Morphological control of bedload transport in modelled gravel-bed rivers.

摘要

The presence of regularly-spaced erosion and deposition sites (pools and bars) in gravel-bed rivers indicates that downstream transport of bedload must be morphologically dependent. Positively-skewed distribution functions (e.g. gamma) are commonly used to describe the transport distances of bed sediment particles, despite being poor approximations of the distribution at channel-forming discharges. Accurate description of the downstream transport distribution is important for the estimation of bedload transport rates, and thus vital to river planning, management, and engineering, yet there have been few studies of the relationship between channel morphology and sediment transport. A series of flume experiments used fluorescent bedload tracers in small-scale models of low-sinuosity gravel-bed meandering rivers to investigate how downstream transport is influenced by the presence of scour pools and point bars, and to examine the relationships between bedload transport and channel morphology.; The time-delayed release of tracers into scour zones resulted in symmetrical distributions with modes centered around the apex of the first downstream point bar, modelled using cauchy functions. The instantaneous introduction of tracers inhibited particle mobility, resulting in bi-modal distributions approximated using combined gamma-cauchy functions, with distribution modes related to the non-movement of bedload grains, and bar deposition respectively. Only at low discharge, with minimal connection between sediment movement and morphology, was downstream bedload transport gamma distributed. There is evidence supporting these findings from previous studies, where symmetrical and multi-modal distributions result from bedload trapping on bars.; At channel-forming discharges, 55% to 75% of the tracer grains were deposited on, or in the vicinity of the first point bar downstream, thus it is reasonable to assume that bedload transport equals the distance from pool to bar (0.50lambda) in meandering channels, for the estimation of transport rates using the morphological method. Bedload was transported to the apex of the point bar, and this was reflected in the lateral dispersion of bedload tracers over the point bar surface. Modes of point bar development display tracer deposition at the point bar apex, and along the bar margin, contributing to downstream and lateral bar growth. A conceptual model of downstream transport is that as transport intensity increases, distributions evolve from positively-skewed (gamma), to bi-modal (combined gamma-cauchy), to symmetrical (cauchy). Bedload transport in gravel-bed rivers is closely tied to the development of bed morphology. This has been previously recognized but not explicitly modelled.