Eddy Diffusion Coefficients of Suspended Particulate Matter: Effects of Wind Energy Transfer and Stratification

Gross sedimentation rates (GSR) have been measured using sediment traps placed at nine different levels above the bed (0•3, 0•5, 0•8, 1•0, 2•0, 4•0, 6•0, 8•0 and 10•0 m). The sediment traps were deployed for 1•25 years and recovered 28 times during the study period. Low average GSR values of 5•5 g m-2 day-1 were obtained at 10•0 m, and high average GSR values of 114•8 g m-2 day-1 were obtained at 0•3 m. An expression for the eddy diffusion coefficient of suspended particulate matter (Ks), based on the measured GSR is given. The expression has been used for modelling of Ks at the different trap levels above the bed. High values (≈42 cm2 s-1) of Ks were obtained at the upper traps, whereas low values (≈2 cm2 s-1) were obtained near the bed. Comparison between level of turbulent energy in terms of shear stress at the boundaries of the water column, i.e. from the wind and the bed flow, showed that wind energy exceeded that of the bed flow by a factor 16. At 5•0 m Ks was positively correlated (r=0•66) to the eddy diffusion coefficient of momentum (Km) derived from the wind energy transfer to the water, giving an average β of 0•5 for Ks =βKm. The density difference between surface and bottom waters has been designated a parameter of stratification, and is discussed in relation to variations of Ks and Km .