Nearshore dynamics of a nourished coast with respect to a neighbouring natural coast

Abstract

The Dutch coast belongs to one the most heavily engineered coasts around the world. In order to protect the coast from structural erosion, the Dutch government decided to dynamically maintain the coastline of 1990, by making use of nourishments. This thesis focuses on the impact of these nourishments on nearshore dynamics, in terms of bar behaviour and volume trends. However, a problem with nourishments is that a proper comparison to ‘natural behaviour’ is always hampered by either time or location. This thesis tackles this problem by comparing a nourished coast to a directly adjacent unnourished site. To do so, 2250 transects from the JARKUS data set, measured perpendicular to the coast of Egmond aan Zee (The Netherlands), were studied. These transects consisted of 45 locations, with a longshore spacing of 250 m and a total length of 11 km, and contained a nourished part and an unnourished part of 5 km and 6 km respectively. The study of exact bar behaviour, in terms of cross-shore location and volume, required a method which could isolate bar positions from these profiles. This method was based on techniques used earlier by Ribas et al. (2010) and Radermacher et al. (2018). It appeared that the intersections of the first derivative with the mean slope is the best approximation of the bar edge. By making use of this strategy, all bars present near Egmond in the period 1964 to 2013 could be found and studied, based on their cross-shore location and volume. This led to the following conclusions: First of all, corroborating other studies on nourishing, erosion trends of the dunes and nearshore area turned into accretion trends. Secondly, implementation of the shoreface nourishments near Egmond locked the bar system landward of the nourishment and ‘froze’ the bar positions for periods up to five years. Next, in the unnourished section (south of the nourished section), Net Offshore Migration (NOM) continued, causing large longshore jumps in cross-shore bar positions, which ultimately resulted in a complete bar switch episode in 2001. Then, after depletion of the outer bar in the unnourished section, the oblique orientation of the whole bar section turned around and 3D structures such as crescent bar shapes and sand waves started to migrate in the opposite direction. Finally, although volume trends in the nearshore area became positive, bar volume did not change, which suggests that sandbars play a minor role in the spreading of sand throughout the surf zone.