Better re-think that siphon stuff unless you have a way to turn off the flow when you need to or you'll drain your pond.
Siphons are great for maintaining the same water level in adjacent ponds, but if you only want to get rid of water that exceeds a certain level in one pond with the system you have, you'll have to raise the discharge port to the water level that you want to maintain in your pond.
He should have a vent where he wants his water level and that vent breaks the siphon
http://ponddampiping.com/syphon1.htmlMaking sure that vent doesn't plug is another story, it wouldn't take much