If it was God’s intent just to wipe out wicked mankind he should have just used one angel to do the job. He went well out of his way to bring about the flood event. I believe his main goal was to change the atmosphere surrounding earth. The rainbow, fermentation and a gradual life span reduction were a result of a new atmosphere.
OR, rather than a quasi-scientific explanation, we can look at the account to realize that it serves three purposes to the Torah:
1) Gives parents a good answer to explain where rainbows come from, when their children ask (and what parent wants to say, "I dunno?" since they've never learned about the refractive properties of water droplets suspended in the air in high-humidity conditions)
2) It reinforces the power of God, and what happens when he's not obeyed (even when he FORGOT to give mankind laws to obey, in the first place! You'd THINK he'd figure that out AFTER Cain killed Abel, and it wouldn't take 1,000 years or so to figure out that men MIGHT be better off administering justice themselves!)
3) It explains why man was alowed to rule over fellow men (remember: YHWH gave Adam domain over ANIMALS, not humans). ALL law codes written in ancient times relied on a supernatural justification to claim their authority, whether it be Code of Hammurabi, or the older Sumerian Codes (Oh, there MAY be an exception I'm not aware of, but I doubt it's wrong to say "ALL").