I personally would also try to run a narrow trench laterally in the center of the dam filled with concrete and maybe some wire mesh. I would make the bottom of it lower than the bottom of the front concrete facing. It would be sort of an anti-seep collar, ditched in the solid core of the dam.
Not that I know anything about it. I'm not an engineer nor do I have experience with flash board systems. That is just the way this uneducated farmer would approach the problem. Use the Idea at your own risk.
And good luck. I have thought about raising my main pond another 4" and it would be nice to somehow use a upside down gate valve on my 8" overflow pipe so I could also adjust the water level just a little. Not a great amount but maybe by 6" or so.
If you attempt, take pictures and document and share. We can learn from your mistakes!!! LOL
