In studying various drainage systems for my own pond, this is the best drawing and explanation of a bottom withdrawal/siphon system I have found on the interwebs:

http://www.georgialandsales.com/pdf/PondSiphonSystem.pdf

This would probably be the least invasive to the dam as it does not need to penetrate much of the core. Just shallow digging with a backhoe. Install it then plug the existing drain pipe with concrete? Perhaps no need for a coffer dam.

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