|
Radu George Dimitriu, National Institute for Marine Geology and Geoecology - GeoEcoMar (Romania)
Gheorghe Oaie, National Institute for Marine Geology and Geoecology - GeoEcoMar (Romania)
Stefan Andrei Szobotka, National Institute for Marine Geology and Geoecology - GeoEcoMar (Romania)
Sorin Corneliu Radan, Geological Institute of Romania (Romania)
Gicu Opreanu, National Institute for Marine Geology and Geoecology - GeoEcoMar (Romania)
Sebastian Dan, National Institute for Marine Geology and Geoecology - GeoEcoMar (Romania)
|
|
The integrated geophysical-geoenvironmental research recently carried out within the Sf. Gheorghe-Vadu littoral zone with a dense geophysical lines network and sediment-water-biota sampling network summing over 4,600 km and respectively over 400 sampling sites. Main results of the high accuracy bathymetry mapping and sedimentological studies in comparison with similar results achieved based on raw bathymetry and sedimentological data acquired in past are presented. The comparison of the bathymetry maps build up for 1985 and 2005 epochs clearly highlights the amplitude and evolution trends of the sedimentary processes that now-a-days shape the shore line and the seabed in the littoral sector. Thus, a generalized shoreward migration trend of the isobaths is noticed. A relatively unchanged hydro-morphological regime was only observed in the Gura Portita-Sacalin Is. sector, at depths of and over 20 m. Intense erosion processes are present in the Sacalin Is.-Sf. Gheorghe sector deepening the seabed with 5÷10 m in the last 20 years. Erosion processes have also been noticed in the Gura Portita-Perisor sector, within the 5÷10 m bathymetry interval. Intense seabed erosion (deepening of 3÷4 m during the last two decades) is also present in the Chituc-Gura Portita sector, for the depths interval ranging from zero to 10 m and beyond. The single relatively stabile sector of the entire investigated littoral zone was found at Vadu. The slightly, seaward shift of the shoreline observed during the last decades in this sector suggests the local dominance of accumulation processes versus the erosion and transport ones.
The seabed of the littoral sector is now-a-days covered by a large spectrum of grain size fractions, which includes sediments from medium sand to silty clay. The coarser sediments (medium sand-silty sand) border the coastal area and the eastern side of the Sacalin Is., until the water depths of 8÷10 m. The rest of the area is covered by fine to very fine bottom sediments, as clayey silt to silty clay. Shelly deposits, which cover large and discontinuous areas, are disposed in parallel position with the shoreline, bordering the eastern side of the area dominated by sandy sediments. Comparing these results with the bottom sediments distribution mapped in past we noticed that the area dominated in past by sandy sediments is now drastically reduced, the finer grain size fractions, from clayey silt to silty clay, becoming dominant.
Among the most important causes that generate these rapid changes of the seabed hydro-geomorphology may be noticed: the historically delta front erosion trend, the diminish of the Danube River sedimentary discharge, the anthropical modified long-shore sediments transport regime and the secular rising of the mean sea level, combined with the regional subsidence of the Danube Delta area.
|
