The effect of cyclic shoreline dynamics on morphological properties of the downdrift coast: Ameland, the Netherlands
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The aim of this Master thesis is to contribute towards knowledge of the downdrift effects of cyclical ebb-tidal delta behaviour at tidal inlets. The north-west coastline of Ameland, the Netherlands, is greatly influenced by the adjacent tidal inlet. It is observed by various authors (e.g. Israel and Dunsbergen, 1999) that the ebb-tidal delta of the Ameland Inlet changes its morphology through a cycle of approximately 50-60 years. This cyclical behaviour permits the bypassing of sand across the ebb tidal delta in the form of a migrating shoals, which weld to the downdrift beach and add a large volume of sand to the beach. It would be expected that sand from the shoal would disperse alongshore in the easterly direction and also toward the tidal inlet. However, there appears to be a longshore limit to this effect. The central part of the island downdrift of the welded shoal experiences chronic erosion which has prompted the need for massive beach nourishment to counter this ongoing erosion. Thus, insufficient longshore dispersal of sand volume from the shoal enables erosion at downdrift locations. This research intends to quantify the morphological changes to the north-west Ameland coastline to understand the transfer of sediment volumes from the attaching shoal to the beaches. The research approach varies in both temporal and spatial scale, combining data collected during a six-week period of fieldwork in Autumn 2010, with the long-term Rijkswaterstaat Jarkus dataset of coastal profiles. Analysis of the short-term fieldwork data examines the measured changes in the surface of the beach over the six-week period of data collection. Transect and spot dGPS measurements were used to create digital elevation models (DEMs) in ArcGIS from which it was possible to derive cross-shore profiles, estimate volumes, and calculate slope. The long-term behaviour of the coastline was interpreted by analysis of the trends (cross-sectional volume; width; slope) in coastal profiles over a long time period. The data used for this analysis is freely-available Jarkus data which was collected, by various means, since 1965 to the present day by Rijkswaterstaat, the governing body responsible for maintaining and protecting the Dutch shoreline. This research has shown that, through time, the shoal is dispersed in such a way that “peaks” in volume and width occur successively from one transect to another in an easterly direction. This appears to be the passage of a sand wave (Schwartz, 2003). The sand wave is preceded by a trend of erosion of around 5 years prior to its appearance at each transect (4 to 7). The sand wave becomes attenuated in form and spread across a wide alongshore width. This attenuation is shown in transects 8 through to 10, where the changes in beach width and volume are reduced compared with the near-field beaches. The volume reaching transect 9 is half of that which was added to transect 5 (from 5000 m3/m to around 2600 m3). The attenuation trend is typical of sand wave behaviour (Schwartz, 2003), and while the volume of the sand wave is conserved the volume is spread over a wider alongshore width and thus smaller changes are seen in the downdrift transects. The results showed that for most of the central transects (RSP 5-8) the shoal volume added was equal or higher than the total volume of the beach: a doubling of the average volume. Transects km 3 to 7 are seen to undergo the most amount of change during the 45 year record, while transects km 8 to 10 are affected in a more gradual manner. A time lag between beach volume and width peaks occurs in transects 3 to 6, ranging from 8 to 11 years in phase difference. In transects 8 to 10 this phase difference is no longer apparent. A bifurcation in sediment transport is proved in both the short- and long-term data. A similar erosion trend was not observed at any other location on the beach, near- or far-field, which suggests that a bifurcation in sediment transport enhances erosion at these transects. The results of the beach volume analysis has shown that a significant volume from the sand wave is yet to reach the eroding locations of central Ameland, and is still dispersing from transects 9-10. It remains to be seen how much of the original volume will nourish the distant beaches, as the attenuation of the shoal between transect 4 and 10 has decreased the available volume for dispersal. Further research and modelling is necessary to quantify the volume changes further downdrift, while understanding of the cyclical dynamics of the Ameland Inlet could further improve the management of the extremely eroding beach at transect 3.