esshup brings up an excellent point on the 45 outlet...it WOULD freeze. A spring loaded, rubber flap valve on a 22.5 degree angle would allow enough water to drain, eliminating the "complete" freeze blockage danger, and preventing air entering. Even IF air entered, a siphon would restart.
BUT, this is a bottom draw system that can be converted to siphon...NOT intended to ever start a siphon unintentionally. I do not think a snow drift, or even a ice covered drift could ever become air tight and not allow enough air into a vent tube to cause siphoning to start. The key, will be burying the pipe in soils deep enough to ever allow freezing anywhere in the pipe where water is always present. Flowing water will always be warm enough in the pipe to keep anything more than peripheral icing over. Like a thickly ice covered river, water stays warm enough to flow.
I appreciate your feedback, Rainman. I tend to agree with you, insofar as I think it unlikely that the vent would become hermetically blocked in the winter, although I plan to give it more thought, just because it's my nature to consider a lot of different 'what-if' situations.
As to a 45-degree trap, it also appears to me that it could freeze solid if/when the water flow rate is too slow to keep it clear of ice, although in my current mode of thought I'm not considering using one of those anyway.
The artesian water well at our Alna house has a stainless steel overflow pipe that extends out into the air, thus being exposed to sub-zero temperatures, and it has never frozen. I suppose that there's enough water flow through it relative to its area, mass, heat conductivity, etc., to keep it warm enough to prevent any ice from forming.
As to the water flowing through the pond under an insulating layer of ice, I don't anticipate a problem, at least not at this point, but being new to this, I plan to do some reading to learn more about ice formation and the factors that influence it to see if I might need to make any alterations, such as pond basin shape, depth, width, water entry, etc., with the goal of dependable flow in the winter and good ice formation.
We live on a tidal, freshwater portion of the Androscoggin River in Brunswick, and a lot of water flows under the ice, so I'm somewhat familiar with that situation. The ice, combined with tidal changes, currents, expansion, spring breakup, etc., is a formidable force. I just finished building a floating dock at the Brunswick house, which has to be removed from the river each year before winter or the ice would destroy it. We have roughly a five- to six-foot tidal range here that moves the ice around, and when the ice breaks up in the spring, the floes floating down the river can do a lot of damage. But that's getting off on a tangent.
Anyway, as you said, any part of the pipes in the dam that would retain water would have to be placed well beneath the frost line. I would also guess that it might be a good idea to place them a little deeper than usual, since I imagine that because of the shape of the dam, the earth in the dam has more surface area exposed to the cold air relative to its mass than the equivalent amount of earth below a flat surface...or at least that's the way it seems to me.
What I envision so far is some kind of simple, low-maintenance bottom-draw system as you mentioned, preferably with no moving parts to depend on for its normal operation, which would allow the flow rate in the drainage below the dam to remain basically normal and consistent. And if I can convert it to a siphon situation if/when I want to drain the pond, that would be great.
I eventually plan to draw up some design ideas and post them to get some opinions on how well they might work...or not. And of course I'd like to get your feedback on them.
