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.
