Sediments grain size and geo-chemical interpretation of three successive cutoff meanders of the Danube Delta, Romania

The paper presents the results of granulometric and geochemical analyses of the superficial bed sediments of the St. George branch, one of three Danube River distributaries. Three cutoff meander reaches (the Mahmudia, Dunavat de Sus (Upper Dunavat) and Dunavat de Jos (Lower Dunavat) meanders, noted here as M, DS and DJ meanders) was selected as an example to analyse the impact of the meanders rectification on the grain size and sediment quality. The cut-offs of the meanders by navigational canals since 1984-1988 caused dramatic changes in the distribution of velocities, discharge and sediment fluxes (Popa, 1997; Panin, 2003, Tiron and Provansal, 2010, Tiron Dutu et al., 2014). The correction of the studied meanders induced different hydro-sedimentary impacts: strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander in two cases (Mahmudia and Dunava( de Jos meanders) and slight modifications in Dunavat de Sus meander, where the natural channel still keeps almost its initial conveyance capacity. Sediment samples were acquired throughout several cross-sections of both natural and artificial canals of the three rectified meanders, in order to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, Skewness) show the predominance of fine fraction (silt and medium/ fine sand), relatively good sorted depending of the sample location (on the natural course of the branch, on the rectification canals or the former meanders. The statistical analysis of chemical data evidenced a high compositional variability, with concentration variation coefficients ranging between 27.4% - Sr and 178.3% - TOC (Total Organic Carbon). Generally, the highest concentrations of terrigenic compounds appear in the samples collected from the rectified natural canals. These results are validated by comparison with aDcp measurements made on each investigated profile.