The goal of this paper is to quantify the uncertainty of sewer discharge associated with unmeasured small scale rainfall variability (i.e. at scales smaller than the usual C-band radar resolution of 1km*1km*5min). Taking this uncertainty into account would enable to improve storm water management in urban areas. A case study is done on a 3,400 ha urban catchment in the county of Seine-Saint-Denis in the North of Paris. A rainfall event that occurred on February 9th, 2009 is studied, with rainfall estimates from the C-band radar of Trappes, located on the West of Paris, and operated by Météo-France. First an ensemble of realistic rainfall fields downscaled to a higher resolution is generated with the help of a multifractal space-time cascade whose exponents are estimated on the radar data. Second the corresponding ensemble of hydrographs is simulated by inputting each rainfall realization into a semi-distributed urban hydrological model. It appears that the dispersion among the ensemble is rather high, with for instance probability distribution of the extremes of both the rainfall and the peak flow exhibit a power-law falloff. In conclusion, we highlight the need to develop the use in urban areas of X-band radars, which provide higher resolution data.