Non-capacity or capacity model for fluvial sediment transport

Abstract

The last several decades have witnessed the development of both capacity and non-capacity models for fluvial sediment transport. While recent investigations of the multiple timescales of fluvial processes have substantiated the understanding of conditional applicability of capacity models, the extent to which a capacity model differs from a non-capacity model remains to be unravelled. This paper presents a comparative investigation of one-dimensional capacity and non-capacity models. As a corollary to the theoretical analyses of the multiple timescales of fluvial processes, this study demonstrates that bed load transport can adapt to local flow sufficiently 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, whereas a capacity model may result in considerable errors at best and become ill-posed at worst because of the requirement for extra boundary conditions. The findings of this work should facilitate the physically enhanced development and applications of mathematical river models.