Deposition in Flowing Water of Fine Sediments under Different Salinity Conditions

While the effect of salinity on the settling of fine sediments has been shown to be important in laboratory experiments, only a few studies have attempted to investigate depositional properties in flumes under different salinity conditions. This paper examines the effect of salinity on the deposition in flowing water of fine sediments collected in the Tagus estuary. The sampling site is located in the middleupper reach of the estuary, where fine-grained sediments are the dominant intertidal material and tidal currents the major transport mechanism. The experiments were conducted in an annular flume (mean diameter 3.7 m, height 0.4 m, width 0.3 m), with initial suspended sediment concentration of 0.7 g l(-1) and five different salinities (0, 2.5, 5, 10 and 15). Two types of experiments were performed: tidal variation for 6.5 hours; and response to sudden reduction in flow velocity (from 0.5 m s(-1) to 0.25 m s(-1)) for 8 hours. Samples were collected every 30 minutes to determine suspended sediment concentration by filtration and grain size by laser diffraction. The experiments with different salinities have yielded similar results, particularly regarding deposition for a velocity of 0.25 m s(-1) (final C = 0.417-0.399 g l(-1), D-90 = 22-19 19 mu m, D-50 = 8-6 mu m, D-10 = 2 mu m). A consistent effect of salinity on the deposition of fine sediments from a flowing suspension could not be observed. Further research should examine deposition at lower flow velocities.