Hydrodynamics

One issue regarding the scope and historicity of Noah's flood is the depth of the flood waters. I recently ran some questions by a field geologist who specializes in fluvial geomorphology. Before reproducing our exchange, I'll quote something I recently said:

Regarding Gen 7:20, the text doesn't say the waters rose to a depth of 15 cubits above the mountains. The Hebrew text simply says the waters rose 15 cubits above, and the mountains were covered.

So "15 cubits above" may well have reference to ground level, which was sufficient to wash over the surrounding hillside. Think of a flood plain or river basin skirted by hills. Keep in mind that "mountain" isn't a technical term in Hebrew, but a synonym for "hill".

With that in mind:

Hays

Is this a correct understanding of the issues:

i) In some river systems, the riverbanks are higher than the surrounding terrain. Due to periodic flooding, which deposits silt and coarse gravel, the riverbanks build up over time. They become high ground in relation to the surrounding terrain. 

Geologist

Yes, the coarse material carried by rivers tends to settle out on the margins of the channel during floods, thereby building levees and high ground right next to the river.  This means that the surrounding valley bottom can readily flood to the level of the levees when the levees do (eventually) overtop in a big enough flood.  The Mesopotamian rivers are classic examples of this kind of river (which tends to be in estuarine environments).

Hays

ii) Is it the case that riverbeds acquire layers of silt? If so, does that mean riverbeds rise/become higher over time?

Geologist

Rivers can aggrade (fill in) or incise (cut down) over time depending on the balance of sediment they receive to the power of the flow to move it. 

A river with a balance between the two will just shunt sediment on downstream. 

Hays

If there was nothing to counteract the accumulation on riverbeds, would that make rivers shallower over time?

Geologist

The transport capacity of the flow keeps the channel open.  Most channels are adjusted (in the width/depth) to carry what is known as the “bankfull” flow, which tends to be close to the annual high flood.  Floods are events that overtop the banks and spill out on to the floodplain or surrounding terrain. 

Hays

iii) Is that offset by (i)? Do rivers retain the same general depth, even if the beds are higher, because the banks are higher? 

Geologist

In aggrading rivers (those with excess sediment) the bed can fill in and the river can shallow — unless the sedimentation on the floodplain raises it (which happens if the floodwaters can spread across the floodplain).  

Hays

iv) This seems to imply that the low ground becomes incrementally lower in relation to the river banks (or levees) as the riverbanks become incrementally higher due to the cumulative effect of flood deposition. 

Geologist

Yes, this can happen when a river aggrades.

Hays

v) The upshot, I take it, is that it takes less volume of water to inundate the surrounding terrain when the terrain is lower than the riverbanks. If the surrounding terrain was higher, it would take more water to submerge the area, or submerge the area at the same depth. 

Geologist

What happens when a river aggrades and builds its levees up higher is that when a big enough flood comes along to overtop the higher levees then the surrounding terrain is inundated under deeper flow.

Hays

vi) Not only depth but breadth. It takes less water for the scope of a flood to be on the same scale if the surrounding area is low ground compared to the riverbanks. 

vii) Is that intensified if there's something like a mountain range (or ridge of hills) to form a barrier that contains the water? 

viii) I've read the claim that "Noah's flood" couldn't be merely regional because Mesopotamian topography is a drainage system, so there's nothing to keep the water building up. It will pour downriver into the Persian Gulf. 

Geologist

I don’t understand the logic of the argument here; a big enough flood there will of course eventually drain into the Persian Gulf, but it could be a monstrous flood while doing so because it can take a lot of time to drain the whole valley bottom after if floods under tens of feet of floodwaters that the levees keep from flowing rapidly back into the channel.  So to me that claim you reference is simply nonsense that demonstrates the writer doesn’t understand what he/she is talking about.