Implications of turbulent shear on clay floc break-up along the Atlantic estuary (Bouregreg), Morocco

Abstract

Water depth, salinity, current, and suspended sediment concentration (SSC) were measured along with the grain size distribution of bed sediment along an estuarine longitudinal section. The floc size increased with increase in the percentage of clay and silt, while decreased with increase in the percentage of sand content of bed sediment. The turbulent shear, G, had a direct effect on floc size with its value increasing with increase in G up to a G value of 15 s−1, while an inverse relation existed between floc size and G at higher G (G > 15 s−1). Further, higher turbulence enabled sand to get resuspended and cause additional shear leading to the break-up of flocs. An attempt was made to modify G to account for the combined effect of water turbulence (G) and shear imparted by sand (Ga) and the impact of the modification of G on the predictability of floc size was evaluated. A new model was developed which explains floc size in terms of sediment concentration (C), salinity gradient (S), and G for different scenarios based on the value of G. Sensitivity analysis was done for observed floc size (FS) and predicted floc size using four approaches: (I) FS α Cx; (II) FS α CxS-y; (III) FS α CxS–yGz for G < 15 s−1 and FS α CxS–yG–z for G > 15 s−1; and (IV) FS α CxS–yGm–z for G > 15 s−1 and Gm = G + Ga, where x, y, and z are determined by calibration. It was observed that the predictability of the floc size improved when the turbulence was modified to account for shear imparted by sand so that the coefficient of determination was increased from 0.78 for model III to 0.89 for model IV. Further, the settling velocity was expressed as a function of suspended sediment concentration, turbulent shear, and salinity gradient. The predictability of settling velocity was improved (R2 increased from 0.77 to 0.86) when the additional turbulence created by sand was incorporated in the non-dimensional empirical equation. The study highlights the influence of sand in causing the break-up of flocs and suggests that for turbulence shear values high enough to resuspend sand, and G has to be modified to account for the additional shear imparted by sand in mixed sediment estuarine environments.