I have a 2.3 acre pond that I am rebuilding a dam currently for. It will be 20-25 ft deep at the dam. I have cut the dam in the center so I can dig out the face and re-core the old dam. WELL, dontcha know we have a 100 year flood here in Ky. The way I see it is shear luck because I get to see it flooded and full and then when the water recedes it will go back to empty. I can mark water levels and it also cleaned out the debris. My thing is I am SO glad that I had this extra holding capacity. It probably saved my house from flood waters. It did bring a question in my head about a syphon drain. If my dam was fixed, syphon in place, and I had a flood again, the water wouldnt pull from the bottom with high flood waters behind my dam. SO- why put the inlet so deep. Wouldnt the syphon only work until the water rises to the height of the inlet? I plan on putting in aeration so the bottom of the pond will be oxygenated. Is a deep inlet needed with a syphon when you have diffusers that deep?

Behind the dam before

http://s821.photobucket.com/user/JasonRu...106032.jpg.html

http://s821.photobucket.com/user/JasonRu...212358.jpg.html

http://s821.photobucket.com/user/JasonRu...758006.jpg.html


Behind the dam after

http://s821.photobucket.com/user/JasonRu...652118.jpg.html

http://s821.photobucket.com/user/JasonRu...534873.jpg.html

Cut through

http://s821.photobucket.com/user/JasonRu...610055.jpg.html

http://s821.photobucket.com/user/JasonRu...420373.jpg.html

Looking up inside

http://s821.photobucket.com/user/JasonRu...701495.jpg.html

Now it looks like

http://s821.photobucket.com/user/JasonRu...81_HDR.jpg.html

http://s821.photobucket.com/user/JasonRu...112433.jpg.html

The bottom withdrawal system goes into siphon mode when the water gets above the vent. See this page for an diagram/explanation:
http://www.georgialandsales.com/pdf/PondSiphonSystem.pdf

I suspect during most of the time when very little or no water is running through the pipe, your aeration system would be needed to keep oxygen at the lower depths. Looks like a fun project!

There are many possible designs on a water control system (siphon plus emergency spillway). It is best to have the option that lets you remove the bottom pond water.