Analysis of the Beach and Dune Development of the Hoogheemraadschap Hollands Noorderkwartier Area

Abstract

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.