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Sediments in combined systems (CS) significantly increase loads emitting via combined sewer overflows. In future prognosticated changes in the storm water characteristics, decreasing popu-lation or water conservation will probably worsen these effects. Various actions can prevent the formation of large quantities of sediments whose amounts change considerably over time and space within the sewer. Information about the whereabouts of deposit can be obtained by visual inspection or mathematical modeling. Both approaches require large efforts. Furthermore, presently available deterministic sewer-quality models with sediment transport were found to be over-parameterized and inherit large uncertainties. Mathematical models as the semi-stochastic sediment transport model are still able to efficiently identify areas with risk of sedimentation. This category of models does not use a single parameter set obtained from calibrating the quality model but varies these parameters instead stochastically within a Monte-Carlo Simulation. The investigation aimed (1) to model the amount of mobile sediments that effect the load distribution during wet weather flow and (2) to quantify the areas with risk of sedimentation within a combined sewerage.A detailed field study showed that readily erodible sediments are most important pollutant based process affecting the load distribution during wet weather flow. Based on this finding, a structured hydrological-stochastic sewer quality model was developed. The simplified conceptual model represents the whole sewer system including the pipes with small cross-sections (less than or equal to DN 300) because in small CS the corresponding part amounts to more than 50% of the total length. The simulations’ results were compared against field data. The results demonstrate validity of the approach. The investigation showed in particular:(1) The simulated total length of sewers with risk of sedimentation of 2’998 m agrees well with monitoring data of 3’121 m.(2) The submodel referring to the transport of gross solids effectively reproduce the deposit observed in reaches with intermittent flow. This allows for evaluation scenarios and management strategies that deal with demographic change or water conservation.