Hi Terry,

Ideally you will not bury the existing sod that deeply; the layer of dead grass could create issues for water movement in the soil and make it more difficult to manage your new grass.  Furthermore, as that material decomposes, the soil level will sink again.  

An alternative to digging the sod out, though, could be to "scalp" the lawn in the depressed area with a weed-whacker, removing the grass down to bare soil.  Rake and remove the cut grass and any thatch and then add new fill to level the trench.  

I hope this is helpful.

Cordially,

John

Hi Terry,

You want the seed barely covered; just enough to keep the seeds moist.  You want the soil to remain damp at all times but not saturated.  You could use grass clippings from elsewhere in the yard to cover the area as a mulch and it will help the seeded section uniformly moist.  Depending on the soil and exposure and whether or not you use mulch, this could be up to several short watering sessions per day.  

Depending on the grass species in the seed mix, 1-2 weeks would be a normal time to see germination beginning.

Good luck!

John