Floodplain chronology and sedimentation rates for the past 200 years derived from trace element gradients, organic compounds, and numerical modeling

This study evaluates the applicability of trace element and organic contaminant data from a floodplain cross-section as the basis for a numerical model of spatial floodplain dynamics. Using threshold values of pollution-sensitive trace elements and market introduction dates of organic xenobiotics, the sampled sediment is assigned to historical phases to develop a sediment chronology. The investigation is based on a 60-m wide core transect from which sediment samples were analyzed to determine grain-size distribution, trace element inventory, and organic xenobiotic content. In addition, floodplain inundation, flow velocities, and the amount of sediment deposited were numerically modeled using Delft3D to verify the analyses results; conversely, the results of the sedimentary analysis served the input data for the model. Changes in floodplain morphology were interpreted on the basis of a digital elevation model (1 m resolution), historical maps from 1865 AD, and field surveys. The architecture of the alluvial sediments was examined in the cores accounting recent floodplain relief and possible historical factors. The results show a broad range of heavy metal pollutants and the presence of 57 volatile organic compounds in a pattern that reflects multiple deposition processes and phases. Based on these results and the model verification, the sediments were assigned to pre-industrial, industrial, and post-industrial phases, and sedimentation rates of 0.6-1.3 cm a(-1) were estimated. The results of this study contribute to a better understanding of the development of small meandering gravel-bed rivers with large floodplains, where suspended sediments predominate.