Addendum: I've been thinking some more about the pipe from the pump to the auxiliary pond. It seams to me that it really doesn't need to pass through the bottom of the new dam, but could pass through the dam above the water level alongside the overflow pipe.
Here is the modified schematic drawing:
Note: Although the drawing shows the red filling pipe below the green overflow pipe, I would actually probably have both at about the same height and above the full pool level. Would that eliminate the need for anti-seep collars?
