My local NRCS agent was not the best of help. He did help determine if the soil was suitable, but that was about it. By going going off my rain gauge and measuring the rise in water level in the pond, I have roughly determined that for every 1" of rain I receive when the ground is saturated and all run off makes it to the pond, I get a 3" rise in water level. Being that it is a 1.5 acre pond, take 1" from the 3" (what fell directly into the pond) leaving 2" which would mean an additional 3 acre of water shed is hitting the pond.
Our land is very flat (compared to you guys up north) and we average 55" of rain per year. We have received over 15" so far this year and are expecting several more inches this weekend.
Behind my berm, about 7' from where my overflow pipe comes out, there is an 8" pipe that goes under my driveway. Roughly 75% of the water that used to drain through it now goes into the pond. When full, it will go through the 6" pipe and back to the 8", under the driveway, and into the gully.
Since I have been here, through 2 hurricanes and several very wet winters, I have only seen the 8" pipe fill up to full flow twice, but never enough to go over the driveway (the top of the pipe is about 12" from the top of the DW).
I plan on having the normal pool level at least 12" from my emergency spill way level. If that is not enough I can lower it by lowering the height of my inner pipe. I may add an additional 6" drain pipe set up as a siphon system at a later point.
