The effect of repeated sand nourishments on long-term nearshore evolution: a case study for Noordwijk aan Zee, the Netherlands
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Sand nourishments are executed along many sandy coasts worldwide to counteract coastal erosion, with the sand added as beach nourishments to the inter- and supratidal beach or as shoreface nourishments to the subtidal nearshore profile. Since the early 1990s beach and shoreface nourishments have been carried out along the Dutch coast, with a total nourished volume of on average 12 Mm3/year. Although we have a reasonable understanding of the temporary effects of individual nourishments on the evolution of nearshore morphology, the long-term effects of repeated nourishments remain unclear. This knowledge is not only crucial for the safety of beachgoers and marine life, but also because it is expected that the number and volume of nourishments will increase in view of expected accelerating sea level rise in the near future. The aim of this thesis is to assess how repeated nourishments impact the long-term evolution of the shoreline and the two subtidal sandbars at Noordwijk aan Zee, The Netherlands, using Argus video imagery available since 1995. Four shoreface and three beach nourishments were realised at the study site between 1998 and 2014. The low tide time-exposure images of the Argus video station were used to monitor the position of the shoreline and sandbars along a 6-km stretch of coast. We found that the sandbars slowly but persistently migrated seaward prior to the first nourishment. The repeated nourishments permanently reduced this offshore migration rate of the bars to only a few metres per year. The sandbars exhibited alternating periods of seasonal to multi-annual offshore and onshore migration that were superimposed on the weak decadal offshore trend. Moreover, the numerous sand nourishments gave rise to a forked shoreline-sandbar morphology. Bar switches, in which the landward part of a sandbar or the shoreline on one side of the bifurcation realigned with the seaward part of a bar on the other side, nullified this large-scale alongshore variability within 1 - 2 years. These switches seem to be a direct result of the repeated nourishments. The 2013-2014 succession of a beach and a shoreface nourishment, for example, led to four bar switches in the following two years, whereas there were only of 12 switches in total in the data set of 24.8 years. In addition, this study confirms that bar switches not solely depend on wave height, but the angle of wave incidence is also crucial for their development. Another observation was that the primarily in-phase coupled low-tide shoreline and inner barline did not directly become more crescentic due to the nourishments. Hence, the presence of associated rip channels also did not increase, but their spatial distribution changed from the inner bar to the low-tide shoreline after repeated beach nourishments. Further research should be conducted to investigate whether these findings also apply to other nourished coasts worldwide.