Assessment of historic flood plain sedimentation rates along the River Meuse in The Netherlands using 137Cs dating with PHAROS.

Deposition of fine sediments during high river discharges is an important factor in flood plain evolution. To assess the effect of river restoration works and climate change on sediment dynamics, a solid understanding of the present and historic sedimentation rates is required. Overbank sedimentation is a process which varies both in time and space, therefore accurate data for sedimentation rates at various locations within a flood plain are required. Episodic 137 Cs deposits offer a way to document rates of overbank deposition over the past 50 years. A laboratory-based device, PHAROS, was designed to help date sediment cores by non intrusively measuring the intensity of the gamma rays emitted from the decay of 137 Cs in 2 cm increments. From a flood plain along the River Meuse (The Netherlands) 23 sediment cores of 1 m length were obtained for PHAROS analysis. From the observed 137 Cs peak depths sedimentation rates were estimated to vary from 0.4 to 2.0 ± 0.2 cm year -1 . The PHAROS derived 137 Cs activity concentrations were calibrated with a regular technique (HPGe). In addition lead isotopes were used to validate the 137 Cs-derived time records.