Theo,
Interesting, you are referring to what Engineer’s call the “freatic line”. In other words there is a predictable wet line inside the structure from the water surface on the front side of the dam to the seepage area on the back slope. This internal wet line can be manipulated prior to construction by incorporating a sand drain inside the structure. This is generally a 2 ft X 2 ft. trench filled with course sand at or slightly below natural grade. Some landowners even install a 4 inch perforated pipe inside this sand envelope to assist the sand trench in draining off any water trying to make its way through the dam. The eventual system works similar to your sewage drain field, only in reverse. The objective, is to control the freatic line and eliminate wet and unstable backslopes. I probably didn’t do a very good job explaining. Suffice it to say, there are ways to counter seepage problems prior to construction.
