I think a flood map can be generated with a single image if it was taken during the flood.
However, if there are areas in the image with very low backscatter (smooth surfaces, permanent water bodies, ect.) you won’t be able to distinguish them from the actual flood. Consequently, over-estimating the flooded areas is very likely to happen.
If that is okay to you, change detection is not needed. But whenever the actual extent of the flood matters, an image without flood is required for comparison.
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