Estimating the transport of bed material at low rate in gravel armoured channels

Classical sediment transport formulae represent high flux conditions. In contrast, bed material transport in most gravel-bed channels remains at very low rates most of the time, with only partial exchange of local bed sediments. Armouring and aggregated clast structures develop that are critical for the persistence of the low transport rate and for the stability of the channel. Classical formulae developed for fully mobile conditions grossly overpredict transport in this state. The transport cannot, in general, be calculated merely by an adjustment of bed material parameterization to reflect the surface armour, though that may work for specific histories of bed development. There is at present no general formulation available for bed material transport in channels with low transport and a structured bed.