jsec,
Isn't this a great forum! Wish I had known about it before starting therapy. After reading your post thought I could share with you how my dam works and how it might work for you.
Background: Spring fed, NO runoff water to the pond. 1.25 acre.
I have about 94" of depth over the ditch that carries my overflow. I built a stand tube (10" pvc) in the pond about 30' from shore. I have a bedrock bottom, so it is supported by a steel tripod up to about 68". At the bottom, a tee into 8" pvc that runs back through the dam (and four seep collars). The top of the standpipe is 4" below my designed pool height. In the dam, I have another tee and 10" standpipe that is set to my designed pool height with a T at the top. The top Tee is set so it is open to the top, and a side port for discharge. This overflow goes through about a 3' piece of 10" and a 45 into a 12" plastic culvert buried down the backside of my dam.
Anyway, now for the good part
. I built a stainless steel flap valve to close off the end of the tee in the bottom of the pond via pulleys on the tripod and a rope to shore. I also have a 10" flange plate cover with a 2" port, and long bolts in the ditch (after buying all that pvc, I was too cheap to buy a 10" gate valve, and regretting it).
Normal (summer) operation: Flange plate in ditch tight, 2" outlet capped. Flapper valve in bottom of pond open, bottom water flows up the standpipe in the dam, and down the culvert to the ditch (with a very pleasing sound and oxygen added).
Fall/winter operation: Close Flapper valve via rope. Loosen flange plate bolts (I should be simply opening a valve
). The water pressure seals the flapper valve shut as soon as the water level in the dam standpipe drops. Level drops 4" for winter (nice skimming action). Overflow pretty much keeps the pipe from freezing in, I also have a homemade bubble tube around the standpipe back to the aerator if I need to unfreeze it.
There you have it, custom from the word go, but I like it. No ugly standpipe visible during the summer; bottom draw (although at 7' the value in that is debatable). Oxygenated overflow that may be a boost in your situation. No losses of minnows or other top dwellers (cage around the bottom to keep the fish in). I had to submit drawings and all my civil calculations to get a small dam permit from the Wisconsin DNR. The 10"/8" combination is to meet my area's 100yr max rainfall/hr with a 60% safety margin. Hopefully my auxiliary overflow will only get wet via rain.
HUGE WARNING! One potentially devastating problem that dawned on me while my pond was filling, and caused me to face my first drawdown (2'). Imagine if you will, your first winter, you close the flapper, and the water drains to the stand pipe height in the pond. Life is good until you realize that the 30' of 8" pvc at the bottom of your pond and your 10" standpipe are now filled with air and are trying to rip your standpipe out of it's moorings!!!
My solution was to draw down, lay heavy monofilement fabric over the pipe, and then hand stack about three tons of rocks on the fabric and pipe, and then coat the whole thing in cement.
If you want more than a mental picture, let me know, I can draw something up to post. I've got lots of pictures too, but they are 35mm.
