If there were no mountains or deep trenches, there would be enough water to cover the Earth but as there are mountains and trenches, there isn't. There is no possible scenario for rain to fall all over the Earth at the same time nor is there any scenario that would allow rain to fall in any one area continuously for forty days and nights.
There is no evidence at all for Noah's flood and plenty of evidence that it never happened. Tree rings, lake varves, coral reef cores and Antarctic ice cores all show a complete lack of a global flood. This, by the way, is accepted by theists and atheists alike including all the mainline Christian churches. Science is for everyone.