Influence of immobile boulders on bedload transport in a steep flume

When applied to mountain torrents, sediment transport formulae habitually overestimate the bedload by several orders of magnitude, even if they have been developed for steep slopes. The reason is that the influence of macro-roughness elements, such as large immobile boulders which have an impact on flow conditions, is not taken into account. Larger roughness elements induce an increased form drag, implying a lower shear stress available for sediment entrainment. Preliminary tests carried out in a laboratory flume with a slope of 6.7%, reveal that the surface occupied by boulders need to be considered in order to obtain a good estimation of the sediment discharge. With 15% of the surface occupied by boulders, the bedload is reduced by 61% compared to transport rate without boulders. The influence of the immobile boulders on flow conditions and sediment transport has been studied as a function of spatial density of the boulders. The results are discussed in comparison with existing empirical bedload transport formula.