I thought fallout tends to degrade the terrain once scrubbed, so a grass tile would end up a plains tile, and a plains a desert.
Flood plains are only on deserts on standardized maps (Pangaea, Continents, Fractal, etc.). There are some maps like Fantasy where you will see all kinds of things like FPs on grassland with 4 food. Rainforest, which is non-standard, has some peculiar tile possibilities. Well, WB you can do pretty much anything..no rules there...it is for modding.