Ryan: Thanks for the info. My syphon pipe intake on the pond side (water side of the dam) is about ten feet below the normal surface level of the pond. The bottom of the syphon pipe running thru the top of the dam is about 3 inches above normal pond level. The bottom of the vent pipe is about 2 inches above normal pond level
If I understand your post correctly, the syphon pipe will begin to run (but not syphon) if the pond level rises above the bottom of the syphon pipe running thru the top of the dam. The syphon action will probably not start unless the water level rises pretty much over the top of the syphon pipe going thru the top of the dam. If the syphon action starts, it will stop when the vent pipe is above the level of the pond.
I am probably going to raise the elevation of my emergency spillway to ensure the level of the pond will engage the syphon action before running off thru the emergency spillway.
If I have understood your advice please confirm that I have.
Thanks very much for your input.
