Pond Boss Forums POND CONSTRUCTION Building a dam 12” siphon system

I’m in the process of building a pond that will be 5-6 acres with a little over 100 acres of watershed. My original plan was to do a stand alone pipe till I found out about a siphon system drainage. My question is, will a 12” siphon be enough for my lake?

I don't know how your NRCS agents are in your area (some are more helpful than others) but mine came out, figured our runoff area, looked at annual rainfall and maximum rainfall for our area, and gave us a size recomendation for overflow pipe capacity.

So calling your local NRCS office might or might not be helpful.

What snrub said, or if not them I would have an engineer run some rough numbers, it shouldn't cost too much and might be money well spent, just my opinion it is definitely on the small side for 100 acres but without more info as to terrain and cover that the acreage has and your annual rainfall average its hard to guess.

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.