OK, I'll finally bite: I take it a sand boil is water percolating up, and producing visible sand?????? I have been wanting to ask that since yesterday. What exactly are sand boils? TIA
You got it. Once water finds a way through, even a trickle....well I don't know of any cases where the engineers stopped it before the water equalized on both sides.
"New Orleans' levee system has been key to survival"
Wednesday, August 31, 2005 By Ann Carrns and Betsy McKay, The Wall Street Journal
It is rare for floodwaters to "overtop" levees, as they usually succumb first to "under seepage," in which water seeps underneath from the river side and emerges on the land side. Sometimes the severe pressure causes soil within the levee to liquefy in a phenomenon known as "sand boils," which often precede structural failure.