MODELLING THE PHYSICAL PROPERTIES OF FINE SUSPENDED SEDIMENTS HOANG-HA NGUYEN (*) LLOYD H.C. CHUA (*) (*) Division of Environmental and Water Resources Eng., School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. This paper presents new formulations for estimating the effective floc density (Δρ f ) and the relative viscosity (μ r ) of mixture of fine suspended sediments. Fitting parameters of the new models were calibrated using data available in the literature. Good comparisons were obtained when the settling velocities estimated based on the proposed models were compared against the settling velocity estimated from existing models found in the literature. The average value of the primary particle size, D p for the data used in the analysis, inferred from the new model for Δρ f was found to vary from 0.05 μm to 100 μm with a mean value of 2.5 μm. The new model for μ r is applicable to mixtures of both non-cohesive and cohesive sediments. Introduction In sediment transport, settling of fine suspended sediments is an important process defining changes in morphology. The effective density and viscosity of sediment-fluid mixtures have been identified as two of main parameters affecting the settling velocity of sediments. If sediments are cohesive they can stick together to form larger flocs, which have a smaller effective density but settle at a higher settling rate than those of individual primary particles. Moreover, the viscosity of the mixture increases if the concentration of sediments increases. Furthermore, the flocculation of cohesive sediments and the enhancement of the viscosity make the settling process become more complicated. Estimating effective floc density (Δρ f ) is a difficult task as there is no direct method available to measure the density of flocs. Floc density (ρ f ) is usually estimated indirectly by using a modified Stokes’ law and the measured settling velocity (w s ) and size of flocs (D f ). Using this approach, many empirical or semi-empirical approximations have been proposed to estimate the density of flocs (e.g., [1, 2]). Recently, the concept of self- similar fractal flocs has been widely used in the estimation of effective floc density. Kranenburg [3] proposed: ( ) F f p w s f D D − ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − = Δ 3 ρ ρ ρ (1) where ρ s and D p are density and size of primary particles, respectively, ρ w is density of water, and F is a constant, fractal dimension. Khelifa and Hill [2] argued that F is not a constant but varies with D f , given by: