Well I think the very first thing to remember is that the writers of Genesis had a very different view of the universe. Something kinda like this. Basically to them Earth was a flat disk which was completely immersed in water. From above the pocket of air we live in was protected from the water by the Firmament of Heaven and from below by the earth. Actually this isn’t quite true, as the oceans are the the tops of the Great Deep. So the Earth didn’t completely close off the Great Deep. Maybe a diving bell would be better comparison.
[QUOTE=Genesis 7:11-12, 17-20, 24]
11 In the six hundredth year of Noah’s life, in the second month, the seventeenth day of the month, that same day were all the fountains of the great deep broken up, and the windows of heaven were opened. 12 And the rain was upon the earth forty days and forty nights.
17 And the flood was forty days upon the earth, and the waters increased and bore up the ark, and it was lifted up above the earth. 18 And the waters prevailed and were increased greatly upon the earth, and the ark went upon the face of the waters. 19 And the waters prevailed exceedingly upon the earth, and all the high hills that were under the whole heaven were covered. 20 Fifteen cubits upward did the waters prevail, and the mountains were covered.
24 And the waters prevailed upon the earth a hundred and fifty days.
[/QUOTE]
So God opened up passages in the disk of the earth and in the Firmament of Heaven and the water came in, from above and below. It was not just a rain. And there is no proportion given, so it may well be that 90%+ of this water came from the Great Deep below the disk of earth. If the diving bell analogy is reasonable, then most of the water would have come from below as the air leaked out of the Windows of Heaven. Once the 40 days were up, all God would need to do is seal off the Windows of Heaven and pump in some new air. This alone would force the water back down into the Great Deep.
The problem here is that given our world is not like that described in the bible. There are no Waters above or below. And there is no Expanse of sky separating them under which all the land is gathered together. There is no Firmament from which two great lights (and the stars) are hung, to mark the seasons, days and years.
In the world we have the flood could not work, at least not according to the rules of physics. If God did it he cheated. In the real world, we aren’t floating in a great body of water. So all that water would have to come from outside. And if that much water falls it would raise the temperature of the surface of the Earth by about 1500° C. Just about all rocks melt at or below 1200° C, so the surface of the earth would all be molten.
That much water would also displace 2/3rds of the atmosphere. In fact it would displace it above the line where Earth’s gravity tends holds it in place. So for about half a year 2/3rds of our atmosphere would be blowing off into space.
Given issues like those, I think the sanitation of dead bodies and washing away of topsoil are minor at best. If it happened at all, God would have had to put Physics on hold while he did it. And that point we can provide no descriptions of what it was like, because the universe Noah was living in was operating by entirely different principles.