The diaspora during the building of the Tower of Babel occurred after the flood. So, Noah survived the flood, the story was passed to his descendants. Noah's descendants tried to build a tower to reach to Heaven.
God confused their tongues. The people spread out with different languages and started different cultures.....all descending from Noah; all with knowledge of the flood.
Well, that clears that right up then.
“all with knowledge of the flood. “
But all of them (except one) forgot about Noah. Seems strange given that he’s the central character and hero of the story.
Not forgetting the Egyptians who have a written history that precedes, is co-existent with, and which continues uninterruptedly after the time of the Flood, yet somehow they were unaware of that catastrophic global event. Not forgetting the Chinese who also have no memory of such a flood.