A series of experiments have been carried out in a laboratory flume to simulate the conditions in a gravel-bed river with a supply of finer, sand-size material under flow conditions where some or all of the material is in motion. The experimental program incorporated three sets of experiments: (1) where flows were just above the theoretical threshold for entrainment of the sand; (2) where flows were at the theoretical threshold for entrainment of the gravel, and (3) where flows were well above the threshold for both transport of sand and gravel. Particle movements were monitored using a new digital photographic and image analysis method that allows detection of mo-tions over periods as small as 0.04 s. The results show that, where flows are capable of moving only the finer material, grain size of the matrix is controlled by a combination of winnowing and particle overpassing. Under these conditions the grain size of the fine material trapped in the pore spaces of the gravel declines downstream. Once the gravel becomes entrained, the processes are very different. Just before the surface gravel layer is eroded, the framework lifts and dilates, causing matrix particles to move down into subsurface pores. Under these conditions matrix grain size increases downstream, because larger matrix particles are more likely to become stuck in pore throats and entrained with the gravel framework. The dilation process is hitherto unreported and has many implications for sediment transport. It appears that lift, not drag, is the important force during the early stages of entrainment and, as a result, fine matrix particles can be less mobile than those of the coarser framework.

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