SEDIMENT- AND MORPHODYNAMICS OF SHOREFACE NOURISHMENTS ALONG THE NORTH-HOLLAND COAST

Abstract

The Dutch coastal maintenance policy which was implemented in 1990 aims at fighting structural erosion by nourishing sand. Initially, the beaches were nourished. The lifetime of these beach nourishments turned out to be relatively short (~2 yrs), especially at identified erosion 'hotspots' along the North-Holland coast (e.g. Egmond aan Zee). Therefore, from 1999 on, an alternative approach was considered, in which beach nourishments were combined with large nourishments on the shoreface (seaward of the outer breaker bar). In recent years, the evolution of individual shoreface nourishments was analyzed on a spatial scale of kilometres and a temporal scale of years, with a focus on their cross-shore development (e.g., Van Duin et al., 2004). In these studies, their alongshore interaction received little attention. The main objective of the present study is to investigate the evolution and lifetime of five large (~1-2 million m3) shoreface nourishments applied between 1999 and 2005 on a spatial scale that includes the larger part of the North-Holland coast (~40km). The analysis of bathymetrical data shows that all five shoreface nourishments evolved into new outer bars with deep troughs on their landward side, that temporarily interrupted the 'autonomous' cyclic offshore migration of breaker bars (e.g. Wijnberg, 1995). Alongshore, the nourishment placed at Camperduin in 2002 connected to the existing outer bar, while the nourishments placed at Egmond aan Zee and Bergen in 1999 and 2000 respectively, remained isolated. The 2004 Egmond and 2005 Bergen nourishments interconnected into a long bar. At Camperduin, the newly formed outer bar remained quite stable in the years after its placement. The bars resulting from the 1999 Egmond aan Zee and 2000 Bergen nourishments, however, almost fully disintegrated within five years, while the interconnected bar of the 2004 and 2005 nourishments is still largely intact after 5 years. The results suggest that the effectiveness of shoreface nourishments is influenced by their shore-parallel length and their ability to connect to surrounding (natural or nourishment) bars. The observed morphodynamic patterns suggest that the 'lost' sand is mainly redistributed to the breaker zone, and further transported alongshore.