Lindsey
As you write, you are on the right track. Since your footing does not rest below grade it would not resist soil weight. I assume from your description that the slab and footing are monolithic. That being the case, a worst case design would be when uplift nearly overcomes the shear...
What is the usual flood recurrence used to check flotation of structures? What safety factors are associated with what periods of occurence? Also, can you recommend references that address bouyancy checks?
