It is a good idea to practise crop rotation with your carrots. Carrots grown in the same spot as last year may be susceptible to attack from pupae that has overwintered in the soil. Moving your crop each year should prevent this from occurring.
There is a Nematode available that can be watered into the soil to kill any larvae that are hiding there. In the Kitchen Garden we also use Carrot Root Fly Traps to give us an idea if the flies are present. This helps you time your application of the nematodes.
When you put your covers on they should be 2 feet high and it’s a good idea to use an insect mesh to stop the flies getting in.
Good luck. Here’s hoping for a root fly free year!