Measuring sediment deposition and accretion on anthropogenic marshland - Part II: The adaptation capacity of the North Frisian Halligen to sea level rise

Low coastlands, marshlands and islands all over the world are challenged by rising water levels due to climatic changes. The adaptation capacity of such lowlands is based on frequent inundations and according sedimentation processes. Exemplarily, a system of small islands west of Northern Germany was investigated over three years. At three out of ten so-called Halligen located in the Wadden Sea, the adaptation capacity of the anthropogenic marshland was determined. The Halligen Hooge, Langeness and Nordstrandischmoor have surface elevations only a few decimetres above mean high water and have to cope with an inundation frequency of nowadays up to 22 times per year. By use of methods introduced in Schindler et al. (2014, this volume) in combination with a Cs-137 and Pb-210 dating campaign on 12 sediment cores, vertical accretion rates were measured and detailed sediment accretion patterns presented. A good agreement was found between the used methods to calculate long term and short term marshland accretion rates. Sediment deposition and vertical marshland accretion is mainly controlled by the high tide events (single storm surges). Coastal protection structures, established in the early 20th century, decrease the inundation frequency and hinder the efficiency of the sediment transport by the tidal channel system on the Halligen. Vertical marshland accretion based on 210Pb dating for the time span 1915-2011 (1.0 +/- 0.3 mm/a, Hooge, 1.2 +/- 0.3 mm/a, Langeness and 2.6 +/- 0.9 mm/a, Nordstrandischmoor) is in disequilibrium with the fast increasing mean high water level (MHW, 5.0 +/- 0.3 mm/a). Projections until 2100 revealed that the extreme values (highest high waters, HHW) tend to rise much faster than the MHW or relative mean sea level (RMSL). Therefore an increasing hazard potential for the Halligen has to be expected if vertical marshland accretion does not accelerate in the future. (C) 2014 Elsevier Ltd. All rights reserved.