mousebob,

What was the diameter of the corrugated pipe that you replaced?

How often did your pond discharge water through your emergency spillway under the prior piping system?

Is your emergency/overflow spillway in good shape?

I believe the design reason for the smaller diameter in the horizontal pipe is the proper engineering practice of saving money on the project. Water flow at the top of the vertical drop inlet will have essentially zero head. Flow through the horizontal section of the pipe will have the head equivalent to the height of the drop inlet. Therefore, the horizontal section will flow significantly more water than the flow through a drop inlet of equal diameter.

Your outlet flow will be dependent on the 15" diameter of your drop inlet pipe.

If you are fairly comfortable with your answers to the three questions posed above, then I think you should be fine!

If the situation is more complicated than that, then please post some more information and perhaps someone with far more expertise than myself will drop into your thread.

I believe the advice to install a trash rack and anti-vortex plate are by far your most important factors at this time to protect your pond. A clogged outlet flows NO water!