The area flooded frequently .. That's what built up the delta south of Basra. .. and basically that's why it was called the Fertile Crescent. Look at a map.While we are having problems dating the flood we have found more than enough evidence that that area had flooded. There have been many villages in the Carmel Mountains discovered that were destroyed by a great flood amongst other evidence. We know the Mediterranean has caused monstrous flooding as well as the Black Sea. What caused these is up for debate...whether rising waters from the climate or sudden tsunami's....we simply do not know.
All we know for certain was that this area was hit multiple times by extreme flooding so its possible that Noah and the flood in the Epic of Gilgamesh were two separate events.
Given the supposed time lapse between those stories that seems the most possible explanation.
There just isn't a way to get any absolute dating on any of this which ultimately is the problem.
The Bible is a pretty accurate history book as far as events go, not scientific by any means but it stays pretty true to course. The problem with the Bible is the lack of detail to nail down specific dating. So if it says there was a great flood there probably was but that's simply nothing more than a starting point to begin researching something and not much help overall.
I think this was done on purpose so we as Christians do not get too focused on the event but recognize the story and lesson that event is supposed to represent.
The source of the flooding was spring snowmelt from the mountains coupled with spring rains. Its not supernatural and it was never global.