Challenges to the representation of suspended sediment transfer using a depth-averaged flux

The sediment saturation recovery process (i.e. the adaptation of suspended sediment concentration [SSC] to local forcing) is the main feature of the non-equilibrium suspended sediment transport (SST) frequently occurring in fluvial, estuarine and coastal waters. In order to quantitatively describe this phenomenon, a series solution is analytically derived, including the evolution of both vertical SSC profile and near-bed sediment flux (NBSF), and is verified by net erosion and net deposition experiments, respectively. The results suggest that the sediment saturation recovery process involves vertically varying fluxes that are not represented correctly by depth-averaging. Consequently, a vertical two-dimensional (2D) combined scheme is established and applied respectively in to a dredged trench and to a sand wave feature to demonstrate this argument. By analyzing the variations of the calculated depth-averaged SSC and NBSF we reveal that the equilibrium state presented by the sediment carrying capacity (SCC) form of the NBSF, which is usually applied in depth-integrated SST models, lags behind the actual dynamic bed equilibrium state. Moreover, the key factor , the so-called saturation recovery coefficient within this form, is not only a function of local Rouse number but also is influenced by the local SSC profile. Finally, a three-dimensional (3D) non-orthogonal curvilinear body-fitted SST model is developed and validated in the Yangtze estuary, China, combined with the in situ hourly hydrographic data from August 14-15, 2007 during spring tide in the wet season. Model results confirm that the vertically varying sediment saturation recovery process, the discrepancies between the actual and SCC form of NBSF and non-constant value of are significant in actual real geomorphic cases. The quantitative morphological change resulting from variations in environmental conditions may not be correctly represented by uncorrected depth-integrated SST models if they do not treat the effects of vertical motion on the sediment saturation recovery process. Copyright (c) 2016 John Wiley & Sons, Ltd.