Re: How deep to dig drainage ditch?
Putting a trench in your asphalt isn't a good idea. The weight of vehicles driving over the edges of the asphalt, along with moisture under it freezing, will destroy the driveway in short order.
I would recommend regrading the surround area so that water naturally drains away from the driveway AND fill the low spot in the driveway with more asphalt to keep water from being able to pool on the driveway.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!