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The flood risk is typically assessed by evaluating a few basic flood scenarios such as HQ30, HQ100 and HQ300. The damage assessment for these scenarios is associated with high uncertainty. This uncertainty is for example due to uncertainty in predicting the course of the event (e.g. potential bridge blockage, failures of protection measures, the amount of transported solid material) or due to limited information about the area and limited model accuracy. To account for these uncertainties, the concept of subscenarios can be used. In this paper, we summarize the principle of risk approximation using basic HQ scenarios and we extend this formulation to account for subscenarios. The procedure is demonstrated through a numerical example of flood risk estimation in a hypothetical mountain torrent.