Is it possible to open it in QGIS?
i tried to do that, but apparently arcgis is needed to read .sdf format. Anyway I have an alternative to export the flooding surfaces to Qgis, but it's not the best way.
- In HEC-RAS after you have your results, go to View-XYZPerspective plots.
- Copy the image to a image editor and save it as png, or another image format.
- In QGIS go to Raster-Georeferencerer-Georeferencerer...
- Import the image and use the tool to georeference matching the cross sections (I had the cross sections in QGIS and then I exported to HEC-RAS as it says in the link Is it possible to export cross sections from QGIS to HEC-RAS).
Then you will have the surfaces in QGIS!.
I'm pretty sure it's not the best way to do that, but in my case it was a good solution. To do that I import the cross sections in UTM coordinate system.