Non-capacity Bed Load Sediment Transport By Turbulent Flows

Sediment transport is an important issue for fluvial hydraulics and morphordynamics. However, limited by the research conditions, the initial studies for sediment transport mostly focused on equilibrium conditions, with assumption of steady, uniform, and capacity state, and the effect of these assumptions or simplifications is far from clear. The recent theoretical work on the multiple time scales of fluvial processes have revealed that bed load sediment transport can adapt to capacity sufficiently rapidly and yet this is not the case for suspended sediment transport, and the approximate applicability of the concept of sediment transport capacity is addressed. As an extension of the theoretical work, this study aims to present a further investigation of the adaption to capacity process for sediment transport.A comparative investigation of one-dimensional capacity and non-capacity models has been made. As a corollary to the theoretical analyses of the multiple timescales of fluvial processes, this study demonstrates that bed load transport can adapt to capacity status rapidly and, accordingly, a capacity model is applicable. However, as bed evolution modifies the flow considerably, a non-capacity model is needed if the flow is to be properly resolved in addition to bed load transport. Furthermore, it takes a long time and space for suspended sediment transport to adapt to capacity. Therefore non-capacity modelling is critical for suspended sediment transport. The findings of this work should facilitate the physically enhanced development and applications of mathematical river models.The distance required for sediment transport to adapt to capacity (adaptation-to-capacity length) of both bed load and suspended sediment transport, in line with reduced or increased sediment supply from the upstream, is computationally studied using a coupled shallow water hydrodynamic model. It is found that the adaptation-to-capacity length generally decreases as the Rouse number increases, irrespective of whether the sediment supply increases or reduces. Quantitatively, the adaptation-to-capacity length of bed load sediment is limited to tens of times of the flow depth, whilst that of suspended sediment increases substantially with decreasing Rouse number and can be up to hundreds of times of the flow depth. The present finding concurs with the recent time scale analysis that bed load sediment transport can adapt to capacity much more rapidly than suspended sediment transport, and it facilitates a quantitative criterion on which the applicability of bed load or suspended sediment transport capacity for natural rivers can be readily assessed. The depth-averaged for fluvial sediment transport modelling is introduced to aeolian sediment transport modeling. Model validity has been assessed using experimental data, and the results show that the major features of aeolian sediment transport can be well captured by the proposed model. Furthermore, based on the integral model, the recent theoretical work on the multiple time scales of fluvial processes is extended to aeolian processes. The results provide a theoretical justification for aeolian saltation that it can be adapted to equilibrium state very rapidly, while it is found that a much longer time and space is needed for aeolian suspension to adapt to equilibrium.Extending the research of non-capacity bed load sediment transport to graded sediment beds, an experimental investigation of bed load transport of uniform and graded sediments under the same range of flow discharge is presented. Comparison between the observed transport rates shows that for degradating cases, sand greatly promotes the transport rate of gravel, and in contrast gravel considerably hinders the transport of sand. Specifically, the promoting impact increases as sand content increases, and the hindering impact increases with the increase of gravel content. Moreover, the promoting and hindering impacts increase greatly with the decrease of flow discharge, which can amount to orders of magnitude. These results are qualitatively corroborated by an analysis of previous equilibrium experimental datasets of graded bed load sediment transport. The present finding facilitates a new basis on which the computation of graded bed load sediment transport can be improved. It also has important implications for fluvial processes subject to land, water, and ecological resources management.