What I've read is that the earth's crust is pretty flexible. The Amazon river basin sinks and rebounds three inches each year under the weight of 30 to 40 feet of floodwater from seasonal rains.
I also read that the glaciers caused the earth to sink many meters in some areas, causing unusually high tides that flowed inland for many miles, and is still rebounding in many areas.
Main article: Isostatic rebound; from Wikipedia
This rise of a part of the crust is due to an isostatic adjustment. A large mass, such as an ice sheet/glacier, depresses the crust of the Earth and displaces the mantle below. The depression is about a third the thickness of the ice sheet. After the glacier melts the mantle begins to flow back to its original position pushing the crust back to its original position. This post-glacial rebound, which lags melting of the ice sheet/glacier, is currently occurring in measurable amounts in Scandinavia and the Great Lakes region of North America
Some believe this widespread rebounding is responsible for small deep earthquakes in areas far from fault zones.
Regarding the flood, we haven't yet constructed a reasonable model of how the flood might have occurred (imo of course), although the biblical story gives many hints. For example, the earth had not received 'rain' until the flood, but instead was apparently watered by (nightly?) dewfall. If this also means that there was no significant groundwater in the earth in the form of the large aquifers present today then the forty days of rain might have had a significant effect and may even have began the flooding process.
Consider a steady rainfall, perhaps started by one or several volcanic eruptions (dirt, previously absent, in the heavy moist air causing widespread rainfall) of a vigorous one inch per hour. This would soak the earth, filling the previously dry ground, with 80 verticle feet of water over the entire land, minus runoff at the seashores. Scientists estimate that the land sank as much as one foot under the weight of three feet of glacial ice, in some areas. Applying the same calculation the rainfall would cause the continents to sink twenty five or so feet, enough to begin massive flooding from the sea in lowland areas. Of course the additional weight would further sink the land and exacerbate the flooding,
It is also interesting to note that the earth's crust is solid rock, floating on molten rock, and barely floating at that. It doesn't take much additional weight to push it down. Of course the downward pressure will displace the liguid magma outward having the further effect of raising the seabeds, perhaps providing the needed water to cover the highest peaks, which may have now sunk considerably under the enormous weight of the incoming waters.
Also of profound interest is the mechanism for the abatement of these waters, now encompassing the entire globe,
the wind. A strong and steady wind can move much water, and the usual limiting factors, such as a containment feature, would be absent from the flood scene allowing plenty of room for the windblown water to move freely away from the land. As the floodwaters returns to the sea the weight presses down on the seafloor causing magma to flow back under the continents and raising them.
Also of interest is the size and scope of the changes needed to complete the flood as recorded in the bible. If you had a globe of twelve inches in diameter representing the earth you would need to deform it about the thickness of two or three sheets of paper to cause global flooding. Such a deformity would not be apparent to the naked eye. Additionally, although mountains look very tall and imposing they are little more than the height of grains of sand on this twelve inch globe.
Or, imagine a teeter-totter representing the dry land and the seafloor. An nearly imperceptible movement of the board is all that is needed to accomplish the movement of water in either direction.
We are so accustomed to thinking of cataclysmic movement of land and water when thinking about the flood that we ignore the information given in scripture. The most notable being the speed of the income floodwater from the sea, which I calculate at 1.6 inches rise per minute based on the duration of 150 days and displacement of 29,000 feet, the height of Mt. Everest.
This inflow is comparable to many incoming tides around the earth, hardly the gulleywashers imagined by most. Here's a video of one that gives a good idea of how fast Noah's flood came in from the sea:
Quand les touristes se font piéger Par la marée montante - YouTube
Notice the clarity of the water as it flows across the sand. At that speed there is no erosion, and as the flood waters flowed gently across many areas there would also be no or little erosion, especially healthy grasslands. It is where the flow of the waters are restricted or flow downhill that serious erosion takes place. A study of the earth's topography will easily reveal those areas.
Another mistake that is made is the assumption that because the flood presented massive amounts of water that massive amounts of erosion and deposition would occur. The reality is that massive amounts of water acted on finite or limited amount of deposition material, and that such erosion and deposition would occur with a much smaller (local?) flood. To demonstrate this for yourself place sand in the bottom of a bucket, then pour it out slowly. Notice that the sediments don't move until the last of the water flows out. It doesn't matter how large the bucket is, the results will be the same. Try the same experiment in your bathtub. Place some dirt or sand in the bottom then fill the tub. Notice that the sediment stops moving soon after the water starts to fill the tub. It doesn't matter how much water you put into the tub, the sediment will only move with the first water in and the last water that drains out.
Anyway that's my current working theory ( always subject to revision of course).