Totally agree with posts above. NRCS was a big help for me. Not sure if they will offer help if they haven’t been involved with design from the start.

4 things will determine the size of the outflow pipe: 1) runoff acres 2) rainfall with a “50 year rain event” which is a maximum rain fall in a 24 hour period expected every 50 years 3) number of surface acres of pond and 4) height of free board until the emergency spillway takes on water.

My plan included a 12” outflow pipe and I questioned the NRCS whether that would be enough. Seemed way too small to me. They assured me the computer design showed it would be.

I have a three acre pond with I think about 120 acres of runoff, two feet of free board and a 12” PVC outflow pipe that is built into the levee angled down at 45 degrees. In the 10 years I have had the pond, I have had two 50 year rain events (one of them about 6 weeks after completion of the project; the NRCS engineer called me the day after the storm to see if we still had a pond). In neither case did the emergency spillway become active. During one of the events, another 6” rise and it would have started to go over emergency spillway.

Watching the outflow from that 12” pipe on the back side of the levee when the water is a foot above it on the inflow side is incredible. The siphoning effect of that water being pulled down hill from above is unfathomable. The amount of water coming out looks like it is a 48” pipe. Not sure if the same dynamics would be seen with a stand pipe or a siphon system.

Again, can’t emphasize enough the quality of engineering from the NRCS office. In addition, through them, they arranged for 50% cost share for construction through the state of Iowa.