You can use the suite of SNAP and Sentinel Toolboxes while analyzing SAR data from Sentinel for some pre-processing and basic image analysis in the context of flood mapping but I am not sure if you can carry out a full fledged flood modelling with it. Anyway take a look at the attached links.

http://www.un-spider.org/links-and-resources/gis-rs-software/sentinel-application-platform-snap

http://www.un-spider.org/advisory-support/recommended-practices/recommended-practice-flood-mapping/step-by-step

http://step.esa.int/main/download/

https://www.disasterscharter.org/image/journal/article.jpg?img_id=90378&t=1418204600404

https://www.disasterscharter.org/web/guest/activations/charter-activations