I know of two alternatives:
The Sumerian tablets had exactly the same story recorded a few thousand years earlier. It could have been passed down and re-written for the Bible. I think there are at least 5 stories of the same tale from different cultures.
Historian point out that the area was prone to floods. Noah would have actually gather 2 of each of his heard on a small boat to just keep his livestock alive. Floated just a few miles. It was his occupation - like any homeowner would do to protect his lively hood. Stories from mouth-to-mouth change after 100's of years.