THE FLOOD LEVEL
The key to understanding the depth of the flood waters is to be found in Gen7v20: The waters rose and covered the mountains to a depth of more than 20 feet. This seems a very strange item of information. We are not told by what depth the highest land was covered but only that it was by more than 20 feet or 7 metres. How did Noah know that? He must have noted the height of the water line on his boat when he disembarked. This was the amount of water the ark needed to float in. It was the draught of the craft. It is the one sure thing an eye witness would know about the flood and it is recorded. I think this is amazing.
If we now look at the account of the rising flood level in Gen7v17to20 it may be possible to deduce the total depth of the flood. I actually think that the AV is a better translation than the NIV in this instance and so will quote from that. Verse 17 states, 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. So the flood level rises to the extent that the ark floats. At this stage we know exactly the height of the water level above the ground where the ark had stood - 7 metres. It took 7 metres of water for the ark to float. Once again the record rings true because this is something Noah would have experienced. He would have known when the ark began to float.
Verse 18 says: And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters. The next change Noah notices is that the ark begins to drift. The floodwater has risen to such an extent that Noah's craft is able to move at the mercy of wind and currents; it is no longer checked by submerged trees and buildings. It now floats above them. Trees in a semi arid area do not grow to a great height. Cultivated olives do not stand above 10 metres. It is probable that another increase in the order of 7 or 8 metres would have allowed the ark to drift.
Verses 19 and 20 state: And the waters prevailed exceedingly upon the earth; and all the high hills that were under the whole heaven, were covered. 15 cubits (7 metres) and upward did the waters prevail; and the mountains were covered. It is not so easy to deduce the rise in level from these verses. Noah watches as the highest land disappears from sight and he knows that he proceeded to float 7 metres above it. The words used to describe the rising level of the flood deal with the total extent of the flood at each stage rather than the rise in depth from stage to stage. The flood gets vaster and vaster as it rises through the same level from stage to stage. I think it likely that the flood level rose by the same amount as in the first two stages. If it had risen at a phenomenally rapid rate to hundreds of metres this would have been observed and commented upon by Noah. It would have been is such stark contrast to the earlier events. Noah says of the floodwaters that floated the ark - they grew great. He writes this of a flood of 7 metres. What would he have written of a flood of 100's of metres!
I think the flood was no deeper than 22 or 23 metres - just enough to clear the low hills and terraces of the Tigris-Euphrates valley. This conclusion does depend upon the dimensions of the ark being given correctly.Roger M. Best has an interesting section on his web site about the size of the ark. He thinks the figures are in spans and not cubits which would reduce the height of the ark by half. This would reduce the depth of the flood to 15 or 16 metres as an increase of 7 or 8 metres would still be needed for the ark once afloat to drift free from obstructions. I am going to continue on the basis that the Biblical dimensions are correct.