Agreed. Didn't some wacko environmentalist nutjob try to blow up a Mississippi levee back in the 90s with a car bomb?
As I recall, all that happened was the levee got an interesting scorch mark and little else. Oh, yeah, all the grass on it for a 500' radius died.
The charge would have to be inside against the concrete liner I would think..?? The water pressure would ensure enough blast force got directed to the concrete to rupture it.