The Building with Nature approach has been gaining ground in hydraulic engineering, increasing the importance of understanding the cross-shore morphodynamic processes. The intertidal zone, where marine and aeolian processes come together, is an important link in the transport of sediment from the sea towards the dunes. The grain size distribution affects the sediment supply in the intertidal zone. This research investigates the effect of marine processes on the cross-shore variations of the grain size distribution in the intertidal zone by using a one-dimensional non linear shallow water XBeach model.
The intertidal zone is subject to shoaling, surf and swash zone processes. The grain size influences the beach slope, the initiation of motion and settling to the bed. The cross-shore sediment transport is the combination of sediment that is stirred up from the bed and subsequently transported. Breaking induced turbulence enhances stirring of sediment from the bed and keeps sediment in suspension. The amount of stirring and the transport direction depends on the wave conditions.
Input and control data for the model study was provided by the Scanex 2020 fieldwork campaign at Noordwijk, the Netherlands. The ADV velocity data combined with a pressure signal has been used for the tidal and incoming wave signal. Cross-shore profiles have been determined in Matlab based on terrestrial laser scans. Soil samples of the intertidal zone were taken with a sand scraper and analyzed with a sieve tower. For the initial grain size distribution is the average distribution of 14 samples on a transect was used. Based on wave, wind and soil sampling data a model period from 29-2-2020 02:00 to 10-3-2020 13:00 was selected.
The XBeach model used is as described by Reniers et al. (2013), but with a time-averaged turbulent kinetic energy and a different implementation of the Riemann boundary. The model consisted of a 176 x 3 grid with a grid size of dx=1 m and dy=5 m. For the initial bathymetry the laser scan of 29-2-2020 02:00 was used. The initial grain size was imposed on all the model grid cells. Additional to the standard run, runs have been performed to research the effect of a storm, the model sensitivity and the effect of aeolian transport.
The model shows a pattern of cross-shore grain size variations with coarser sediment from x=20 to x=56 m, finer sediment from x=57 to x=105 m and fluctuating grain size from x=106 to x=136 m compared to the initial grainsize. After 24 h a grain size pattern establishes with a clear deposition of fine sediment on the upper beach. The pattern remained stable for nearly the full model period. After 200 hours the fines become less prominent and move onshore. On the intratidal scale sediment becomes coarser when submerged and finer when emerged, except near the high water line where fine sediment is deposited.
The model reproduced the same pattern of grain size variations over the cross-shore as was found in the soil samples of 10-3-2020. As the cross-shore grain size pattern remained stable during the model period, processes on the spring-neap time scale or storm time scale seem to govern the cross-shore variations of the grain size. For the aeolian transport this would imply that for this model period the fine sediment supply is controlled on the same time scales. Nevertheless, considering that aeolian transport could have resulted in coarsening of the fines in the upper intertidal zone, processes over a single tide, could be more important than was visible in the model result.

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Dunes are the primary sea defence along the Dutch coast. This research investigated the development between 1965 and 2021 of the beaches and dunes of the Hoogheemraadschap Hollands Noorderkwartier (HHNK) area which stretches from IJmuiden to northern Texel. This research used an advanced data analysis on the annual coastal elevation data. The coastline of the HHNK area is maintained by several different measures including nourishments, planting and removal of grass, placing reed fences and building regulations. The aim of this research was to relate the investigated developments of the beaches and dunes to these measures and natural processes.
A literature study was performed first, to get acquainted with the research area and the processes that drive the beach and dune development in the area. In four phases, the developments and their relationships with the drivers were investigated for several subsections of the research area that showed similar behaviour. In phase 1, the coastal profile data was collected for multiple transects spanning the research area. Characteristic parameters describing features of the dune and beach were derived from the coastal profile data. In phase 2, the collected profile data was decomposed into spatial and temporal patterns by a principal component analysis. In phase 3, the transects of the research area were categorised on the temporal development of their coastal profile using the results of the spatial and temporal decomposition. This created subsections of the coast that showed a similar morphological behaviour. In phase 4, the morphological behaviour of several subsections was investigated and related to the natural and human drivers. The relationships between the drivers and the developments were investigated for several subsections, varying in morphological behaviour and drivers, to gain insight into these relationships for the entire research area.
The coastal categorisation resulted in 36 subsections which were mainly continuous in space with the exception of four clusters. Seven subsections were studied in more detail, by investigating the development of several characteristic parameters derived from the coastal profiles, like dune volume, beach slope and shoreline location among others. The shoreline location and beach width showed a strong correlation with the nourishments. The beach slope and width were expected to influence the dune volume changes, which was not supported by the results. The beach width is assumed to be larger than the critical fetch length and the variations in beach slope had a marginal effect on the transport capacity. The presence of beach pavilions limited the dune growth in both height and volume, but regularly moving the pavilions did allow for a seaward migration of the dune front. The effect of small beach houses on the dune volume change was found negligible. Maintenance works of HHNK influenced the dune development locally. The placement of reed fences caused seaward migration of the dune front and the creation of blow-outs increased the dune volume behind the most seaward dune. The strongest dune volume increase was found at coastal areas behind shoals that reduced the incoming wave energy.
The coastal categorisation succeeded in grouping transects with a similar development of their modified coastal profiles. Several modifications were proposed to improve the categorisation of the original profiles and to take into account smaller scale processes. Nourishment was distinguished as most important driver for the development of the coastal area, which has maintained the shoreline location and caused a trend break in several other characteristic parameters. Human influences like buildings, coastal structures and reed fences have a strong local effect on the development of the beaches and dunes. Further research could take into account wave and wind climate, water level variations and grain size quantitatively to get a more comprehensive study of the relationships between the development and the drivers. ... Read more

Coastal dunes are dominant features along much of the world’s sandy coastlines serving as the first line of protection against coastal flooding. Besides this primary purpose, the coastal dunes also provide a variety of other functions such as the supply of drinking water, nature conservation and recreational areas. With the secondary functions in mind, the Dutch coastal management strategy changed in 1990 from erosion control and stabilization of the coastline (reactive) to a policy of dynamic preservation (pro-active) with the introduction of a law called ‘’Dynamic preservation of the Dutch coast’’. This new dynamic strategy naturally induced irregularities in dunes that were completely stabilized before. One of these irregularities is the formation of a blowout. A small depression or hollow in the foredunes formed by wind erosion or wave impact may grow in time as sediment from the beach and foredunes is transported into the back dunes. In several Dutch cases such a blowout feature was artificially initiated as the exchange of sediment from the coastal system into the back dunes enhances the biodiversity significantly. Due to the high importance of the coastal dune performance as flood protection, good understanding and prediction of the (dynamic) coastal dune system is desired. Lately, general interest in this topic increased even more due to new societal challenges, such as decisions on coastal development for longer time scales, more complex management settings and sustainability. Coastal dunes and blowouts are shaped by wind induced sediment transport (aeolian sediment transport), biological - and hydrodynamic processes. To better understand and increase the predictability of dynamic systems, this study focuses on simulating the development of artificially initiated blowout features by including the combination of these relevant processes in the numerical model AeoLiS. This process-based model was originally developed to simulate aeolian sediment transport in supply-limited conditions, such as coastal areas. The study describes simulations of several academic dune formations to validate the applicability of the different (new) processes that were included in the numerical model. Finally, all processes are combined in simulations of a practical case that is used to compare numerical model results to field data on the development of multiple artificially initiated blowout features in the Dutch coast at Meijendel. ... Read more