7.3.4: Bar cycles over years
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In Sect. 7.3.2 on beach states, we have discussed the three-dimensional behaviour of bars under antecedent and concurrent wave forcing conditions. This behaviour is either forced or self-organisational behaviour of the bar around its mean position of that particular instant. However, that mean position varies over time as we will discuss here.
If we study the behaviour of bars on longer timescales, i.e. years, we observe that bars generally and on average move offshore under more energetic conditions and may move a little back onshore during less energetic but skewed waves. The net offshore movement generally shows a cyclic behaviour; the initial bar formation is in the in- tertidal zone, after which the bars move offshore and grow in size until a maximum is reached somewhere around the initiation of the surf zone, after which they gradually decrease in size and amplitude and finally disappear at the end of the active shoreface profile. Along the Holland coast, this cycle varies between 4 to 5 years at the South Holland coast (SH) and around 15 years at the North Holland (NH) coast. The cyclic behaviour is indicated in Figs. 7.14 and 7.15 for the NH and SH coast, respectively.
The NH transect 7001827 between Callantsoog and Petten demonstrates an undisturbed, stationary bar movement (Fig. 7.14, top right). On the contrary, near Bergen aan Zee (transect 7003350) the initial 15 year bar cycle stagnates around 2000 (Fig. 7.14, bottom left). When comparing the 3D images of 1990 and 2019 (Fig. 7.14, middle) the sandy reinforcement of the Hondsbossche and Pettemer sea defence – completed in 2018 – is clearly visible.
The SH transect 8006900, which is just south of Zandvoort, exhibits an initially rapid offshore bar migration (4 year cycle) that gradually slows down in time to a 5 year cycle from 1990–2010 and a still longer cycle since 2010 (Fig. 7.15, top right). Near Katwijk (transect 8008850) the initial rapid offshore bar migration (5 year cycle) stag- nates around 2000 (Fig. 7.15, bottom left). The 3D images (Fig. 7.15, middle) clearly show the nourishment intervention known as the Sand Engine, comprising an unprecedented \(21.5\ Mm^3\) unit commands concentrated mega-nourishment, which has been implemented in the summer of 2011 (Sect. 10.7.6).
We believe that the changes to the autonomous bar cycle in both NH and SH are the cause of the Dutch coastline preservation strategy, which consists of frequent sand nourishments on the beach and shoreface to maintain the coastline in its 1990 position. The difference in cycle time between NH and SH is most probably explained by the fact that the shoreface slope is steeper and the bars are larger on the NH coast (see Fig. 7.5), such that more energy is needed to induce sufficient sediment transport. The importance of this behaviour is most probably that the bar’s position will influence the energy dissipation that energetic waves will undergo. This may lead to regional differences in shoreline response (see List and Farris results in Sect. 7.3.3).