The dynamics of foredune restoration measures

Abstract

This thesis focuses on the development of dune vegetation and dune morphology in two coastal areas in the Netherlands. The dunes on the northern coast of Terschelling and west of Heemskerk, that are under the influence of blowouts. These blowouts occurred as dynamic coastal manage-ment was implemented to increase flood protection by the dunes, this was done by a learn-by-doing concept. These measures have increased floodprotection, as dune crest height increased. But now, after more than 30 years of developing, the question arises whether the blowouts have benefited the coastal morphology and ecology, and if there is a relationship between the two. Therefore, two blowout systems were analysed over longer periods of time: the artificial blowout area on the northern coastline of Wadden Island Terschelling (1997-2016) and a single naturally developed blowout west of Heemskerk (2007-2016). The blowouts have different origins, as Ter-schelling was artificially created by digging eight notches into the dunes and removing vegetation. Whereas the blowout in Heemskerk initiated and developed naturally by letting the area develop freely. These study areas were analysed based on the development of both morphology and vegetation using airborne LIDAR elevation data and Landsat satellite imagery. Overall, the dune volume of both areas increased: 54% for the hinterland of Terschelling and 7% for the area behind the much smaller blowout near Heemskerk. The mapping of vegetation using NDVI resulted in maps, which indicate the amount vegetation cover of the blowouts with bare soil for Terschelling; hinterland initially decreased (to 0,3 in 2003) but is now increasing (to 0,4 from 2004 to present). As annual volume change decreased in the blowout area, from 5.700m3/year to 1.500m3/year (from 1997 LIDAR). Depositing sand both in front and behind of the blowout; resulting in the disappearance of original foredunes from pre-1996 and growth of the secondary dunes. For Heemskerk, the NDVI is high and is decreasing (from 0,4 in 2004 to 0,2 in 2016 in the blowout area), as the blowout is still progressing landward; a large lobe is forming behind the blowout. The blowout still had an annual volume change of 1.900 m3/year in 2015. The results of both locations substantiate the fact that blowouts increase throughput of sand from the beach further into the dune system. The seasonality of this phenomena could be analysed more accurately with other imagery, such as aerial photography or newer satellite imagery (ESA’s Sentinel 2). As Sentinel has more images available (one every eight days) and the resolution is higher (10 by 10 versus 30 by 30 of Landsat).