Understanding Water Pressure in Mississippi Flooding and Levee-Dwelling Homes

[RickVS]

WRT the Mississippi flooding, you see pictures of homes with levees surrounding them that the homeowners built. It keeps the water out. Here is my question. The ground beneath the flood waters has to be soaked. Wouldn't this water permeate the dry ground right next to it under the levee like when a paper towel absorbs water and the water spreads as it gets soaked up? And if the ground under and inside the levee is soaked, then how come the enormous pressure of the surrounding waters don't just percolate up through the ground and fill in the area inside the levee until the water level, and pressure, is equal to the water levels surrounding it? These homes, for the most part, are bone dry, or at least dry enough to not pool water (with one exception that I saw, but even then it just pooled and that was it). Thanks.

 

[SpectraCat]

WRT the Mississippi flooding, you see pictures of homes with levees surrounding them that the homeowners built. It keeps the water out. Here is my question. The ground beneath the flood waters has to be soaked. Wouldn't this water permeate the dry ground right next to it under the levee like when a paper towel absorbs water and the water spreads as it gets soaked up? And if the ground under and inside the levee is soaked, then how come the enormous pressure of the surrounding waters don't just percolate up through the ground and fill in the area inside the levee until the water level, and pressure, is equal to the water levels surrounding it? These homes, for the most part, are bone dry, or at least dry enough to not pool water (with one exception that I saw, but even then it just pooled and that was it). Thanks.

I think that does happen, it just happens very slowly, and the floodwaters generally recede before it can happen to any appreciable extent. For example, the polders (reclaimed farmland that lies below sea level) in Holland need to be pumped out periodically to keep them dry.

 

[Studiot]

For example, the polders (reclaimed farmland that lies below sea level) in Holland need to be pumped out periodically to keep them dry.

We have an even more esoteric system where I live.

Good answer, by the way.

 

[pallidin]

Wouldn't this water permeate the dry ground right next to it under the levee like when a paper towel absorbs water and the water spreads as it gets soaked up?

Perhaps, but this is very site-specific. That is, the degree of permeation(thus saturation) is generally dependent on porosity.
Not sure about the area where your referring to, but I've seen ground with well over 70% solid rock, or large concentrations of solid gravel, or other soil conditions/compositions which do not lend themselves to ready saturation(due to extremely low porosity)

 

[PJC01]

I can provide some insight into the phenomenon of water pressure and its interaction with levees in the context of Mississippi flooding. The key concept to understand here is the concept of hydraulic head, which refers to the total energy of a water system, including both its pressure and elevation.

In the case of a levee, the hydraulic head is highest on the side facing the flood waters, as this is where the water has the most potential energy to exert pressure. However, on the other side of the levee, the hydraulic head is much lower, as the water is contained and has less energy. This difference in hydraulic head creates a pressure gradient, where water will naturally flow from an area of high pressure to an area of low pressure.

Now, in order for water to permeate the dry ground next to the levee, it would need to overcome this pressure gradient and flow from the high pressure side to the low pressure side. However, the levee acts as a barrier, preventing this flow and keeping the water contained. Additionally, the ground beneath the levee is often compacted and reinforced to prevent water from seeping through.

As for the dryness of the homes inside the levee, this can be attributed to a combination of factors. Firstly, the levee acts as a physical barrier, preventing water from entering the area. Secondly, the homes themselves are typically built on higher ground, further reducing the potential for water to seep in. And finally, the homes may also have drainage systems in place to redirect any water that does enter the area.

In summary, while the ground beneath the flood waters may be soaked, the levee and other barriers prevent this water from permeating the dry ground next to it. The pressure of the surrounding flood waters is also counteracted by the lower hydraulic head on the other side of the levee, preventing the water from percolating up and filling the area inside the levee. I hope this helps to clarify the science behind this phenomenon.