You are correct that the elbow at the bottom forms your trap. It sounds likely you'll start to siphon when the water reaches above the pipe and could possibly even start a siphon when the pipe is half full. One of the siphon system manufacturers could tell you for sure at what point a siphon would start if you give them the full specs of your system.
Keep in mind it really isn't all that important to start a siphon unless you absolutely need the added capacity to remove excess water from your pond. You will still get the benefits of removing water from the bottom of the pond in contrast to a conventional overflow even if water is only trickling out.
