Doug,
The answer depends on a lot of factors including the soil conditions outside of where you want the bamboo to stay. If you plan to fill the ditch with mulch or equivalent then 30" should be enough. Bamboo tend to run where the conditions are best for them so if you only fertillize and water near the center of where you want them to stay it will reduce the amount of runners into nearbye poorer drier soil. Heavy much also tends to keep the rhizomes closer to the surface.

I prefer soaker hoses.

Mike near Brenham TX