Chris,
I just finished my pond as well. I built an overflow just like the one you drew in your first diagram. I did not want the "drain" inside the water because this would require me to get into the water to adjust it. I used 6" Sch 40 pipe.
Right now I am filling the pond for the first time. The overflow pipe is a lot taller than it needs to be. Once the water gets to where I want it, I will then cut the excess pipe off.
When it came to the anti-siphon, I was told that it was not necessary unless there was about 10 feet or more of hydrostatic pressure on the pipe. I will never have more than about 4 feet of hydrostatic pressure on the pipe so I did not use one.
At 4 feet I would have about 1.75 pounds of hydrostatic pressure. With a 6" pipe, this is only 50 pounds of force "pushing" the pipe. The pipe I have is 40 feet long and I doubt that 50 pounds will overcome 40 feet of friction.
I don't know what the valve is called, but it slipped over the 6" pipe and had a threaded rod that when turned it mated up the valve to the seat.
The guy that conculted me on this builds catfish ponds for a living so I assumed he knew what he was talking about.
Good luck with it all. I hope you enjoy watching your pond evolve as much as I have enjoyed watching mine!!!
