Portela, L.I., Ramos, S. and Trigo-Teixeira, A., 2013. Effect of salinity on the settling velocity of fine sediments of a harbour basin. In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.)

Salinity is known to increase the cohesion of clay minerals, and thus the flocculation of fine-grained sediments in suspension. However, the influence of salinity on the settling velocity of natural particles is often unclear, due to organic and biological aggregation and other controlling factors. This paper examines the effect of salinity on the settling velocity of fine sediments collected in a harbour basin in the Tagus estuary. The sediment sample consists of silt- and clay-sized particles (D₁₀ = 2 μm; D₅₀ = 9 μm; D₉₀ = 37 μm). Quartz is the main constituent and illite the main clay mineral. The experiments were conducted in a 2.25-m high settling column, for a constant initial concentration of fine sediment (1.5 g l⁻¹) and different salinity values (0, 5, 10, 15 and 30). During each experiment, samples were collected at 10 vertical levels and at 10 time instants. Settling velocities were calculated on the basis of the time- evolution of suspended sediment concentrations. At the start of the experiments, after cessation of turbulence, settling velocities increased in direct relation with the increase in salinity, the maximum values ranging between 0.3 mm s⁻¹ under freshwater conditions and 0.9 mm s⁻¹ for a salinity of 30. After 5 hours, the proportion of the initial sediment remaining in suspension was 45% under freshwater conditions and only about 10% for salinities between 10 and 30. Using the mass-weighted mean settling velocity to describe the deposition fluxes, it is concluded that settling increases by a factor of 6.5 between freshwater and marine conditions.