The same way, sedimentation of liquid drops in another liquid differs from sedimentation of solid particles. See: V.G. Levich, PHYSICOCHEMICAL HYDRODYNAMICS, Printice Hall, N.J. 1962.
Superficial velocity is defined in that way. It's the velocity of a fluid if I suppose there is only the fluid under consideration flowing. So find the ratio of volumetric flow rate to the total cross sectional area.