In response to the question about the siphon system ,I saw my siphon handle the latest hurricane here in mid west Ga. quite nicely.
I have a 2 acre pond fed by a live stream with ,a 10" pvc siphon,on Monday (Sept 27) w/3.8"s of rain,the pond being at full pool filled over the pipe but the siphon action started and with in about 3 hours had pulled the level back down almost to normal pool.In 2 years I don't think my spillway has ever seen pond water and it seems to me that this is exactly what you are looking for in a water management system.
Don't be too concerned with your inital cost when sizing your pipe as the peace of mind is priceless.
