Consider installing a stoplog structure (flashboard riser) on the inlet side of the culvert, perhaps with steel channel and removable boards. Commercially available ones in that size might break the bank, so you may want to build your own. The trick will be sealing things up, but expanding wood is pretty good at that. This kind of set up lets you control the water level by adding or removing boards.

http://wisconsinflowgate.com/

http://www.glasssteelinc.com/stoplogs.html

https://bid.houseauctioncompany.com/lot-...;current_page=0

https://www.briggsconcrete.com/

Last edited by RAH; 02/26/20 08:56 AM.