Mike, your dam crossover pipe functions the same as if it were a straight drain pipe through your dam.
If you wanted your normal pool level at X freeboard, that should be where the inner bottom of your crossover pipe should be. As water rises above normal, your pipe drains normally. If you set the inlet height of your VENT pipe at say, 5 inches above normal level, you would still not get a siphon started until the pool was above the TOP of your crossover pipe and has evacuated enough air in the system to start a siphon (because your vent inlet is under water and "sealed") The siphon would theoretically break when the pool was back to 5" above normal pool and the pipe becomes a normal drain again..in reality, due to whirlpooling, a 5" rise set on the vent will break at 7-9" above normal pool
I just went down to the dam again and the water is rising slowly. For some reason the 10" pipe just cant keep up with the water. I sized the pipe based on recommendations from one of the internet companys I wont mention. They asked what my watershed was I told them about 70 acres and it's stream fed. The size of pipe recomended was 8". I bought 10" for that added comfort. Now I see that regardless, the 10" cant keep up with the rainfall we've been having. It's probably going to cost anoughter $1500.00 to add a second 10" siphon bringing the total of this adventure to $6500.00 That just sounds crazy to me. If I add another 2ft of fill to the dam I'll prob be around 9000.00
