In the Saturday posts on oil reservoirs and why drawing down the oil too quickly can cause problems, I talked about channeling.  There is a visual example of how this comes about in the failure of the repaired levee in New Orleans.   When the levee was repaired they filled the hole with a combination of fine and coarse rock which was dumped at the site and raised into a wall that filled the opening and stopped the water flowing into the city.  However the pressure of the water in the canal was sufficient for it to percolate through some of the passage ways that were left through the rock in that berm and so, by Thursday evening there were pictures on some news channels,  from the site, of water flowing out through bottom of the wall, and this before the greater surge and failure that came later.  (This is the residual permeability that was talked about in the posts back then).