Yes, that's a valid concern, seepage under the berm(or roadway). At least two solutions : a clay berm as a base instead of the silt/gumbo they have now, which means importing HUGE amounts of impervious clay on barges over many years(for 300 miles of levees around N.O,); or deep concrete walls, say 6 to 10 ft deep(8" thick), on both sides of the roadway(from 20 to 25 ft wide), basically a big post-tensioned box beam that has buoyancy in and of itself. Thus it would exert little weight-pressure on the gumbo as you'd have vertical dead man anchors holding it down against that positive buoyancy. That still leaves the problem of a road panel strong enough to carry semitrucks yet light enough to pop right up, via buoyancy and side piano hinges, in a tsunami wave, that's what I'm designing right now... The objective, in this instance, is a ring road entirely around a re-built New Orleans that provides a 20 to 25 ft high sea wall for any future hurricane storm surge; and it operates automatically by natural forces. W=P
Best wishes in your endeavour!