Hi Ark, welcome to Pond Boss! I can't answer your question so I won't but I'm sure of the expert members will chime in. Sounds like you have put quite a lot of thought into this project. I like your double spillway idea.

Wow a 13 acre pond, dang that it one nice size pond! How much total acreage do you have?

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)