It depends on what the clog is ?
You can approach this in a few different ways - PUSH - PULL - FLUSH
PUSH - get 2 10' lengths of 1-1/2" PVC, ABS, or even galvanized- AND a coupler to join the 2 pieces together. - Take the makeshift ramrod and push the obstacle through.
FLUSH - get 2 10' lengths of 1/2" PVC, galvanized, ? - a coupler to join the 2 , an adapter to go to garden hose male, garden hose female.
Assemble parts, hook garden hose on one end, sprayer head on other end (straight, twist open style)
Turn on water and adjust sprayer for a strong stream- insert into culvert, and work it in and out (the sprayer will blast a hole in the obstruction) REPEAT After you get a couple holes blasted through let the water run (on the uphill side) and the remainder of the blockage should wash out.