Roundy, a lot of information is needed to estimate the flow of your siphon.

In "normal" mode, the pipe acts like any normal, gravity drain pipe, but when in siphon mode, the weight of the water, head pressure, pressurizes your pipe and flow increases dramatically.

Contrary to what was said about where the inlet is, the inlet location has no bearing on flow. Head pressure, (and pipe pressure) is determined by total height of the water level to the outlet, vertically.

With a 50 foot long pipe, and a 10 foot drop using smooth bore PVC and a minimal roughness coefficient, I'd conservatively estimate typical discharge flow around a minimum of 5000 GPM in "full siphon" mode, and about 500 GPM, maximum, in normal gravity drain mode....a 100 foot long pipe, but with 5 feet more of drop (15'), your flow would drop about 10% due to friction.


Last edited by Rainman; 02/12/18 10:15 PM. Reason: correct mistake on head pressure calculation..TY Highflyer!