Stress partitioning for bedload transport in rivers with immobile boulders

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. In order to analyse the impact of such boulders on sediment transport, preliminary tests have been carried out on a tilting flume. The impact of boulder spatial density has been studied. To increase the efficiency of flood risk mitigation measures, there is a need to improve current hazard assessment methods, starting from the rainfall space-time distribution, following the chain of event down to the inundation risk in the floodplains. Sediment transport predictions are needed to route sediment through river networks, model river incision into bedrock, restore river functionality and habitat, and mitigate debris flows initiated from channel-beds.