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Pre- and post-nourishment morphologic behaviour along the Dutch and Danish North Sea coast

Barmentloo, D.S.G. (2018) Pre- and post-nourishment morphologic behaviour along the Dutch and Danish North Sea coast.

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Abstract:In the shoreface of the Dutch and Danish coast nearshore sandbars are present. These sandbars play a vital role in the nearshore morphology; due to the decreased depth at the bar crest, waves break and dissipate a part of their energy before reaching the coast. The sandy North Sea coast of The Netherlands and Denmark are both prone to erosion, especially in case of storm events. Sandbars are important to reduce this coastal erosion. Amongst other protection measures, shoreface nourishments are applied along both coasts aimed at counteracting erosion so that the coastline is maintained and the probability of flooding is decreased. In case of a shoreface nourishment, sand is supplied to the coastal zone, commonly around -6m MSL (mean sea level). A shoreface nourishment influences also the migration and position of nearshore sandbars. In this research, eigenfunction analysis of cross-shore transect data measurements has been performed to investigate the influence of shoreface nourishments on nearshore morphologic behaviour, including sandbar migration. By using eigenfunctions analysis, dominant modes of variation (eigenfunctions) have been determined. The temporal component corresponding to the eigenfunctions (weightings) enables to examine the development of the nearshore morphology over time in terms of the shape of the eigenfunctions. The first eigenfunction, the most dominant pattern of variation, strongly resembles the time-averaged profile while the second and third eigenfunction generally account for migrating nearshore sandbars. Pre-nourishment morphologic behaviour This research concludes that the cyclic offshore bar migration along the Holland coast, as previously observed by Wijnberg (1995), remained present after 1990 until the application of the first shoreface nourishments. North and South of the IJmuiden harbour moles, offshore bar migration is observed, though on a completely different timescale. Bar cycle return periods of 15 (range: 12-18) and 4 (range: 3-4) years are observed respectively. Along the Danish Midtjylland coast (km. 80-156), generally offshore migrating shore-oblique sandbars are observed. The sandbars have lengths of approximately 6-10 km and are generally attached to the shore in the north and extend seawards in the south. Due to the oblique orientation and relatively coarse resolution of the data used in this study, it seems like the sandbars are migrating northward. However, the apparent northward movement is likely the effect of offshore migration of the shore-oblique bars, combined with bar decay at the most offshore (southern) point and development of a new bar close to the shore in the north. The bar cycle return period is estimated between 8-12 years. Compared to the consistent bar migration along the Holland coast, the observed bar migration pattern along the Danish coast is more variable and less consistent. Along major parts of the Danish west coast, the shoreface steepened up to 50% over the last century, combined with over 200m coastal retreat. The eigenfunction analysis used showed no off- or onshore bar migration pattern along the Danish coast from Hanstholm to Fjaltring (km. 0-79). Post-nourishment morphologic behaviour After the application of shoreface nourishments, reduced offshore migration, stagnation and temporal onshore migration of the offshore moving sandbars is observed. This effect is present for almost all shoreface nourishments along the analysed Holland coast with offshore migrating bars (km. 30-90). The duration of affected bar behaviour due to a single nourishment in this area ranges from (at least) 13 years to only 1 year. This period of 13 years is significantly longer compared to many previous studies of individual nourishments along the Holland coast. Investigating the effect of single nourishments on the bar migration is often complicated due to the application of a subsequent nourishment shortly after the first nourishment. The application of multiple nourishments north of the IJmuiden harbour moles (transect km. 30-40) led to 9 years (2007-2016) of bar stagnation. Hence, the current nourishment practice seems to cause stagnation of the sandbars. The interval between the nourishments is not large enough to make offshore migration of the sandbars possible. Results of this research show that along the Holland coast (repeated) nourishments influence the offshore bar migration up to 2km alongshore from the borders of the nourished section. Generally, the alongshore influence is very limited and bar switches occur directly at the borders of the nourished section. In the area where no migrating sandbars are observed (between Den Helder and the former Pettemer Zeewering), the nourishments cause significant and long term (> 8 years) flattening of the shoreface. In Denmark the observed pre-nourishment bar migration forms a less clear pattern, complicating the analysis of post-nourishment bar behaviour. Only at one location a clear bar signal has been observed over an 8-year period in which nourishments were applied frequently. In this particular case, the bars were relatively stable during the nourishment period. After this period, bar started migrating offshore. No consistent relation between nourishment implementation characteristics and bar migration has been observed. Generally, large nourishments do affect the offshore migration for a longer period. However, also after one relatively small nourishment a long period of bar stagnation has been observed. Causal relationships are difficult to determine due to the many variables and non-linear relations between them (e.g. sediment diameter size and distribution, nourished volume per running metre, total nourished volume, wave-variability (storm-events), placement depth and the pre-nourishment morphologic behaviour).
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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