A sediment routing model for steep torrent channel networks called SETRAC has been developed and extensively tested. SETRAC has been applied to well document case studies on flood events in torrential catchments with substantial sediment transport in the Austrian and Swiss Alps. The influence of form roughness, armouring and transport by size fractions on modelling bedload transport is discussed. The simulation results show the importance of form roughness losses when computing bedload transport in torrents and mountain streams. Neglecting from roughness losses results in an overestimation of the observed bedload transport by a factor of about 10 on average.