it's diversity that will hold the bank together. so the more kinds of things you can plant there, the better.

I'd start at the bottom of the ditch with water-tolerant species like flags, the native daylily species, and I'm sure there's something native to your area that tends to grow at the base of creekbeds. Primrose actually loves to live creekside, so you may be able to get them to adapt to the 'run-off' lifestyle. woodland phlox as well as the creeping form make good plants for naturalizing, and Lamium is a plant I never fail to plug- it goes from near full sun to pretty deep shade, and the silver and green foliage is always attractive.

I'd put in more berry-bearing bushes...blueberries if your ground will support them (acidic soil is a must for them) or check for native species...

http://www.tneppc.org/

this also has a list of things that your local aggies wish you would avoid, since they're better suited to the area than the natives.

Mulberries will live anywhere in TN, and the birds love them! Same with dogwoods, redbuds, etc. I highly recommend the natives. Be aware that that culvert is going to be a magnet for weed seeds, and if you bring the birds, they are going to poop their last lunch of privet seeds in your culvert. Not to cast doom and gloom, but I have a steep hill on my property and it's an ongoing battle.