[Silent A]

I'm a hydrologist, so I'll speak of the flood.

First, what exactly do you have in mind when you think of a "biblical great flood"? Can any large flood in the Middle East circa 4000 BCE qualify? Does it have to be something much bigger than all the floods that have occurred in recorded history? Or, is it something pretty close to a literal interpretation of Genesis, a worldwide flood total produced by about 40 days of rain?

If it's the first or second, it should be obvious that we'd find evidence of some kind of qualifying flood.

If it's the latter you have some serious problems. First, there simply isn't enough water on Earth to drown the entire planet in several thousand metres of water (remember, mountains were supposedly completely covered). Second, the rate of rainfall required is at least 3 metres/hour (based on 3000 metres of rain over 40 days). Only once has this kind of rainfall rate been observed: In 1970 it rained at a rate of about 2m/hour in Guadeloupe. However, this intensity only lasted about one minute. This kind of intensity would have produced massive erosion scars in every locale across the surface of the Earth. The evidence would be so obvious that no one could deny that such an event once took place. However, such scars are relatively rare - everyone doesn't have a Grand Canyon in their back yard.

So we have a problem: any truly "Biblical" flood would have left a lot more evidence, but if we look for a biggest-ever-flood we're left with a relatively mundane event that is hardly miraculous - and impossible to tie directly to the Noah story.