You could also buttress your weir boards from behind. Put the butt end of some 2x10s on the concrete of your outlet and the other end on your weir boards.
I think the ice is going to put the most pressure on your top board. You should be able to horizontally brace a 4x4 across your culvert and use that to create the anchor to buttress for your top weir boards.
That should still allow plenty of water bypass during a freak winter flood. Just take it all out when your ice breaks up this spring.
Hey thanks, well looks like running the aerator all night freed it right up! Now it's flowing and that was my worry - that the ice was just going to lift up my weir boards. Some great ideas thanks very much. For next season it should be flowing quite well all winter.
I also checked my DO today and was at about 3 - 4ppm down about 4 feet, the RBT and BRT are still cruising at 10 feet and lower so they must be doing all right.