We are going to modify our dam to a siphon system. We currently have just an emergeny spillway and a 6" bottom drain and valve for a 50 acre lake. I understand the siphon principal. My question is "does the water have to get completely over the main pipe to get into siphon mode?" In other words, I can have multiple 12" pipes or one 24" pipe. If we use a 24" will we need a 24" rise to get into siphon, or will a partial rise with a submerged vent get us into siphon? My gut says it must mostly fill the pipe tp get there.

it must be over the main pipe to go into siphon mode.

I put in a 4" bottom withdrawal siphon spillway in last September and I've been experimenting with different heights of the air inlet relative to the main pipe. I currently have the air inlet at about the apex (the bottom of the main pipe at the highest point). This is working well for me so far. Once water starts flowing through the pipe, the air inlet is already blocked off. It does build up slight air pressure in the pipe as water starts to flow, but it does start to siphon prior to being over the top of the main pipe.

When I placed the air inlet above the main pipe, the siphon would start, but all of the air in the pipe would never purge and the siphon would only be pushing out about half of what it is capable of.

I'm still experimenting with better ways to release the air pressure faster so that the siphon starts sooner. Yesterday, I drilled a 1/16" hole into the clean-out cap (the highest point in the system) and countersunk it with a 3/16" bit 2/16" deep and then placed a BB in the hole. It's supposed to rain next week, so I'll be able to test this theory to see if the air releases without letting any air back in when siphoning.

You might look into a back flow valve. Don't know if it will seal air or not.