I am no expert but I would think your head pressure in your drain pipe would be equal to the water depth. When your water goes up you have only 6 inches of head pressure, not very much.
You would need to replace with a syphon system if you have enough slope on the back side of your dam. Then a 3 inch pipe would pull a lot more water out.
I love the siphon system, but it would get in the way of my dogs entering the water.
