11' or deeper is nice to have, but not that important under average pond circumstances. I can think of two benefits...longer life as it fills in over time and a larger holding capacity given that it's aerated.

I also think the freeboard needs to be investigated further, but we are lacking those details.

If I had to make that pond 2-3' deeper, with my very limited knowledge of doing so (read - I have no authority on the subject), I would get another piece of pipe like the drain pipe (size and style if possible) to extend it on the pond side. Then...weld it, epoxy it, tar it, wrap it, tar it again, etc....and then build a form around it a concrete the whole works. I'd bet that it would outlast the existing rusty drain pipe. Like I said...I have no experience doing this and would not recommend raising the water level until all the info is explored.

A second thought would be to put in an all new pipe 2-3" higher and have the old pipe dug back, sealed off, and abandoned. Once again, I have no idea how to seal it off besides "weld it, epoxy it, tar it, wrap it, tar it again, etc"...lol

Here's a flip and post of your photo...

[Linked Image from forums.pondboss.com]
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