Paul Seely writing about The Flood at BioLogos.org, says we can trust the theological lessons of Gen 5—9 even if the Flood story is not historical.
Although the human author probably did not make a sharp distinction between Legend and History, the account [of Noah’s Flood] was factual to him. But because of the light we have received from modern science, we must think of it as parabolic. Some, however, still raise the question, How can we believe the moral-theological lessons in the account if we reject its historicity since the lessons are based on the assumption that the account is historical fact?
The answer is that we are reading the account over the shoulders of the ancient Israelites to whom it was addressed. They believed it was factual. This was a naïve belief, but they had no reason to question the account’s historicity. We must remember that their understanding of the natural world was that of little children. As the conservative nineteenth-century Princeton theologian Charles Hodge said, they believed the sky was solid, the earth was flat, and the sun literally moved.1 As for an anthropologically universal Flood, second millennial Mesopotamians believed it was an important historical fact, and this tradition may well have been passed down to the Israelites through the Mesopotamian patriarchs beginning with Abraham.
Given these inherited naïve “scientific” and traditional beliefs, it was pedagogically wise for God to speak to them in terms of those beliefs.2 We can thus appropriate the moral-theological lessons which are still valid for us while ignoring the accommodated ancient Near Eastern “science” and traditions upon which they are based. These now outmoded concepts are in the text only because the account was not written to us but to the ancient Israelites.