Search

Delta systems, as final receptors of the fluvial systems are under considerable and increasing influence of multiple anthropogenic stresses, such as hydropower plant developments in the basins, extraction of groundwater in delta plains, embankment of banks, dredging of the navigation channel, the land use change, channel engineering, that affect the sediment connectivity. Along the Sulina Canal, the cut-off program and construction of the groins and dikes had important responses in grain size distribution between the main channel, the rectified meanders, and the lakes of the Danube Delta. Two field campaigns (at high and low waters) were made in May and October 2023, focusing on the bed sediment composition and lithological constituents. Sediment samples were acquired throughout several cross-sections, to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, and Skewness) show the predominance of fine fraction (medium and fine sand, and silt), moderately and poorly sorted. The data were compared and larger modifications were found between the two analysed periods in the distribution of sediments. [ABSTRACT FROM AUTHOR]

The pressure control of the anthropogenic factors has consequences on the fluvial systems, generating structural changes. The hydrology, hydraulics and sediment load of the rectified meandering systems depend on the hydrologic connectivity with the main stem of the river. The former meanders can be permanently, temporarily disconnected with the main channel producing interruptions in the transfer of fresh water and thus affecting the morpho-sedimentological processes and the biodiversity. This paper aims to investigate the distribution of the water and suspended sediment fluxes between the former meanders and the artificial man-made canals along the St. George distributary and thus, the hydrological connectivity. Understanding these complex human pressures are of high importance for reaching/maintaining the ecological status of the Danube. Herein, we made investigations along the St. George branch on many sub-systems river-channel-lake site type, formed by cutoff meanders, connective channels and lakes to observe how much the water and sediment inputs to the delta depressions is affected by the structural changes of the meander's physiography. [ABSTRACT FROM AUTHOR]

Microplastics represent one of the most important concerns of the actual society. With a long environmental persistence, these plastic based particles are transported by the flow and deposited in sedimentary systems. This paper describes the distribution of microplastics in fluvial and lacustrine environments of the Danube Delta. Taking into account that the River Danube and its main distributaries (Kilia, Sulina and St. George) are the primary pathways along which MPs are transferred to the Black Sea, the superficial sedimentary material could testify the pollution degree. Microplastic concentrations identified previously by Pojar et al. (2021a) and the data of flow velocities and fluxes at different hydrological regimes were considered to characterise the MPs distribution, as their abundance and morphology change due to the discharge and currents speed. Our results show that moderate to higher concentrations have been found in the first and second rank locations, related closely with the main distributaries. Complementary, we discuss how hydrodynamics can elucidate the spatial distribution of MPs in the Danube Delta, as well as their path and behaviour in a regional-scale aquatic environment. [ABSTRACT FROM AUTHOR]

Major (CaCO3, TOC, and Fe2O3), minor (MnO and TiO2) and trace (Zr, Cr, V, Zn, Cu, Ni, As, Pb, and Hg) elements from surface riverbed sediments were determined at 66 locations in deltaic environment, along St. George distributary and in the interdistributary depressions of the Danube Delta. The studied area is characterized by terrigeneous, either non-carbonated or low calcareous sediments, quite poor in organic matter. Most of the studied heavy metals showed high spatial variability, generally with higher concentrations in interdistributary depressions and adjacent canals, as well as on the meander Mahmudia (M1). Ni, Cu, and Cr showed concentrations frequently exceeding the quality criteria set in the Romanian legislation. Contamination Factor (CF) and Pollution Load Index (PLI) were used to assess the level of heavy metal pollution in the studied area. [ABSTRACT FROM AUTHOR]

In the Danube Delta, on the Sulina branch, the morphology, sediment, and bedform characteristics were investigated. Three-dimensional (3D) bathymetry, flow velocity, suspended-load concentration, and liquid and solid discharge data were acquired throughout several cross sections along the Sulina channel, in order to investigate the distribution of water and sediment discharges and their influence against the river bed. A single observation (in February 2007) was made regarding the geometry, sediment composition, and hydraulic conditions under which the dunes grew and degenerated. The investigation focuses here mostly on the geometrical parameters of these bedforms, such as height, length, as well as grain size characteristic of the sediment and water dynamics. Based on in-site measurements, different hydraulic parameters were calculated, such as bed shear stresses and Reynolds number. During the field campaign, the measured water mean velocity was from v = 0.22-1.13 ms-1. At the same time, the measured range of shear stresses within the dune field formation was from π0 = 2.86 N·m-2 (on the cutoffs) to 8.62 N·m-2 (on the main channel). It was found that the correlation between height (H) and length (L) of the Sulina branch dunes describes the formula: H = 0.093L0.5268. The bedforms of the Sulina channel are, in general, developed in fine sand (D50 between 0.06 and 0.35 mm). [ABSTRACT FROM AUTHOR]

Sediment-associated pollutants are transported by rivers as suspended load and bed load under specific flow conditions. In particular conditions part of pollutants (e.g. metals) could be incorporated into the alluvial deposits, and over time, became a major source of the river contamination. The hydrology, the riverbed morphology and the characteristics and quality of river sediments of the Lower Danube have been studied yearly in the 2009 - 2013 period using different methodologies (single and multi-beam bathymetry, ADCP measurements of current velocity and suspended load, sediment sampling using grabs and corers, laboratory analyses). Yearly measurements and sampling have been carried out on more than 70 cross-sections on the Lower Danube course, between km 1072, where the river enters the Romanian territory, and the Black Sea. From each cross-section, one to three sediments samples were analysed for determining the sediment grain size, mineralogy and chemical composition using different laboratory methodologies, as X-ray fluorescence and AAS techniques for Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti, V and Zn. The obtained multiannual data allowed identifying the main anthropogenic pollution by heavy metals zones along the river. The Romanian tributaries of the Danube have been sampled and analysed separately for evaluating their role in the general environmental state of the region. The influence of the Danube River water, sediment and pollutants supply to the NW Black Sea has been also studied. [ABSTRACT FROM AUTHOR]