Abstract
Barrier sandbars and barrier islands are geologically young, highly dynamic and represent a complex coastal system that includes a number of different but closely related sedimentary depositional environments with geomorphologic elements of varying origin, genesis and evolution. Barrier sandbars are exposed ridges of sand that are built offshore by wave action. The so-called Kniepsand on the island of Amrum represents one of the widest beaches in Europe. The Kniepsand originally belongs to a system of sandbars lining the North-Frisian coast. Some of these sandbars are already attached to the mainland (e.g. St. Peter-Ording-Sand and Westerhever-Sand). The offshore sandbars of Japsand, Norderoogsand and Suderoogsand are located in front of the western coastline of the North-Frisian Island and the Halligen and fulfil the function of natural coastal defence dissipating the energy of the incoming deep-water waves of the North Sea. Barrier sandbars are usually investigated through the use of aerial photos and borehole data. Therefore, the processes of evolution and the internal structure of sandbars are often unknown. That is why this study chooses an integrated approach using high-resolution ground-penetrating radar (GPR) and sedimentological analyses of shallow sediment cores drilled at selected sites along the radar profiles. A geophysical Survey Systems Inc. radar system, SIR-2000 coupled with a 200 MHz antenna, was used. Based on these data a sedimentary model was generated that describes the process of barrier sandbar migration and the attachment to the Pleistocene island core. According to historical maps and nautical charts of the sixteenth and seventeenth century, the Kniepsand used to be a solitary barrier sandbar located well in front of the islands west coast without any connection to its Pleistocene core consisting of Saalian moraine deposits. The presented model shows how the Kniepsand has welded onto the Pleistocene island core of Amrum. Before the barrier sandbar was connected to the island, tidal flat deposits had been accumulated in a low energy environment. Tidal flat deposits show a general coarsening upward trend and turn into overlying coarser grained beach deposits. Old cliffs formed through several storm surges are also preserved in GPR data. Normally, the former structure of the barrier sandbar will be lost as its sediment is welded onto the island core to form the contemporary sandy beach. The study indicates that GPR-data make it possible to reveal the structure of the former sandbar and to figure out and understand the barrier sandbar attachment process.