Hold on. Let’s back this truck up. I’ll try to be more clear this time. We don’t know which “actual event” may be behind the various flood myths of Mesopotamia. There is evidence of large floods at several places and times, ranging from Ur ca. 3500 B.C. to Kish ca. 2600 B.C. Writing, specifically cuneiform, was invented during this same approximate time frame. Any one of these floods could be the “original” flood that inspired the Mesopotamian myths about a flood. Alternatively, since rivers tend to flood and floods occasionally reach historic levels, one single flood event may not lie behind all the various myths. Since the area experienced several major floods over the course of hundreds of years, stories passed down about the various events probably merged in the re-telling. That’s the background of the historical event(s). As for the Mesopotamian flood stories, there are many (Gilgamesh, Atrahasis, Sumerian King List, Sumerian deluge), and Genesis 6-9 is the last to be written.
Now, the “regional flood” approach can take two directions from here. One version of this interpretation simply identifies one of the above floods as “Noah’s Flood,” and afterward the story was passed down intact through Noah’s descendants until it was written down several thousand years later. This seems to be your position. The other version, represented by Walton, Longman, et al., says that one (or all) of these disastrous floods provided the “nugget of truth” behind Noah’s flood, but the details of the story in Gen. 6-9 are examples of literary “hyperbole,” which the author employed to make a greater point about God.
I don’t think the first version of the “regional flood” interpretation is sustainable. The name “Noah” is a Hebrew word, and the Hebrew language did not yet exist when the floods in question happened. Also, the idea that the story was passed down orally for nearly 2000 years without alteration strikes me as wishful thinking, if not silly.
The second interpretation of the regional flood is simply that an actual flood served as the inspiration for Genesis, and that fact somehow preserves the historicity of the account. If that sounded a little sarcastic, that’s because it was. This is where I depart from Walton and Longman. If the flood at Ur or Kish was the “real” flood behind Genesis, it also was the flood behind Gilgamesh, Atrahasis, and all the other Mesopotamian myths. But, in that case, we must apply the same reasoning to those myths as to Genesis, so what has been proved? Only that the Gilgamesh epic has as much historical basis as Genesis. Do we therefore regard the Gilgamesh epic as being “historically true” in the same sense as Genesis? Obviously not. And if we do not accept such reasoning in the case of Mesopotamian myths, why do we think it is valid in the case of Genesis 6-9?