other are analysed for 17 coastal sections (‘kustvakken’).
Our results show an average increase of dune volume between 1965-2017 of 8.2 (σ =9.4) m3/m/year. During the same period an average nourishment volume of 31.7 m3/m/year is supplied. Over time, a slight increase of the RT is observed, due to an increase of dynamics at the front and at the top of the foredune. The percentage of dynamic managed foredunes are increased from 11% in 1990 to 51% in 2017. According to our analysis, nourishment and foredune volume are correlated (=0.61). In addition, we found a clear correlation between foredune management and the corresponding RT. Insight in the effects of nourishments and foredune management are crucial to come up with effective integral coastal management approaches, which allow coastal safety and ecological development to go hand-in-hand.