I see a hole or two in your flow logic. You say that the 18,000 gpm pump cannot keep up at the extremes. Generically speaking, with little to no head pressure at the pipe inlet, an 8" pipe can carry only 950 gpm...a 12" pipe might be upwards of 4700 gpm with decent head pressure.
Either you pump is not pushing 18,000 gpm or your pipe sizes are too small for the extreme rain events.
I don't pretend to be an expert in fluid dynamics, but I suggest you look deeper into this.
I can say that pump specs are typically advertised at maximum capacity, meaning that the pump has NO lengths of hoses attached but somehow magically receives the water. This spec is to sell pumps not to be very practical, but should lend an idea of what it can do. Even at half spec, 9,000 gpm is more than a fairly level and short run of 12" pipe can carry.
How much water shed does your pond have?
Point of reference...My 1/4 acre pond with about 15 acres of watershed can overtake my 15" drain pipe. It has not gone over the dam yet, but it will eventually. Before renovation, it had a 18 inch pipe and overflowed a time or two. I, now, have extra freeboard to help during those extreme rains, but that only buys me time.
Hopefully I am missing something, but a pond drain expert may be a good idea.
