On the factors affecting the settling velocity of fine suspended sediments in a shallow estuary

Abstract

Of the factors that affect settling velocity (w _s), the concentration of suspended sediments (SSC), C, and turbulence (root-mean-square velocity gradient), G, have been identified as being of major importance. This work reveals the important effect on w _s of the vertical salinity gradient, s, (defined as the difference between surface and bottom salinity divided by water depth). w _s was obtained by use of the Rouse equation, and ranged between 0.1 and 2.47 mm/s. w _s was found to have an exponential relationship with SSC for low SSC; an inverse relationship was observed for high SSC, because of hindered settling. The study revealed that w _s was inversely related to s and its effect was observed only for high values of s. A new model is proposed in which w _s is explained in terms of C, G, and s. Two scenarios were considered on the basis of the stratification parameter ∂S/S ₀ (the ratio of the difference between the surface and bottom salinity divided by the mean salinity): ∂S/S ₀ > 0.1 and ∂S/S ₀ < 0.1. The effect of s on w _s was negligible when ∂S/S ₀ < 0.1. Regression analysis was performed on observed w _s (using the Rouse equation) and calculated w _s using three approaches: (1) $w_\text{s} \propto \,C^{x}$, (2) $w_\text{s} \propto \,C^{x} G^{y}$, and (3) the new model $w_\text{s} \propto \,C^{x} G^{y} s^{ - z} , {\text{for}}$ ∂S/S ₀ > 0.1, and coefficients of determination of 0.59, 0.65, and 0.86 respectively were obtained. The new model has the advantage that it is dimensionally stable and the empirical coefficient is dimensionless.