Particle path length distributions in meandering gravel-bed streams: Results from physical models

In gravel-bed rivers with well-defined pool-bar morphology, the path length of transported bed particles must be, at least during 'channel-forming' flows, equal to the length scale of the morphology. This is the basis for some methods for estimating bed material transport rates. However, previous data, especially from field tests, are often strongly positively skewed with mean much shorter than the pool-bar spacing. One possible explanation is that positively skewed distributions occur only in channels lacking distinct pool-bar topography or only at lower discharges in pool-bar channels. A series of flume experiments using fluorescent tracers was used to measure path length distributions in low-sinuosity meandering channels to assess the relation with channel morphology and flow conditions. At channel-forming flows, 55 to 75 per cent of the tracer grains were deposited on the first point bar downstream of the point of tracer input, with 15 per cent passing beyond the first bar. Path length distributions are symmetrical with mean equal to the pool-bar spacing and can be described with a Cauchy distribution. In some cases there was a secondary mode close to the point of tracer introduction; this bimodal distribution fits a combined gamma-Cauchy distribution. Only when discharge was reduced below the channel-forming flow were frequency distributions unimodal and positively skewed with no relation to the pool-bar spacing. Thus, path length distributions become more symmetrical, and mean path length increases to coincide with pool-bar spacing, as flow approaches channel-forming conditions. This is a substantial modification of existing models of particle transfer in gravel-bed rivers. Copyright (C) 2003 John Wiley Sons, Ltd.