Framework dilation, winnowing, and matrix particle size: The behavior of some sand-gravel mixtures in a laboratory flume

A series of experiments have been carried out in a laboratory flume to simu late 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 mot ion. The experimental program incorporated three sets of experiments: (1) w here 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 hows were well above the threshold for both tran sport of sand and gravel, Particle movements were monitored using a new dig ital photographic and image analysis method that allows detection of motion s over periods as small as 0.04 s, The results show that, where flows are c apable of moving only the finer material, grain size of the matrix is contr olled by a combination of winnowing and particle overpassing. Under these c onditions the grain size of the fine material trapped in the pore spaces of the gravel declines downstream. Once the gravel becomes entrained, the pro cesses are very different. Just before the surface gravel layer is eroded, the framework lifts and dilates, causing matrix: particles to move down int o subsurface pores. Under these conditions matrix grain size increases down stream, 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 far sediment transport. It appears that lift, not drag, is the important force during the early stage s of entrainment and, as a result, fine matrix particles can be less mobile than those of the coarser framework.