Rangersedge,
I used a 4" temp. siphon on my small pond & it worked well the first time I drained it. However the pond filled back up before I could clear the original drain so I tried it again. Everything went well for 3 days then it just stopped. I had seen a 6" turtle in the pond & suspect it got stuck on the inlet stopping the siphon which allowed air to gurgle up the outlet. It is much more difficult to start the siphon when the water level is 20' down the dam face.
Recommend: 1)Use a basket or some device on the inlet. 2)If you float the inlet be sure it's suspended well below the surface to prevent sucking air when it creates a whirlpool. 3)6" gate valves are expensive (I found one at a salvage co. for $125.00) & I think unnessesary. PVC ball valves will work fine.
I like your idea for starting the syphon ie: combo. of valves ect.
