Because stakes are concentrated in urban environment, flood modelling should be carried out there with high accuracy. However, uncertainties remain important because of: -the estimate of the inputs, these inputs being either rainfalls, or flows coming from the upstream basin or from the exchanges with the sewage network; -the complexity of the processes, which includes a frequent change of the flow regime -from subcritical to supercritical and oppositely -, the difficulty in predicting the flow

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... distribution at the street junctions, the oscillations created by waves reflecting against , for instance, obstacles or walls of houses; -the parameters of the flooded area: variable roughness, very irregular topography, moving obstacles in the streets, water entering the built-up areas, etc. An assessment of these uncertainties is presented for a few examples coming from laboratory experiments or from the two-dimensional modelling of flood events in French towns. These latter results can be used in order to estimate the range of uncertainty during field studies.