GPR imaging of recent coastal-dune development : effects of sand nourishments : a contribution to the projects 'Kustlijnzorg' and 'Kustveilig'

Since 1965, the development of the coastal zone has been monitored by means of coastal profiling (called Jarkus). Decadal shoreface, beach and foredune development can be analyzed using this morphometric database. The Jarkus database contains elevations for cross sections spaced apart 250 m (corresponding with the beach pole grid). These elevations are measured annually along the entire Dutch coast (cf. Arens & Wiersma, 1984). The profiles stretch from about 1000 m seaward of the low-tide line up to at least the dune foot. The supratidal part of the profiles has been measured by airborne laser altimetry (LIDAR) since 1996. Earlier profiling of the land part was achieved by topographic levellers and aerial photography. Annual measurements extend landward to at least the first top of the (fore)dune. Year-to-year variability in measuring and processing techniques can hamper straightforward analyses of the net changes of profiles (Arens, 2009). Additionally, the Jarkus database solely represents annual registrations of in the topography of the beach-dune profile. Hence, little can be said on the actual processes resulting in the observed year-toyear variability. This paper addresses these processes by linking Jarkus with groundpenetrating radar (GPR) and to meteorological observations. This approach has been applied to data from a non-nourished beach that has been subject to long-term net erosion and from an actively and frequently nourished beach that has experienced recent coastline progadation. The primary aims are to determine 1) the origin of sedimentary elements within the beach foredune area, 2) the fate of the sand volumes nourished in the past, and 3) the decadal-scale preservation potential of sand in the dune foot and on the adjacent backshore in light of the established nourishment policy. A secondary aim is to demonstrate temporal and spatial accretion and erosion variability along nourished coasts.