you need a larger vertical pipe connecting to the horizontal pipe. it is called a riser. like ben said, there is a ratio. also needs a cross in there to break the vortex. my old pond had a 12" pipe vertical and horizontal. it never carried but about a 1/3 pipe full at discharge. my new pond has a 10" horizontal and a 12" vertical. it carries a full pipe during heavy rains.


Scott Hanners