I am renovating a pond that I had built about 30 years ago. The pond is about 0.6 acre surface area and is 18' deep at its deepest spot, but is generally pretty deep throughout most of the pond. The pond had filled with sediment which I have removed. There was 6' of sediment at the bottom of the pond in it's deepest spot. The pond was built with a 6" galvanized pipe as a surface withdrawal spillway. I would like to install a bottom withdrawal spillway to improve the water quality and to prolong the life of the pond. The original galvanized spillway is probably nearing the end of it's life, so I thought I might use 4" PVC to build the new bottom withdrawal spillway and feed it thru the existing galvanized pipe. The galvanized pipe would be sealed to prevent water from entering. This would allow me to not have to cut thru the dam to install the spillway outflow pipe. I am in east central Iowa. My question is if the 4" pipe is large enough for what I am trying to do. Any opinions would be appreciated.
I hope you have a good emergency spillway. A 4 inch pipe is not nearly big enough as a primary spillway, but is big enough to use to control the normal water level. What you need is a hybrid bottom siphon / surface spillway. You will want it to be dry during freezing weather, so a valve at the top, but slightly on the downhill side and one at the downhill end will be necessary to control and start / stop the siphon. You would close the valve at the open, downhill end, open the valve at the top, fill the pipe on the downhill slope, then close the valve at the top and open the one at the downhill end to start the siphon. You would need a wye in the pipe with a valve at the high water mark that would be normally open to keep the water level where you want it. When not in siphon mode, all the valves should be open. In siphon mode, the top valve and the one at the high water level would be closed. The downhill end of the 4" pipe should be considerably longer than the end going into the pond.
If I can figure out how to put a drawing on here, I will. Edit: I took a picture of the drawing with my phone. Don't know how I would post a pdf otherwise.
Thanks for the responses. I can see the need to keep an open dry pipe during freezing conditions, so the system does not freeze closed. John- I do have a good, wide emergency spillway that has never been used in the 30 years since the pond was built. That is why I was thinking I could save some digging by installing a 4" pvc outlet pipe thru the current surface spillway (6" corrugated metal). It may be safer to not change the sizing for the spillway and to cut thru the dam at the current full water level to install 6" PVC for the bottom withdrawal spillway.
