ABSTRACT
A gravel-bed river is formed of a wide range of grain sizes. The vertical sorting of these differently sized grains, due to spontaneous percolation and kinetic sieving, has implications for sediment transport, fluvial ecology, stratigraphy and river morphology. Previous experimental research on vertical grain size sorting has mainly focused on the general categorisation of behaviour. However, in order to develop more accurate models of riverbed behaviour, we require an improved understanding of how and why the grains undergo sorting. The process of spontaneous percolation is controlled by limits on the size ratio between the overpassing, fine and static, coarse grains, whereby a decreasing grain size ratio allows less infiltration. Whereas kinetic sieving, involving mobile grains, is not subject to such limits and therefore exhibits more complex behaviour. Using these results, significant steps can be made towards understanding the process of grain size sorting in alluvial riverbeds.
