Bill -
I'm a bit nervous about a spillway. Mainly with erosion occurring underneath the concrete or the boulder lined one. It'd be nice to have a waterfall type spillway, but I think it might cause more issues in the long run. I even thought about taking a large diameter pipe, cutting it in half and making a U-shaped trough down the hillside. Like a spillway....sound crazy? It'd be easy to look for leakage and monitor the flow.
I was thinking the siphon design would be easiest and cheapest to install? The link above, pretty much sells the siphon system as a kit, in my view. I can do a quick CAD drawing and hopefully get a PE to sign it and be on my way. I don't know if I will have to do any calculations. Not sure how to address that. ODNR hasn't asked for anything calcs (yet). That might be as simple as referencing flow charts for various pipe diameters and just going one size up to show that there is no issues.
