The risk assessment for extreme floods is a key point to anticipate and to dimension a set of measures to improve the resilience of the territories. Because extreme floods can seldom be observed, the calculation of hazard in the flooded areas comes from an extrapolation of models calibrated on more frequent events. This latter extrapolation can be based on a detailed representation of the processes that may influence the flood features or on a more simplified approach that permits much faster calculations. For the specific cases of the Rhône and Gardons valleys in the Southern France, an estimation of the uncertainty is carried out in order to compare a detailed representation and a simplified approach. called "Additional Depths Method" that guesses the flooded areas of a reference flood are accurately estimated. Then, the topography of the valley above the reference flood level permits to define a set of cross sections in which a uniform flow is calculated for a flow discharge calculated as the difference between the extreme flow and the reference flow. Finally, the limits of the flooded areas are drawn starting from the water elevation interpolated between the values obtained at the cross sections. This simplified approach is encapsulated in a plug-in of the QGIS software. The uncertainty linked to the simplified approach can vary a lot from as low as 10% up to 100%. The plug-in is convenient in the more favourable cases in which the hypothesis of a 1D flow can be kept and the topography agrees with the reference map.