HERBIE OVERFLOW METHOD

[ca1ore]

The water will only be at the bottom of the horizontal part of the elbow and the hole will only be under water if the primary drain fails. When water first goes over the top of the emergency drain the hole will make noise alerting you to the problem. When the water completely covers the elbow it will convert to a full siphon.

Precisely! Stand pipe will only ever function as an open channel; dual elbows plus air hole on the top will function equally well as an open channel but also convert to a siphon. You may never need it to; but it's nice to know it can.