The ramp at the RiverPark (right below Chickamauga dam) is great. Be advised though, it can get a bit tricky due to the current at times. Never had a problem there if you can handle your boat.
BE CAREFUL below the dams if you aren't familiar with the waters there. Lots and lots of rock piles and other things that can cause a bad day.
EDIT: I just saw that the ramp there is closed currently. Sorry about that.
The only other ramp that I am familiar with is way down stream at Sullivans. Long ride from there to the dam though.
You could consider putting in at the ramp immediately above Chichamauga dam and locking through. Not a difficult thing to do but takes a little time sometimes.