Quote Originally Posted by buad hai View Post
Based on some comments here I decided to test my little overflow drain. I put a half inch garden hose into the pond and turned it on full blast. No matter how long I left it on the level of the water never went above the top of the outlet pipe.

Over the last 12 hours it got a better test: torrential rain showers beginning about 6 PM last night and ending about 6 AM this morning. The flooding was so severe that I had to abandon my morning run. So, I detoured to the pond instead. Sure enough, the pond water level was right at the bottom of the overflow pipe.

It works!
BH. Just a thought, have you put a filter over the mouth of the outlet pipe? It might prevent snails and other creatures getting wedged down the pipe.