One of the key aspects in urban drainage (water quality) modelling is the accurate simulation of the input into such models. The modelling of surface sediment build-up, erosion and wash-off is discussed in this paper. An analysis and a comparison of the quality modelling tools of the commercial urban drainage software packages HydroWorks (Wallingford Software, UK) and Mouse (DHI, Denmark) reveal important differences between, and incompatibilities in, both models. The analysis is performed using concentration–duration–frequency (CDF) relationships. A generalised model, accounting for the common model principles used in both models and incorporating the model principles lacking, is proposed.