Thanks for the responses. I can see the need to keep an open dry pipe during freezing conditions, so the system does not freeze closed. John- I do have a good, wide emergency spillway that has never been used in the 30 years since the pond was built. That is why I was thinking I could save some digging by installing a 4" pvc outlet pipe thru the current surface spillway (6" corrugated metal). It may be safer to not change the sizing for the spillway and to cut thru the dam at the current full water level to install 6" PVC for the bottom withdrawal spillway.