Observations of shear-augmented diffusion processes and evaluation of effective diffusivity from current measurements in Corpus Christi Bay

Studies on the process of diffusion in fluids have shown that in the presence of a shear structure within the current field, the observed spreading of a marked fluid can be augmented significantly. Shear-diffusion becomes the dominant diffusion process after a time Tn, the initialization time has elapsed. Given the existence of a vertical shear structure within the flow field, the characteristic vertical mixing time, Tc having an inverse relation to vertical turbulent diffusivity, Kz governs this initialization time.This study focuses on the observation of shear-augmented diffusion process in a shallow wind-driven body of water leading up to the development of numerical algorithms for obtaining an effective diffusivity, Ke from shear-current measurements at spatial scales ∼1000 m. This was part of a series of dye-tracer experiments conducted in Corpus Christi Bay, Texas. Numerical estimates are provided for Tn using the value of Kz determined from the temporal current fluctuations based on velocity correlation function, Rτ   of the velocity time-series. An algorithm was developed by discretization of an equation of the form Ki=ui′2¯TcIi (i=x,y) which was then used in the evaluation Ke based on numerical estimates of Tc and the shear coefficient Ii.It was found that in the presence of shear-current structure, predicted Ke values along two orthogonal directions were ∼104 and 105 cm2/s, respectively, about 10–20 times higher than estimates obtained based on turbulence alone and confirmed through visual observations and statistical estimates of the size of the diffusing dye cloud.