As others have already mentioned, take a deeper look at the sheer impossibility of a global flood and you'll find that the flood story is just another myth with no more truth in it than the exploits of the ancient Greek gods.
Purely in terms of the "rainbow promise", do you think God magically altered the laws of physics to allow light refraction only after the flood? Silly. It's just a tale invented by people from long ago trying to make sense of the world around them without the benefit of science.
A great resource for information regarding the flood is located here: http://www.talkorigins.org/faqs/faq-noahs-ark.html
This was what led to me losing my faith in the god of the bible, because if there's one false myth what's to say that the other outlandish stuff (talking snakes, sun standing still, people rising from the dead) is true?