Modeling sediment transport in estuarine environment: Effects of tidal asymmetry, lateral circulation and sediment-induced stratification.

摘要

Effects of tidal asymmetry, lateral circulation and sediment-induced stratification on suspended sediment transport are investigated with numerical modeling studies. The two-equation turbulence closures can appropriately present the effect of sediment-induced stratification under sub-saturation conditions and predict the reduction of friction velocity of similar order as that observed in laboratory experiments. Under saturation condition, an unrealistic top laminar layer is formed because internal wave mixing is missed in two-equation closures.;In short tidal embayments, the local phase lag is an arc tangent function of the ratio between tidal period and sediment settling time that is the time taken for a particle to settle from surface to bottom. It increases with decreasing settling velocity. The direction of local residual sediment flux is controlled by tidal velocity asymmetry and settling lag. The introduction of nonlocal sediment transport results in more complicated relations.;In the partially mixed estuary of the Passaic River, the lateral circulation shows significant tidal variation due to the tidal straining. During highly stratified ebb tides, the lateral circulation in an asymmetric transverse section exhibits a two-layer structure which is driven by a lateral asymmetry in vertical mixing. The tidally averaged lateral advection acts as a driving forcing for producing lateral structure of sub-tidal estuarine circulation. It strengthens outflow on the shoal and compensates outflow over the thalweg. Sediment budget indicates that lateral advection dominates horizontal advection and produces a net transport of sediment from the deep channel to the shoal. Also, the lateral entrapment of sediment occurs at a certain range of grain size. For longitudinal sediment transport, the lateral circulation acts as a mechanism for exporting sediment from the estuary.