The classical solid transport theory has been used to analyse the experimental results obtained from the No.13 combined sewer trunk of Marseille for more than two years. This study demonstrates that the sediment transport phenomena in a combined sewer trunk are nothing other than the classical ones. A numerical model has been established according to the analyses. Based on a permanent now regime, this model considers not only the effects of the real channel geometry, non-uniform particles size, but also the coexistence of mineral and organically materials. Some particular sediment transport phenomena such as the armouring of bed have also been taken into account. It also shows that although the influence of the suspension particles is not necessarily considered, the simulation including the variations of particle density with each granular fraction may be improved.