Instead of "forty days and forty nights"

That would cause like every river in the world to flood in one powerful/huge blast, which would destroy many homes and buildings. It'd also kill many.

You're forgetting that the destruction of all life on the earth wasn't the only thing that was happening during the flood. Noah and all the animals and his family were in the arc. God was seeing Noah's patience and faith in him during this time. That wouldn't have happened if the flood had only been one day.

ppl that say that would cause the rivers to flood , kill people etc...was that not what the 40 days and 40 nights of rain intended for...to destroy most life forms......hmm prolly god had a small odubt about himself being able to succesfully delete all life forms in one day:)