What kind of damage do sandboils cause?

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It is covered in rocks ...

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I thought those were sandbags in that picture.

As to the threat, I think it shows a weak spot in the levee, or in the case of the River Rd pics (the ditch), a spot where water is seeping up from underground. Scary--I drive right by there every day!

Its natural. Just means that water is moving through the sand underneath the levee and pushing out the other side.

If to much water starts pushing through and it starts pushing dirt out, then it can cause the levee to collapse. Luckily water moves through sand pretty easily.

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what kind of threat does this pose?

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The big one...a breache.

Sand boils mean that the water pressure has found a sand stringer of some kind to force its way through. It is like seepage, for the most part. If they find it and seal it, which is easy to do, no problem. If it goes long enough, it can weaken the levee, which is evident when the water going through is muddy.