Speaking from experience with my dad's lake and ponds, I was always amazed at how much water could come through in floods and would overwhelm the drainage culverts/ etc. often undercutting them. I remember tons of rip-rap getting carried away like little pebbles. You could hear them knocking against each other when they got washed off.
When you design the drainage/ overflow culverts/ pipes. Take whatever you think you need and quadruple it. It still won't be enough!
Same goes for digging it out- make it twice as deep as you think you'll need, and make sure to consider stumps if you need to clear trees before filling/ and you will be swimming.
Stumps in my dad's lake were still there after 30+ years, as were lawnmowers, Jimmy Hoffa, etc..