A numerical study of lateral grain size sorting by an estuarine front

A two-dimensional non-hydrostatic model of baroclinic flow was applied to an estuarine cross-section. The model was driven by lateral variations in density and produced the classical bilateral cross-sectional recirculation of an axial convergent front. Simulations of the erosion, transport and deposition of sediment by the frontal secondary flows were applied to a range of grain sizes. The model predicted considerable lateral variation in grain size deposition across the frontal recirculation zone, analogous to the lateral grain size sorting which occurs in river meanders. The sorting primarily took place between the frontal surface convergence and the bankward limit of recirculation. A series of sensitivity tests revealed that the contribution of the front to lateral grain size sorting was strongly influenced by the lateral channel slopes and lateral density gradient. The results from this numerical study support previous suggestions (based on observations of near-surface discontinuities in sediment concentration across the frontal interface) that fronts may act as sieves within the estuarine sediment transport system.