Sediment transport in steep channels with large roughness elements

Most sediment transport equations overestimate the bedload transport rate by several times when applied to mountain rivers. This is due to the fact that the presence of large relatively immobile boulders, which disrupt the flow, is generally not taken into account. Sediment transport in steep channels with boulders was herein investigated using 41 laboratory experiments carried out on a tilting flume for three slopes (S = 6.7%, 9.9%, and 13%). Sediment transport, mean flow velocities and morphology-describing variables were measured regularly during the experiments. The sediment transport capacity is clearly decreasing with dimensionless boulder distance and is better estimated in terms of critical discharge for incipient motion of mobile sediments than in terms of bed shear stress. A sediment transport formula based on excess discharge relative to a critical value, which depends not only on the channel slope but also on the boulder spatial density, is herein developed.