Thanks for the reply. The siphon intake has about 8 to 10 feet of head pressure on it. A 10� siphon will flow 2,480gpm with 8ft of head pressure. The old 15� drain pipe could flow a maximum of 1,600gpm with 1ft of head pressure. This would give me an increase of 55% in flow capacity compared to the original stand-pipe system. It was really shooting the water out, but just could not keep up with the inflow. With this size pipe I should be able to handle a 150 acre water shed. Not sure whats up there. My spillway is on the far right side of the dam and travels to the stream that is the discharge. It's considerably lower than where my siphon exits. It looks like someone just lined the entire thing with rip rap. I'm going to clear all the treebranches and junk out this week. Once the rain stopped the pond started to drop pretty quick and right now its back at normal pool just 13hrs later.
Here is a video of the amount of water I'm talking about. This was happening at the same time the 10" pipe was pumping out over 2000 gallons a minute.
you tube pond overflow Video of siphon working
10" siphon running video 
